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Nearest Milky Way Spiral Arm Closer than Previously Thought
(Added 12/30/05) Astronomers have determined laid to resta problem in galactic astronomy: How far away is the Earth's closest spiral arm? New measurements that were made with the planet-wide consortium of radio telescopes, the VLBA, has found that the Perseus arm lies 1.95±0.04 kpc (6.36±0.13 thousand light-years) away from Earth.
Previous techniques involve modeling how fast stars and nebulae within the Galaxy should travel based upon their distances from the center. But, this method has many problems associated with it, with the largest being that the measured velocity from Earth can be a faulty indicator of true speed, and some clusters can have anomalously high or low velocities.
The Perseus arm, the closest spiral arm of our Galaxy to Earth, has previously had its distance measured by observing ultra-luminous stars known as Type O. The distances derived from these are approximately 7.2 thousand light-years. However, measurements based upon the velocities of material observed in the arm result in a distance of 27.7 thousand light-years.
This study used a massive star-forming region known as W3OH in the Perseus arm. The astronomers used the VLBA to measure the slight shift in its apparent location based upon the movement of Earth, known as "parallax." This is the phenomenon of aligning your index finger with a distant target, closing one eye and then switching, and the object appears to move. This is how your brain determines the 3-D nature of the world arround you, and by very careful measurements of this shift, astronomers can measure distances in our Galaxy.
The extremely accurate parallax measurement of W3OH puts the distance to the Perseus arm near the minimum, with the O stars measurements. Other implications are, based upon the accuracy of these measurements, the VLBA could be used to measure parallaxes out to a distance of about 33 thousand light-years, which is a factor of 100 better than the most accurate and comprehensive study to-date, which was made by the Hipparcos satellite.
Adapted from the information in: Xu, Y. et al. "The Distance to the Perseus Spiral Arm in the Milky Way" (2005), which is available at http://lanl.arxiv.org/pdf/astro-ph/0512223.
First Triple Asteroid Discovered
(Added 12/26/05) One of the thousands of minor planets orbiting the Sun has been found to have its own mini planetary system. Astronomer Franck Marchis (University of California, Berkeley) and his colleagues at the Observatoire de Paris (France) have discovered the first triple asteroid system - two small asteroids orbiting a larger one, where the main asteroid has been known since 1866 as 87 Sylvia.
"Since double asteroids seem to be common, people have been looking for multiple asteroid systems for a long time," said Marchis. "I couldn't believe we found one."
The discovery was made with Yepun, one of ESO's 8.2-m telescopes of the Very Large Telescope Array at Cerro Paranal (Chile), using the outstanding image sharpness provided by the adaptive optics NACO instrument. Via the observatory's proven "Service Observing Mode," Marchis and his colleagues were able to obtain sky images of many asteroids over a six-month period without actually having to travel to Chile.
One of these asteroids was 87 Sylvia, which was known to be double since 2001 from observations made by Mike Brown and Jean-Luc Margot with the Keck telescope. The astronomers used NACO to observe Sylvia on 27 occasions over a two-month period. On each of the images, the known small companion was seen, allowing Marchis and his colleagues to precisely compute its orbit. But on 12 of the images, the astronomers also found a closer and smaller companion, indicating that the asteroid was not double, but a triple system.
Because 87 Sylvia was named after Rhea Sylvia, the mythical mother of the founders of Rome, Marchis proposed naming the twin moons after those founders: Romulus and Remus. The International Astronomical Union approved the names.
Sylvia's moons are considerably smaller than itself, orbiting in nearly circular paths and in the same plane and direction. The closest and newly discovered moonlet, orbiting about 710 km from Sylvia, is Remus, a body only 7 km across and circling Sylvia every 33 hours. The second, Romulus, orbits at about 1360 km in 87.6 hours and measures about 18 km across.
The asteroid 87 Sylvia is one of the largest known from the main asteroid belt, and it is located about 3.5 times further away from the Sun than Earth. The wealth of details provided by the NACO images show that 87 Sylvia is shaped like a lumpy potato, measuring 380 x 260 x 230 km. It is spinning at a rapid rate, once every 5 hours and 11 minutes.
The observations of the moonlets' orbits allow the astronomers to precisely calculate the mass and density of Sylvia. With a density only 20% higher than the density of water, it is likely composed of water ice and rubble from a primordial asteroid. "It could be up to 60% empty space," remarked co-discoverer Daniel Hestroffer (Observatoire de Paris, France).
"It is most probably a 'rubble-pile' asteroid," Marchis added. These asteroids are loose aggregations of rock, presumably the result of a collision. Two asteroids smacked into each other and got disrupted. The new rubble-pile asteroid formed later by accumulation of large fragments while the moonlets are probably debris left over from the collision that were captured by the newly formed asteroid and eventually settled into orbits around it. "Because of the way they form, we expect to see more multiple asteroid systems like this."
Marchis and his colleagues reported their discovery in the August 11 issue of the journal Nature, simultaneously with an announcement that day at the Asteroid Comet Meteor conference in Armação dos Búzios, Rio de Janeiro state, Brazil.
Adapted from the information on http://www.eso.org/outreach/press-rel/pr-2005/pr-21-05.html.
Finally, an Image of an Extra-Solar Planet
(Added 12/26/05) On April 30, an international team of astronomers reported confirmation of the discovery of a giant planet, approximately five times the mass of Jupiter, that is gravitationally bound to a young brown dwarf. This puts an end to a year-long discussion on the nature of this object, which started with the detection of a red object close to the brown dwarf.
In February and March of this year, the astronomers took new images of the young brown dwarf and its giant planet companion with the NACO instrument on ESO's Very Large Telescope in northern Chile. The planet is near the southern constellation of Hydra and approximately 200 light-years from Earth.
"Our new images show convincingly that this really is a planet, the first planet that has ever been imaged outside of our solar system," explained Gael Chauvin, astronomer at ESO and leader of the team of astronomers who conducted the study.
"The two objects - the giant planet and the young brown dwarf - are moving together; we have observed them for a year, and the new images essentially confirm our 2004 finding," remarked Benjamin Zuckerman, UCLA professor of physics and astronomy, member of NASA's Astrobiology Institute, and a member of the team. "I'm more than 99% confident." The separation between the planet and the brown dwarf is 55 times the separation of the Earth and Sun.
Anne-Marie Lagrange, another member of the team from the Grenoble Observatory in France, looked towards the future: "Our discovery represents a first step towards one of the most important goals of modern astrophysics: To characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets."
Last September, the same team of astronomers reported a faint reddish speck of light in the close vicinity of a young brown dwarf. The feeble object, now called 2M1207b, is more than 100 times fainter than the brown dwarf, 2M1207A. The spectrum of 2M1207b presents a strong signature of water molecules, confirming that it must be cold. Based on the infrared colours and the spectral data, evolutionary model calculations led to the conclusion that 2M1207b is a 5 Jupiter-mass planet. Its mass can be estimated also by use of a different method that focuses on the strength of its gravitational field; this technique suggests that the mass might be even less than 5 Jupiters.
At the time of its discovery in April 2004, it was impossible to prove that the faint source is not a background object (such as an unusual galaxy or a peculiar cool star with abnormal infrared colours), even though this appeared very unlikely. Observations with the Hubble Space Telescope, obtained in August 2004, corroborated the VLT/NACO observations, but were taken too soon after the NACO ones to conclusively demonstrate that the faint source is a planet.
The new observations show with high confidence that the two objects are moving together and hence are gravitationally bound. "Given the rather unusual properties of the 2M1207 system, the giant planet most probably did not form like the planets in our solar system," says Gael Chauvin. "Instead it must have formed the same way our Sun formed, by a one-step gravitational collapse of a cloud of gas and dust."
Adapted from the information on http://www.eso.org/outreach/press-rel/pr-2005/pr-12-05.html.
Giant Star Cluster Found Nearby
(Added 12/26/05) Stars are generally born in small groups, mostly in open clusters that typically contain a few hundred stars. From a wide range of observations, astronomers infer that the Sun itself was born in one such cluster, some 4.5 billionyears ago. In some active ("starburst") galaxies, scientists have observed violent episodes of star formation, leading to the development of super star clusters, each containing several million stars. Such events were common during the Milky Way's childhood, more than 12 billion years ago: The many galactic globular clusters - which are nearly as old as our Galaxy - are thought to be the remnants of early super star clusters.
All super star clusters so far observed in starburst galaxies are very distant. It is not possible to distinguish their individual stars, even with the most advanced technology. This dramatically complicates their study and astronomers have therefore long been eager to find such clusters in our neighbourhood in order to probe their structure in much more detail. Now, a team of European astronomers has finally succeeded in doing so, using several of ESO's telescopes at the La Silla observatory (Chile).
The open cluster Westerlund 1 is located in the Southern constellation Ara (the Altar). It was discovered in 1961 from Australia by Swedish astronomer Bengt Westerlund, who later moved from there to become ESO Director in Chile (1970-74). This cluster is behind a huge interstellar cloud of gas and dust which blocks most of its visible light. The dimming factor is more than 100,000 -- and this is why it has taken so long to uncover the true nature of this particular cluster.
In 2001, the team of astronomers identified more than a dozen extremely hot and peculiar massive stars in the cluster called "Wolf-Rayet" stars. They have since studied Westerlund 1 extensively with various ESO telescopes. From these observations, they were able to identify about 200 cluster member stars. To establish the true nature of these stars, the astronomers then performed spectroscopic observations of about one quarter of them. These observations have revealed a large population of very bright and massive, quite extreme stars. Some would fill the solar system space within the orbit of Saturn, and others are as bright as a million Suns.
Westerlund 1 is obviously a fantastic stellar zoo, with a most exotic population and a true astronomical bonanza. All stars identified are evolved and very massive, spanning the full range of stellar oddities from Wolf-Rayet stars, OB supergiants, Yellow Hypergiants (nearly as bright as a million Suns) and Luminous Blue Variables (similar to the exceptional Eta Carinae object).
All stars so far analysed in Westerlund 1 weigh at least 30-40 times more than the Sun. Because such stars have a rather short life - astronomically speaking - Westerlund 1 must be very young. The astronomers determine an age somewhere between 3.5 and 5 million years.
"If the Sun were located at the heart of Westerlund 1, the sky would be full of stars, many of them brighter than the full Moon", commented Ignacio Negueruela of the Universidad de Alicante in Spain and member of the team. The large quantity of very massive stars implies that Westerlund 1 must contain a huge number of stars. "In our Galaxy," explained Simon Clark of the University College London (UK) and one of the authors of this study, "there are more than 100 solar-like stars for every star weighing 10 times as much as the Sun. The fact that we see hundreds of massive stars in Westerlund 1 means that it probably contains close to half a million stars, but most of these are not bright enough to peer through the obscuring cloud of gas and dust". This is ten times more than any other known young cluster in the Milky Way.
This super star cluster now provides astronomers with a unique perspective towards one of the most extreme environments in the Universe. Westerlund 1 will certainly provide new opportunities in the long-standing quest for more and finer details about how stars, and especially massive ones, do form.
Adapted from the information on http://www.eso.org/outreach/press-rel/pr-2005/pr-08-05.html.
New Evidence for How Stars Form Near Massive Black Holes
(Added 12/26/05) The supermassive black hole at the center of the Milky Way has surprisingly helped spawn a new generation of stars, according to observations from NASA's Chandra X-ray Observatory. This novel mode of star formation may solve several mysteries about the supermassive black holes that reside at the centers of nearly all galaxies.
"Massive black holes are usually known for violence and destruction," remarked Sergei Nayakshin of the University of Leicester, United Kingdom, and coauthor of a paper on this research in an upcoming issue of the Monthly Notices of the Royal Astronomical Society. "So it's remarkable that this black hole helped create new stars, not just destroy them."
Black holes have earned their fearsome reputation because any material -- including stars -- that falls within the event horizon is never seen again. However, these new results indicate that the immense disks of gas known to orbit many black holes at a "safe" distance from the event horizon can help nurture the formation of new stars.
This conclusion came from new clues that could only be revealed in x-rays. Until the latest Chandra results, astronomers have disagreed about the origin of a mysterious group of massive stars discovered by infrared astronomers to be orbiting less than a light year from the Milky Way's central black hole, AKA Sagittarius A*, or Sgr A*. At such close distances to Sgr A*, the standard model for star formation predicts that gas clouds from which stars form should have been ripped apart by tidal forces from the black hole.
Two models to explain this puzzle have been proposed. In the disk model, the gravity of a dense disk of gas around Sgr A* offsets the tidal forces and allows stars to form; in the migration model, the stars formed in a star cluster far away from the black hole and migrated in to form the ring of massive stars. The migration scenario predicts about a million low mass, sun-like stars in and around the ring, whereas in the disk model, the number of low mass stars could be much less.
Nayakshin and his coauthor, Rashid Sunyaev of the Max Plank Institute for Physics in Garching, Germany, used Chandra observations to compare the x-ray glow from the region around Sgr A* to the x-ray emission from thousands of young stars in the Orion Nebula star cluster. They found that the Sgr A* star cluster contains only about 10,000 low mass stars, thereby ruling out the migration model.
"We can now say that the stars around Sgr A* were not deposited there by some passing star cluster, rather they were born there," explained Sunyaev. "There have been theories that this was possible, but this is the first real evidence. Many scientists are going to be very surprised by these results." Because the Galactic Center is shrouded in dust and gas, it has not been possible to look for the low-mass stars in optical observations. In contrast, X-ray data have allowed astronomers to penetrate the veil of gas and dust and look for these low mass stars.
The results suggest that the "rules" of star formation change when stars form in the disk of a giant black hole. Because this environment is very different from typical star formation regions, there is a change in the proportion of stars that form. For example, there is a much higher percentage of massive stars in the disks around black holes. And, when these massive stars explode as supernovae, they will "fertilize" the region with heavy elements such as oxygen. This may explain the large amounts of such elements observed in the disks of young supermassive black holes.
Adapted from the information on http://chandra.harvard.edu/press/05_releases/press_101305.html.
How Much Neon Exists in the Sun?
(Added 12/26/05) NASA's Chandra X-ray Observatory survey of nearby sun-like stars suggests there is nearly three times more neon in the sun and local universe than previously believed. If true, this would solve a critical problem with understanding how the sun works.
"We use the sun to test how well we understand stars and, to some extent, the rest of the universe," explained Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. "But in order to understand the sun, we need to know exactly what it is made of," he added. It is not well known how much neon the sun contains. This is critical information for creating theoretical models of the sun. Neon atoms, along with carbon, oxygen and nitrogen, play an important role in how quickly energy flows from nuclear reactions in the sun's core to its edge, where it then radiates into space.
The rate of this energy flow determines the location and size of a crucial stellar region called the Convection Zone. The zone extends from near the sun's surface inward approximately 125,000 miles. The zone is where the gas undergoes a rolling, convective motion much like the unstable air in a thunderstorm. "This turbulent gas has an extremely important job because nearly all of the energy emitted at the surface of the sun is transported there by convection," Drake described.
The accepted amount of neon in the sun has led to a paradox. The predicted location and size of the solar convection zone disagree with those deduced from solar oscillations. Solar oscillations - known also as "helioseismology" - is a technique astronomers previously relied on to probe the sun's interior. Several scientists have noted the problem could be fixed if the abundance of neon is in fact about three times larger than currently accepted.
Attempts to measure the precise amount of neon in the Sun have been frustrated by a quirk of nature; neon atoms in the Sun give off no signatures in visible light. However, in a gas heated to millions of degrees, neon shines brightly in x-rays. Stars like the sun are covered in this super-heated gas that is betrayed by the white corona around them during solar eclipses. However, observations of the sun's corona are very difficult to analyze.
To probe the neon content, Drake and his colleague Paola Testa of the Massachusetts Institute of Technology in Cambridge, AM, observed 21 sun-like stars within a distance of 400 light-years from Earth. These local stars and the sun should contain about the same amount of neon when compared to oxygen. However, these close stellar kin were found to contain on average almost three times more neon than is believed for the sun. "Either the sun is a freak in its stellar neighborhood, or it contains a lot more neon than we think," Testa said.
These Chandra results reassured astronomers the detailed physical theory behind the solar model is secure. Scientists use the model of the sun as a basis for understanding the structure and evolution of other stars, as well as many other areas of astrophysics.
"If the higher neon abundance measured by Drake and Testa is right, then it is a simultaneous triumph for Chandra and for the theory of how stars shine," said John Bahcall of the Institute for Advanced Study, Princeton, NJ. Bahcall is an expert in the field who was not involved in the Chandra study. Drake is lead author of the study published in this week's issue of the journal Nature.
Adapted from the information on http://chandra.harvard.edu/press/05_releases/press_072705.html.
Flash Flares in Young Solar Systems
(Added 12/26/05) New results from NASA's Chandra X-ray Observatory imply that X-ray super-flares torched the young Solar System. Such flares likely affected the planet-forming disk around the early Sun, and may have enhanced the survival chances of Earth.
By focusing on the Orion Nebula (M42) almost continuously for 13 days, a team of scientists used Chandra to obtain the deepest X-ray observation ever taken of this or any star cluster. The Orion Nebula is the nearest rich stellar nursery, located just 1,500 light years away.
These data provide an unparalleled view of 1400 young stars, 30 of which are prototypes of the early Sun. The scientists discovered that these young suns erupt in enormous flares that dwarf - in energy, size, and frequency - anything seen from the Sun today.
"We don't have a time machine to see how the young Sun behaved, but the next-best thing is to observe Sun-like stars in Orion," explained Scott Wolk of Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. "We are getting a unique look at stars between one and 10 million years old - a time when planets form."
A key result is that the more violent stars produce flares that are a hundred times as energetic as the more docile ones. This difference may specifically affect the fate of planets that are relatively small and rocky, like the Earth. "Big X-ray flares could lead to planetary systems like ours where Earth is a safe distance from the Sun," remarked Eric Feigelson of Penn State University in University Park, and principal investigator for the international Chandra Orion Ultradeep Project. "Stars with smaller flares, on the other hand, might end up with Earth-like planets plummeting into the star."
According to recent theoretical work, X-ray flares can create turbulence when they strike planet-forming disks, and this affects the position of rocky planets as they form. Specifically, this turbulence can help prevent planets from rapidly migrating towards the young star. "Although these flares may be creating havoc in the disks, they ultimately could do more good than harm," said Feigelson. "These flares may be acting like a planetary protection program."
About half of the young suns in Orion show evidence for disks, likely sites for current planet formation, including four lying at the center of proplyds (proto-planetary disks) imaged by Hubble Space Telescope. X-ray flares bombard these planet-forming disks, likely giving them an electric charge. This charge, combined with motion of the disk and the effects of magnetic fields should create turbulence in the disk.
The numerous results from the Chandra Orion Ultradeep Project appeared in a dedicated issue of The Astrophysical Journal Supplement in October, 2005. The team contains 37 scientists from institutions across the world including the US, Italy, France, Germany, Taiwan, Japan and the Netherlands.
Adapted from the information on http://chandra.harvard.edu/press/05_releases/press_051005.html.
How Heavy-Weight Stars Bulk Up
(Added 12/26/05) The most massive stars in our galaxy weigh as much as 100 small stars like the Sun. How do such monsters form? Do they grow rapidly by swallowing smaller protostars within crowded star-forming regions? Some astronomers thought so, but a new discovery suggests instead that massive stars develop through the gravitational collapse of a dense core in an interstellar gas cloud via processes similar to the formation of low mass stars.
"In the past, theorists have had trouble modeling the formation of high-mass stars and there has been an ongoing debate between the merger versus the accretion scenarios." explained astronomer Nimesh Patel of the Harvard-Smithsonian Center for Astrophysics (CfA). "We've found a clear example of an accretion disk around a high-mass protostar, which supports the latter while providing important observational constraints to the theoretical models."
Patel and his colleagues studied a young protostar 15 times more massive than the Sun, located more than 2,000 light-years away in the constellation Cepheus. They discovered a flattened disk of material orbiting the protostar. The disk contains 1 to 8 times as much gas as the Sun and extends outward for more than 30 billion miles - eight times farther than Pluto's orbit.
The existence of this disk provides clear evidence of gravitational collapse, the same gradual process that built the Sun. A disk forms when a spinning gas cloud contracts, growing denser and more compact. The angular momentum of the spinning material forces it into a disk shape. The planets in our solar system formed from such a disk 4.5 billion years ago.
Evidence in favor of high-mass accretion has been elusive since massive stars are rare and evolve quickly, making them tough to find. Patel and his colleagues solved this problem using the Submillimeter Array (SMA) telescope in Hawaii, which offers much sharper and highly sensitive imaging capabilities compared to single-dish submillimeter telescopes. SMA is currently a unique instrument that makes such studies possible by allowing astronomers to directly image the dust emission at submillimeter wavelengths and also to detect emission from highly excited molecular gas.
The team detected both molecular gas and dust in a flattened structure surrounding the massive protostar HW2 within the Cepheus A star formation region. SMA data also showed a velocity shift due to rotation, supporting the interpretation that the structure is a gravitationally bound disk. Combined with radio observations showing a bipolar jet of ionized gas, a type of outflow often observed in association with low-mass protostars, these results support theoretical models of high-mass star formation via disk accretion rather than by the merging of several low-mass protostars.
"Merging low-mass protostars wouldn't form a circumstellar disk and a bipolar jet," said co-author Salvador Curiel of the National Autonomous University of Mexico (UNAM), who is on sabbatical leave at CfA. "Even if they had circumstellar disks and outflows before the merger, those features would be destroyed during the merger."
Adapted from the information on http://cfa-www.harvard.edu/press/pr0527.html.
(Added 12/26/05) In 1987, earthbound observers saw a star explode in the nearby dwarf galaxy called the Large Magellanic Cloud. Astronomers eagerly studied this supernova - the closest seen in the past 300 years - and have continued to examine its remains. Although its blast wave has lit up surrounding clouds of gas and dust, the supernova appears to have left no core behind. Astronomers now report that even the sharp eyes of the Hubble Space Telescope failed to locate the black hole or ultracompact neutron star they believe was created by the star's death 18 years ago.
"We think a neutron star was formed. The question is: Why don't we see it?" asked astronomer Genevieve Graves of UC Santa Cruz, first author on the paper announcing these results.
When a massive star explodes, it leaves behind some sort of compact object, either a city-sized ball of subatomic particles called a neutron star, or a black hole. The outcome depends on the mass of the progenitor star. Smaller stars form neutron stars while larger stars form black holes.
The progenitor of supernova (SN) 1987A weighed 20 times as much as the sun, placing it right on the dividing line and leaving astronomers uncertain about what type of compact object it produced. All observations to date have failed to detect a light source in the center of the supernova remnant, leaving the question of the outcome unanswered.
Detecting a black hole or neutron star is challenging. A black hole can be detected only when it swallows matter because the matter heats up and emits light as it falls into the black hole. A neutron star at the distance of the Large Magellanic Cloud can be detected only when it emits beams of radiation as a pulsar, or when it accretes hot matter like a black hole.
"A neutron star could just be sitting there inside SN 1987A, not accreting matter and not emitting enough light for us to see," mused astronomer Peter Challis (CfA), second author on the study.
Observations have ruled out the possibility of a pulsar within SN 1987A. Even if the pulsar's beams were not aimed at Earth, they would light the surrounding gas clouds. However, theories predict that it can take anywhere from 100 to 100,000 years for a pulsar to form following a supernova because the neutron star must gain a sufficiently strong magnetic field to power the pulsar beam. SN 1987A may be too young to hold a pulsar.
As a result, the only way astronomers might detect the central object is to search for evidence of matter accreting onto either a neutron star or a black hole. That accretion could happen in one of two ways: Spherical accretion in which matter falls in from all directions, or disk accretion in which matter spirals inward from a disk onto the compact object.
The Hubble data rule out spherical accretion because light from that process would be bright enough to detect. If disk accretion is taking place, the light it generates is very faint, meaning that the disk itself must be quite small in both mass and radial extent. Also, the lack of detectable radiation indicates that the disk accretion rate must be extremely low - less than about one-fifth the mass of the Moon per year.
In the absence of a definitive detection, astronomers hope to learn more about the central object by studying the dust clouds surrounding it. That dust absorbs visible and ultraviolet light and re-radiates the energy at infrared wavelengths. "By studying that reprocessed light, we hope to find out what's powering the supernova remnant and lighting the dust," explained Graves. Future observations by NASA's Spitzer Space Telescope should provide new clues to the nature of the hidden object. Additional observations by Hubble also could help solve the mystery. "Hubble is the only existing facility with the resolution and sensitivity needed to study this problem," said Kirshner.
Adapted from the information on http://cfa-www.harvard.edu/press/pr0515.html, and based upon a research paper online at http://arxiv.org/abs/astro-ph?0505066.
(Added 12/26/05) When the distant planetoid Sedna was discovered on the outer edges of our solar system, it posed a puzzle to scientists. Sedna appeared to be spinning very slowly compared to most solar system objects, completing one rotation every 20 days. Astronomers hypothesized that this world possessed an unseen moon whose gravity was slowing Sedna's spin. Yet Hubble Space Telescope images showed no sign of a moon large enough to affect Sedna.
New measurements by Scott Gaudi, Krzysztof (Kris) Stanek and colleagues at the Harvard-Smithsonian Center for Astrophysics (CfA) have cleared up this mystery by showing that a moon wasn't needed after all. Sedna is rotating much more rapidly than originally believed, spinning once on its axis every 10 hours. This shorter rotation period is typical of planetoids in our solar system, requiring no external influences to explain. "We've solved the case of Sedna's missing moon. The moon didn't vanish because it was never there to begin with," remarked Gaudi.
Sedna is an odd world whose extreme orbit takes it more than 45 billion miles from the Sun, or more than 500 A.U. (where one astronomical unit (A.U.) is the average Earth-Sun distance of 93 million miles). Sedna never approaches the Sun any closer than 80 A.U., and it takes 10,000 years to complete one orbit. In comparison, Pluto's 248-year-long oval orbit takes it between 30 and 50 A.U. from the Sun.
"Up until now, Sedna appeared strange in every way it had been studied. Every property of Sedna that we'd been able to measure was atypical," said Gaudi. "We've shown that Sedna's rotation period, at least, is entirely normal."
Sedna appears unusual in other ways besides its orbit. First and foremost, it is one of the largest known "minor planets," with an estimated size of 1,000 miles compared to Pluto's 1,400 miles. Sedna also displays an unusually red color that is still unexplained.
Initial measurements indicated that Sedna's rotation period was also extreme - extremely long compared to other solar system residents. By measuring small brightness fluctuations, scientists estimated that Sedna rotated once every 20-40 days. Such slow rotation likely would require the presence of a nearby large moon whose gravity could apply the brakes and slow Sedna's spin. As a result of this interpretation, artist's concepts released when Sedna's discovery was announced showed a companion moon. One month later, images taken by NASA's Hubble Space Telescope demonstrated that no large moon existed.
In true detective fashion, Gaudi and his colleagues re-investigated the matter by observing Sedna using the new MegaCam instrument on the 6.5-meter-diameter MMT Telescope at Mount Hopkins, Ariz. They measured Sedna's brightness looking for telltale, periodic brightening and dimming that would show how fast Sedna rotates. As noted by Matthew Holman, one of the members of the CfA team, "The variation in Sedna's brightness is quite small and could have been easily overlooked." Their data fits a computer model in which Sedna rotates once every 10 hours or so. The team's measurements definitively rule out a rotation period shorter than 5 hours or longer than 10 days.
While these data solve one mystery of Sedna, other mysteries remain. Chief among them is the question of how Sedna arrived in its highly elliptical, eons-long orbit. "Theorists are working hard to try to figure out where Sedna came from," said Gaudi. Astronomers will continue to study this strange world for some time to come.
Adapted from the information on http://cfa-www.harvard.edu/press/pr0510.html, and based upon a research paper online at http://arxiv.org/abs/astro-ph/0503673.
First Light from Extra-Solar Planet
(Added 12/26/05) Two teams of astronomers announced that they have directly detected light from two known planets orbiting distant stars. This discovery opens a new frontier in the study of extrasolar planets. Researchers now can directly measure and compare such planetary characteristics as color, reflectivity, and temperature.
A team led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics (CfA) published their detection of the planet TrES-1 in the June 20 issue of The Astrophysical Journal. A team led by Drake Deming of the Goddard Space Flight Center (GSFC) published their observations of the planet HD 209458b in the March 22 online issue of Nature.
"It's an awesome experience to realize we are seeing the glow of distant worlds," said Charbonneau. "When I first saw the data, I was ecstatic." Each of the two target planets periodically crosses in front of and behind its star. When in front, the planet partially eclipses the star and blocks a small portion of the star's light. Similarly, the system dims slightly when the planet disappears behind its star since the star blocks the planet's light. By observing this "secondary eclipse," astronomers can tease out the faint signal of the planet from the overwhelming light of the nearby star.
Charbonneau and his colleagues used the Infrared Array Camera (IRAC), a Smithsonian-developed instrument aboard NASA's Spitzer Space Telescope, to observe TrES-1 in the infrared region of the spectrum. Deming and his associates used Spitzer's Multiband Imaging Photometer for Spitzer (MIPS) to observe HD 209458b.
"Planets like TrES-1 are tiny and faint compared to their stars, but the one thing they can't hide is their heat," explained Charbonneau. "We are like detectives. Previous clues told us the planet must be there, so we put on our 'infrared goggles' and suddenly, it popped into view." Infrared offers an advantage because the star outshines the planet by a factor of 10,000 in visible light, while in the infrared the star is only about 400 times brighter, making it easier to pick out a planet's feeble light. Astronomers compare the challenge to trying to spot a firefly buzzing next to a searchlight.
Using Spitzer data combined with previous measurements, Charbonneau and his colleagues confirmed that TrES-1, which orbits its star at a distance of 4 million miles, has a temperature of about 1,450 °F (1060 K). They also calculated that the planet has a reflectivity of only 31%, meaning it absorbs the majority of the star's light that falls on it.
CfA researcher Guillermo Torres modeled the dynamics of the TrES-1 system to constrain the planet's orbit. He determined that the orbit has been made very nearly circular by the tidal effect of the nearby star, as expected.
Charbonneau is quick to point out that the achievement of directly detecting an extrasolar planet's light is only the beginning. "We've caught our first 'firefly.' Now we want to study a swarm of them." Astronomers expect the Trans-Atlantic Exoplanet Survey (TrES) network, which spotted TrES-1, to locate additional "hot Jupiters." That ground-based network is designed to spot planets orbiting bright stars, which can be more easily studied with Spitzer and other instruments. By comparing many "hot Jupiter" planets, researchers hope to determine what gases their atmospheres contain and how their composition was affected by when and how they formed.
Adapted from the information on http://cfa-www.harvard.edu/press/pr0509.html.
(Added 12/26/05) A monstrous cosmic explosion last December showed that Earth is in more danger from real-life space threats than from hypothetical alien invasions. The gamma-ray flare, which briefly outshone the full moon, occurred within the Milky Way galaxy. Even at a distance of 50,000 light-years, the flare disrupted Earth's ionosphere. If such a blast happened within 10 light-years of Earth, it would destroy the much of the ozone layer, causing extinctions due to increased radiation.
"Astronomically speaking, this explosion happened in our backyard. If it were in our living room, we'd be in big trouble!" explained Bryan Gaensler (Harvard-Smithsonian Center for Astrophysics), lead author on a paper describing radio observations of the event. Gaensler headed one of two teams reporting on this eruption at a special press event today at NASA headquarters. A multitude of papers are planned for publication.
The giant flare detected on December 27, 2004, came from an isolated, exotic neutron star within the Milky Way. The flare was more powerful than any blast previously seen in our galaxy. "This might be a once-in-a-lifetime event for astronomers, as well as for a neutron star," remarked David Palmer of Los Alamos National Laboratory, lead author on a paper describing space-based observations of the burst. "We know of only two other giant flares in the past 35 years, and this December event was one hundred times more powerful."
NASA's newly launched Swift satellite and the NSF-funded Very Large Array (VLA) were two of many observatories that observed the event, arising from neutron star SGR 1806-20, about 50,000 light years from Earth in the constellation Sagittarius.
Neutron stars form from collapsed stars. They are dense, fast-spinning, highly magnetic, and only about 15 miles in diameter. SGR 1806-20 is a unique neutron star called a magnetar, with an ultra-strong magnetic field capable of stripping information from a credit card at a distance halfway to the Moon. Only about 10 magnetars are known among the many neutrons stars in the Milky Way.
"Fortunately, there are no magnetars anywhere near the Earth. An explosion like this within a few trillion miles could really ruin our day," said graduate student Yosi Gelfand (CfA), a co-author on one of the papers. The magnetar's powerful magnetic field generated the gamma-ray flare in a violent process known as magnetic reconnection, which releases huge amounts of energy. The same process on a much smaller scale creates solar flares. "This eruption was a super-super-super solar flare in terms of energy released," added Gaensler.
Using the VLA and three other radio telescopes, Gaensler and his team detected material ejected by the blast at a velocity three-tenths the speed of light. The extreme speed, combined with the close-up view, yielded changes in a matter of days. Spotting such a nearby gamma-ray flare offered scientists an incredible advantage, allowing them to study it in more detail than ever before. "We can see the structure of the flare's aftermath, and we can watch it change from day to day. That combination is completely unprecedented," said Gaensler.
Adapted from the information on http://cfa-www.harvard.edu/press/pr0506.html.
(Added 12/26/05) Using the MMT Observatory in Tucson, AZ, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) are the first to report the discovery of a star leaving our galaxy, speeding along at over 1.5 million miles per hour (2.4 million km/hr). This incredible speed likely resulted from a close encounter with the Milky Way's central black hole, which flung the star outward like a stone from a slingshot. So strong was the event that the speedy star eventually will be lost altogether, traveling alone in the blackness of intergalactic space.
"We have never before seen a star moving fast enough to completely escape the confines of our galaxy," said co-discoverer Warren Brown (CfA). "We're tempted to call it the outcast star because it was forcefully tossed from its home." The star, catalogued as SDSS J090745.0+24507, once had a companion star. However, a close pass by the supermassive black hole at the galaxy's center trapped the companion into orbit while the speedster was violently flung out. Astronomer Jack Hills proposed this scenario in 1998, and the discovery of the first expelled star seems to confirm it. "Only the powerful gravity of a very massive black hole could propel a star with enough force to exit our galaxy," explained Brown.
While the star's speed offers one clue to its origin, its path offers another. By measuring its line-of-sight velocity, it suggests that the star is moving almost directly away from the galactic center. Its composition and age provide additional proof of the star's history. The fastest star contains many elements heavier than hydrogen and helium, which astronomers collectively call metals. Less than 80 million years were needed for the star to reach its current location, which is consistent with its estimated age.
The star is traveling twice as fast as galactic escape velocity, meaning that the Milky Way's gravity will not be able to hold onto it. Like a space probe launched from Earth, this star was launched from the galactic center onto a never-ending outward journey. It faces a lonely future as it leaves our galaxy, never to return.
Adapted from the information on http://cfa-www.harvard.edu/press/pr0505.html.
Planet Formation Around a Brown Dwarf?
(Added 12/26/05) Using the Spitzer Space Telescope, a team of astronomers led by Kevin Luhman (Harvard-Smithsonian Center for Astrophysics) has discovered a protoplanetary disk around a surprisingly low-mass brown dwarf. This remarkable finding raises the possibility of planet formation around objects that themselves have planetary masses. Moreover, the presence of a disk suggests that terrestrial planets could form and thrive while orbiting an object too small to shine via nuclear fusion.
"It's an exciting possibility -- one that hasn't been explored extensively because this is the first evidence for the building blocks of planets around such a small object," explained Luhman. The team's findings were published in the February 10 issue of The Astrophysical Journal Letters.
The brown dwarf in question, OTS 44, is located approximately 500 light-years away in the southern constellation Chamaeleon. OTS 44 weighs in at around 15 Jupiter masses, placing it near the dividing line between brown dwarfs (generally defined as objects of 15-70 Jupiter masses) and planets. At a temperature of 3,600 °F (2300 Kelvin), OTS 44 is the coolest and least massive brown dwarf known to have a circumstellar disk.
Although the team cannot measure the total mass of the disk, it likely contains enough matter to make one small gas giant or several Earth-sized planets. "This brown dwarf and its disk could eventually evolve into a miniature version of our solar system," remarked Luhman. Due to the brown dwarf's low temperature, an Earth-sized world would have to orbit much closer to the brown dwarf than the Earth from the Sun in order to be as warm as Earth. Theorists estimate that liquid water could exist on the surface of a planet about 1 to 4 million miles from the brown dwarf. The disk of OTS 44 extends beyond both sides of this "habitable zone."
Without nuclear fusion to sustain it, the brown dwarf will gradually cool and dim. If an Earth-sized world forms near the brown dwarf, it will be scorching at first, then grow cooler and more hospitable over time. Since the brown dwarf cools more slowly as it gets older, such a planet could remain in the habitable zone for an extended time, raising the intriguing possibility that life might evolve.
The researchers plan to search for similar disks around other nearby brown dwarfs. Spitzer revealed the disk of OTS 44 in only 20 seconds of observing time. Further searches may locate similar disks around even smaller central objects of 10 Jupiter masses or less. The team detected OTS 44's circumstellar disk using Spitzer's Infrared Array Camera, or IRAC. IRAC data showed an excess of infrared emission at long wavelengths-the signature of a dusty disk that absorbs radiation from the brown dwarf, heats up, and re-radiates the energy in the infrared.
Adapted from the information on http://cfa-www.harvard.edu/press/pr0504.html.
Two Uranian Moons Discovered, Along with Two Rings
(Added 12/23/05) To the surprise of astronomers, NASA's Hubble Space Telescope has photographed a pair of new rings around the distant planet Uranus. The largest is twice the diameter of the planet's previously known rings. The new rings are so far away that they are being called Uranus's "second ring system."
In addition, Hubble has spied two small satellites, one sharing its orbit with one of the newly discovered rings. Even more surprisingly, precise analysis of the data reveals that the orbits of Uranus's family of inner moons have changed significantly in the last decade. Collectively, these new discoveries mean that Uranus has a densely packed, rapidly changing, and possibly unstable dynamical system of orbiting bodies. "The new discoveries dramatically demonstrate that Uranus has a youthful and dynamic system of rings and moons," remarked Mark Showalter of the SETI Institute. "Until now, nobody had a clue the rings were there, we had no right to expect them."
The image above shows two sets of images taken with Hubble, one in 2003 and the other in 2005. The fully annotated image shows the two new rings, given the temporary designations R/2003 U1 and U2, and the two new moons, which have already been named Mab and Cupid.
Since dust in such an orbit is expected to be depleted by spiraling away, the rings must be continually replenished with fresh material. Showalter and collaborator Jack Lissauer of the NASA Ames Research Center propose that the outermost ring is replenished by a 12-mile-wide companion satellite, named Mab, which they first saw in 2003 using Hubble. Meteoroid impacts continually blast dust off the surface of Mab, and the dust then spreads out into a ring around Uranus. Mab's ring receives a fresh infusion of dust from each impact. In this way, nature "balances the books" by keeping the ring supplied with new dust while older dust spirals away or bangs back into the moon.
Showalter and Lissauer have measured numerous changes to the orbits of Uranus's inner moons since 1994, when their motions were derived from earlier Hubble and Voyager observations. "This appears to be a explained or chaotic process, where there is a continual exchange of energy and angular momentum between the moons," says Lissauer. "The changes in the last ten years are small, but the thing about chaos is that small changes build up exponentially with time. As a result, this suggests that the entire system is orbitally unstable." Lissauer's calculations predict that that moons would begin to collide within a few million years, which is extraordinarily short compared to the 4.5 billion year age of the Uranian system. Perhaps the most unstable moon of all is tiny Cupid, whose orbit brings it within 500 miles (800 km) of the moon Belinda.
Showalter and Lissauer propose that their discovery of a second ring, which orbits closer to the planet, provides further evidence for collisional evolution of the system. This ring orbits in the midst of the moons but has no visible body to re-supply it with dust. "This ring may be the telltale sign of an unseen belt of bodies a few feet to a few miles in size," said Showalter. He proposes that the collisional disruption of a moon in Uranus's past could have produced the debris ring they now observe.
Hubble's exquisite sharpness and sensitivity uncovered the rings in a series of 80 four-minute-long exposures of Uranus taken in August 2004. The team later recognized the faint new rings in 24 similar images taken a year earlier. Recent images from September 2005 reveal them more clearly than ever.
Showalter also found the rings in archival images taken during Voyager 2's flyby of Uranus in 1986. Uranus' first nine rings were discovered in 1977 during stellar occultation observations of the planet's atmosphere. During the Voyager encounters, two other inner rings and ten moons were discovered. However, no one noticed the new outer rings because they are extremely faint and much farther from the planet than anyone had expected. Showalter was able to find them by a careful analysis of nearly 100 Voyager images.
Because the new rings are nearly transparent, they will be easier to see when they are tilted more edge-on. This means that the new rings will increase in brightness every year as Uranus approaches its equinox, when the Sun will shine directly over Uranus' equator. When that happens, in 2007, all of the rings will be tilted edge-on to Earth and the new rings will be much easier to study.
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/33/.
Mass of Sirius' Companion is Measured
(Added 12/23/05) For astronomers, it has always been a source of frustration that the nearest white dwarf star is buried in the glow of the brightest star in the nighttime sky. This burned-out stellar remnant is a faint companion of the brilliant blue-white Dog Star, Sirius, located in the winter constellation Canis Major. Now, an international team of astronomers has used the keen eye of NASA's Hubble Space Telescope to isolate the light from the white dwarf, Sirius B.
The new results allow them to measure precisely the white dwarf's mass based on how its intense gravitational field alters the wavelengths of light emitted by the star. Such spectroscopic measurements of Sirius B taken with a telescope looking through the Earth's atmosphere have been severely contaminated by scattered light from the very bright Sirius.
"Studying Sirius B has challenged astronomers for more than 140 years," remarked Martin Barstow of the University of Leicester, U.K., who is the leader of the observing team. "Only with Hubble have we at last been able to obtain the observations we need, uncontaminated by the light from Sirius, in order to measure its change in wavelengths."
Accurately determining the masses of white dwarfs is fundamentally important to understanding stellar evolution. The sun will eventually become a white dwarf. White dwarfs are also the source of Type Ia supernova explosions that are used because of their brightness to measure distances to distant galaxies and the expansion rate of the universe. Measurements based on Type Ia supernovae are fundamental to understanding 'dark energy,' a dominant repulsive force stretching the universe apart. Also, the method used to determine the white dwarf's mass relies on one of the key predictions of Einstein's theory of General Relativity; that light loses energy when it attempts to escape the gravity of a compact star.
Sirius B has a diameter of 7,500 miles, less than the size of Earth, but it is much more dense. Its powerful gravitational field is 350,000 times greater than Earth's, meaning that a 150-pound person would weigh 50 million pounds standing on its surface. Light from the surface of the hot white dwarf has to climb out of this gravitational field and is stretched to longer, redder wavelengths of light in the process. This effect, predicted by Einstein's theory of General Relativity in 1916, is called gravitational redshift, and is most easily seen in dense, massive, and hence compact objects whose intense gravitational fields warp space near their surfaces.
Based on the Hubble measurements of the redshift, made with the Space Telescope Imaging Spectrograph in February 2004, the team found that Sirius B has a mass that is 98% that of the sun. Sirius itself has a mass of two times that of the sun and a diameter of 1.5 million miles. The Hubble observations have also refined the measurement of Sirius B's surface temperature to be 44,900° Fahrenheit. Sirius itself has a surface temperature of 18,000° Fahrenheit.
At 8.6 light-years away, Sirius is one of the nearest known stars to Earth. Stargazers have watched Sirius since antiquity. Its diminutive companion, however, was not discovered until 1862, when it was first glimpsed by astronomers examining Sirius through one of the most powerful telescopes of that time.
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/36/.
Stellar Observations Force White Dwarf Model Revision
(Added 11/20/05) Astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope are taking advantage of a once-in-a-lifetime opportunity to watch an old star suddenly stir back into new activity after coming to the end of its normal life. Their surprising results have forced them to change their ideas of how such an old, white dwarf star can re-ignite its nuclear furnace for one final blast of energy. Computer simulations had predicted a series of events that would follow such a re-ignition of fusion reactions, but the star didn't follow the script; events moved 100 times more quickly than the simulations predicted.
"We've now produced a new theoretical model of how this process works, and the VLA observations have provided the first evidence supporting our new model," said Albert Zijlstra, of the University of Manchester in the United Kingdom. Zijlstra and his colleagues presented their findings in the April 8 issue of the journal Science.
The astronomers studied a star known as V4334 Sgr, in the constellation Sagittarius. It is better known as "Sakurai's Object," after Japanese amateur astronomer Yukio Sakurai, who discovered it on February 20, 1996, when it suddenly burst into new brightness. At first, astronomers thought the outburst was a common nova explosion, but further study showed that Sakurai's Object was anything but common.
The star is an old white dwarf that had run out of hydrogen fuel for nuclear fusion reactions in its core. Astronomers believe that some such stars can undergo a final burst of fusion in a shell of helium that surrounds a core of heavier nuclei such as carbon and oxygen. However, the outburst of Sakurai's Object is the first such blast seen in modern times. Stellar outbursts observed in 1670 and 1918 may have been caused by the same phenomenon.
Astronomers expect the Sun to become a white dwarf in about five billion years. A white dwarf is a dense core left after a star's normal, fusion-powered life has ended. A teaspoon of white dwarf material would weigh about 10 tons. White dwarfs can have masses up to 1.4 times that of the Sun; larger stars collapse at the end of their lives into even-denser neutron stars or black holes.
Computer simulations indicated that heat-spurred convection (or "boiling") would bring hydrogen from the star's outer envelope down into the helium shell, driving a brief flash of new nuclear fusion. This would cause a sudden increase in brightness. The original computer models suggested a sequence of observable events that would occur over a few hundred years.
"Sakurai's object went through the first phases of this sequence in just a few years - 100 times faster than we expected - so we had to revise our models," Zijlstra explained. The revised models predicted that the star should rapidly reheat and begin to ionize gases in its surrounding region. "This is what we now see in our latest VLA observations," Zijlstra added. "It's important to understand this process. Sakurai's Object has ejected a large amount of the carbon from its inner core into space, both in the form of gas and dust grains. These will find their way into regions of space where new stars form, and the dust grains may become incorporated in new planets. Some carbon grains found in a meteorite show isotope ratios identical to those found in Sakurai's Object, and we think they may have come from such an event. Our results suggest this source for cosmic carbon may be far more important than we suspected before."
The scientists continue to observe Sakurai's Object to take advantage of the rare opportunity to learn about the process of re-ignition. They are making new VLA observations just this month. Their new models predict that the star will heat very quickly, then slowly cool again, cooling back to its current temperature about the year 2200. They think there will be one more reheating episode before it starts its final cooling to a stellar cinder.
Adapted from the information on http://www.nrao.edu/pr/2005/sakurai/.
First-Ever Measurement of a Galaxy's Proper Motion
(Added 11/20/05) Astronomers using the National Science Foundation's Very Long Baseline Array (VLBA) have measured the motion across the sky of a galaxy nearly 2.4 million light-years from Earth. While scientists have been measuring the motion of galaxies directly toward or away from Earth for decades, this is the first time that the transverse motion (called proper motion by astronomers) has been measured for a galaxy that is not a satellite of our own Milky Way Galaxy.
An international scientific team analyzed VLBA observations made over two and a half years to detect minuscule shifts in the sky position of the spiral galaxy M33. Combined with previous measurements of the galaxy's motion toward Earth, the new data allowed the astronomers to calculate M33's movement in three dimensions for the first time. M33 is a satellite of the larger galaxy M31, the well-known Andromeda Galaxy that is the most distant object visible to the naked eye. Both are part of the Local Group of galaxies that includes the Milky Way.
"A snail crawling on Mars would appear to be moving across the surface more than 100 times faster than the motion we measured for this galaxy," said Mark Reid, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA. In addition to measuring the motion of M33 as a whole, the astronomers also were able to make a direct measurement of the spiral galaxy's rotation. Both measurements were made by observing the changes in position of giant clouds of molecules inside the galaxy. The water vapor in these clouds acts as a natural maser, strengthening, or amplifying, radio emission the same way that lasers amplify light emission. The natural masers acted as bright radio beacons whose movement could be tracked by the ultra-sharp radio "vision" of the VLBA.
Reid and his colleagues plan to continue measuring M33's motion and also to make similar measurements of M31's motion. This will allow them to answer important questions about the composition, history and fates of the two galaxies as well as of the Milky Way. "We want to determine the orbits of M31 and M33. That will help us learn about their history, specifically, how close have they come in the past?" Reid explained. "If they have passed very closely, then maybe M33's small size is a result of having material pulled off it by M31 during the close encounter," he added.
Accurate knowledge of the motions of both galaxies also will help determine if there's a collision in their future. In addition, orbital analysis can give astronomers valuable clues about the amount and distribution of dark matter in the galaxies.
The direct measurement of M33's transverse angular spin is the first time such a measurement has been done accurately. In the 1920s, some astronomers thought they had measured the spin of spiral galaxies, but their results proved to be in error. More recently, radio astronomers have measured the Doppler shift of hydrogen gas in galaxies to determine the spin speed, which, when combined with the angular spin, gives a direct estimate of the distance of the galaxy.
The astronomers' task was not simple. Not only did they have to detect an impressively tiny amount of motion across the sky, but they also had to separate the actual motion of M33 from the apparent motion caused by our Solar System's motion around the center of the Milky Way. The motion of the Solar System and the Earth around the Galactic center, some 26,000 light-years away, has been accurately measured using the VLBA over the last decade.
"The VLBA is the only telescope system in the world that could do this work," Reid said. "Its extraordinary ability to resolve fine detail is unmatched and was the absolute prerequisite to making these measurements."
Reid worked with Andreas Brunthaler of the Max Planck Institute for Radioastronomy in Bonn, Germany; Heino Falcke of ASTRON in the Netherlands; Lincoln Greenhill, also of the Harvard- Smithsonian Center for Astrophysics; and Christian Henkel, also of the Max Planck Institute in Bonn. The scientists reported their findings in the March 4 issue of the journal Science.
The VLBA is a system of ten radio-telescope antennas, each with a dish 25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the VLBA spans more than 5,000 miles, providing astronomers with the sharpest vision of any telescope on Earth or in orbit. Dedicated in 1993, the VLBA has an ability to see fine detail equivalent to being able to stand in New York and read a newspaper in Los Angeles.
Adapted from the information on http://www.nrao.edu/pr/2005/m33motion/.
Gamma Ray Burst Yields Magnetar Secrets
(Added 11/20/05) A giant flash of energy from a supermagnetic neutron star thousands of light-years from Earth may shed a whole new light on scientists' understanding of such mysterious "magnetars" and of gamma-ray bursts. The blast from an object named SGR 1806-20 came on December 27, 2004, and was first detected by orbiting gamma-ray and X-ray telescopes. It was the brightest outburst ever seen coming from an object beyond our own Solar System, and its energy overpowered most orbiting telescopes. The burst of gamma rays and X-rays even disturbed the Earth's ionosphere, causing a sudden disruption in some radio communications.
While the intensely bright gamma ray burst faded away in a matter of minutes, the explosion's "afterglow" has been tracked by the VLA and other radio telescopes for weeks, providing most of the data needed by astronomers trying to figure out the physics of the blast.
A magnetar is a superdense neutron star with a magnetic field thousands of trillions of times more intense than that of the Earth. Scientists believe that SGR 1806-20's giant burst of energy was somehow triggered by a "starquake" in the neutron star's crust that caused a catastrophic disruption in the magnetar's magnetic field. The magnetic disruption generated the huge burst of gamma rays and "boiled off" particles from the star's surface into a rapidly-expanding fireball that continues to emit radio waves for weeks or months.
The VLA first observed SGR 1806-20 on January 3, and it has been joined by other radio telescopes in Australia, the Netherlands, and India. Scientific papers prepared for publication based on the first month's radio observations report a number of key discoveries about the object. Scientists using the VLA have found:
The excitement isn't over, either. "The show goes on and we continue to observe this thing and continue to get surprises," said Greg Taylor, an astronomer for NRAO and the Kavli Institute of Particle Astrophysics and Cosmology in Stanford, CA.
One VLA measurement may cause difficulties for scientists trying to fit SGR 1806-20 into a larger picture of gamma ray bursts (GRBs). GRBs, seen regularly from throughout the Universe, come in two main types -- very short bursts and longer ones. The longer ones are generally believed to result when a massive star collapses into a black hole, rather than into a neutron star as in a supernova explosion. The strength and short duration of SGR 1806-20's December outburst has led some astronomers to speculate that a similar event could be seen out to a considerable distance from Earth. That means, they say, that magnetars may be the source of the short-period GRBs.
That interpretation is based to some extent on a previous measurement that indicates SGR 1806-20 is nearly 50,000 light-years from Earth. One team of observers, however, analyzed the radio emission from SGR 1806-20 and found evidence that the magnetar is only about 30,000 light-years distant. The difference, they say, reduces the likelihood that SGR 1806-20 could be a parallel for short-period GRBs. In any case, the wealth of information astronomers have gathered about the tremendous December blast makes it an extremely important event for understanding magnetars and GRBs.
Adapted from the information on http://www.nrao.edu/pr/2005/sgrburst/.
(Added 11/19/05) Using NASA's Hubble Space Telescope to probe the ninth planet in our solar system, astronomers discovered that Pluto may have not one, but three moons.
If confirmed, the discovery of the two new moons could offer insights into the nature and evolution of the Pluto system, Kuiper Belt Objects with satellite systems, and the early Kuiper Belt. The Kuiper Belt is a vast region of icy, rocky bodies beyond Neptune's orbit.
"If, as our new Hubble images indicate, Pluto has not one, but two or three moons, it will become the first body in the Kuiper Belt known to have more than one satellite," explained Hal Weaver of the Johns Hopkins Applied Physics Laboratory, Laurel, MD. He is co-leader of the team that made the discovery. Pluto was discovered in 1930. Charon, Pluto's only confirmed moon, was discovered by ground-based observers in 1978. The planet resides 3 billion miles from the sun in the heart of the Kuiper Belt.
The candidate moons, provisionally designated S/2005 P1 and S/2005 P2, were observed to be approximately 27,000 miles (44,000 kilometers) away from Pluto. The objects are roughly two to three times as far from Pluto as Charon. The team plans to make follow-up Hubble observations in February to confirm that the newly discovered objects are truly Pluto's moons. Only after confirmation will the International Astronomical Union consider names for S/2005 P1 and S/2005 P2.
The Hubble telescope's Advanced Camera for Surveys observed the two new candidate moons on May 15, 2005. "The new satellite candidates are roughly 5,000 times fainter than Pluto, but they really stood out in these Hubble images," said Max Mutchler of the Space Telescope Science Institute and the first team member to identify the satellites. Three days later, Hubble looked at Pluto again. The two objects were still there and appeared to be moving in orbit around Pluto.
"A re-examination of Hubble images taken on June 14, 2002 has essentially confirmed the presence of both P1 and P2 near the predicted locations based on the 2005 Hubble observations," said Marc Buie of Lowell Observatory, Flagstaff, AZ, another member of the research team.
The team looked long and hard for other potential moons around Pluto. "These Hubble images represent the most sensitive search yet for objects around Pluto," said team member Andrew Steffl of the Southwest Research Institute, "and it is unlikely that there are any other moons larger than about 10 miles across in the Pluto system."
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/19/.
(Added 11/19/05) New NASA Hubble Space Telescope images of the distant planet Neptune show a dynamic atmosphere and capture the fleeting orbits of its satellites. Images were taken in 14 different colored filters probing various altitudes in Neptune's deep atmosphere so that scientists can study the haze and clouds in detail.
The natural-color view of Neptune (left), common to naked eye telescopic views by amateur astronomers, reveals a cyan-colored planet. Methane gas in Neptune's atmosphere absorbs most of the red sunlight hitting the planet, making it look blue-green. The image was created by combining images in red, green, and blue light. On April 29 and 30, 2005, Hubble images were taken every 4-5 hours, spaced at about a quarter of Neptune's rotational period.
Neptune's subtle features are more visible in the enhanced-color view (top right). Images taken in special methane filters show details not visible to the human eye (bottom right). The features seen in this enhanced image must be above most of the sunlight-absorbing methane to be detectable through these special filters.
The planet is so dark at the methane wavelengths that long exposures can be taken, revealing some of Neptune's smaller moons. Clockwise from the top (in composite image at left), these moons are Proteus (the brightest), Larissa, Despina, and Galatea. Neptune had 13 moons at last count.
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/22/.
Hidden Planet Shape's Star's Dust Disk
(Added 11/19/05) The top view, taken by NASA's Hubble Space Telescope, is the first visible-light image of a dust ring around the nearby, bright young star Fomalhaut (HD 216956). The image offers the strongest evidence yet that an unruly planet may be tugging on the dusty belt. The left part of the ring is outside the telescope's view. The ring is tilted obliquely to our line of sight.
The center of the ring is about 1.4 billion miles (15 A.U.) away from the star. The dot near the ring's center marks the star's location. Astronomers believe that an unseen planet moving in an elliptical orbit is reshaping the ring.
The view at bottom points out important features in the image, such as the ring's inner and outer edges. Astronomers used the Advanced Camera for Surveys' (ACS) coronagraph aboard Hubble to block out the light from the bright star so they could see the faint ring. Despite the coronagraph, some light from the star is still visible in this image, as can be seen in the wagon wheel-like spokes that form an inner ring around Fomalhaut.
The suspected planet may be orbiting far away from Fomalhaut, near the dust ring's inner edge, between 4.7 billion and 6.5 billion miles (50-70 A.U.) from the star. Only Hubble has the exquisite optical resolution to resolve that the ring's inner edge is sharper than its outer edge, a telltale sign that an object is gravitationally sweeping out material like a plow clearing away snow. The ring is in the Fomalhaut system's frigid outer region, about 12 billion miles (133 A.U.) from the star. This distance is much farther than our outermost planet Pluto is from the Sun. The ring's relatively narrow width, about 2.3 billion miles (25 A.U.), indicates that an unseen planet is keeping the ring from spreading out.
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/10/.
(Added 11/19/05) These images reveal the dynamic nature of Saturn's auroras. Viewing the planet's southern polar region for several days, NASA's Hubble Space Telescope snapped a series of photographs of the aurora dancing in the sky. The snapshots show that Saturn's auroras differ in character from day to day, as they do on Earth, moving around on some days and remaining stationary on others. But compared with Earth, where auroral storms develop in about 10 minutes and may last for a few hours, Saturn's auroral displays always appear bright and may last for several days.
The observations, made by Hubble and the Cassini spacecraft, while en route to the planet, suggest that Saturn's auroral storms are driven mainly by the pressure of the solar wind - a stream of charged particles from the Sun - rather than by the Sun's magnetic field.
The aurora's strong brightening on January 28, 2004, corresponds with the recent arrival of a large disturbance in the solar wind. The image shows that when Saturn's auroras become brighter (and thus more powerful), the ring of light encircling the pole shrinks in diameter.
Seen from space, an aurora appears as a ring of glowing gases circling a planet's polar region. Auroral displays are initiated when charged particles in space collide with a planet's magnetic field. The charged particles are accelerated to high energies and stream into the upper atmosphere. Collisions with the gases in the planet's atmosphere produce flashes of glowing energy in the form of visible, ultraviolet, and infrared light.
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/06/.
Mars Exploration Rover Opportunity Finds Iron Meteorite on Mars
(Added 01/29/05) NASA's Mars Exploration Rover Opportunity has found an iron meteorite, the first meteorite of any type ever identified on another planet.
The pitted, basketball-size object is mostly made of iron and nickel according to readings from spectrometers on the rover. Only a small fraction of the meteorites fallen on Earth are similarly metal-rich. Others are rockier. As an example, the meteorite that blasted the famous Meteor Crater in Arizona is similar in composition. "This is a huge surprise, though maybe it shouldn't have been," remarked Dr. Steve Squyres of Cornell University, Ithaca, NY, principal investigator for the science instruments on Opportunity and its twin, Spirit.
The meteorite, dubbed "Heat Shield Rock," sits near debris of Opportunity's heat shield on the surface of Meridiani Planum, a cratered flatland that has been Opportunity's home since the robot landed on Mars nearly one year ago. "I never thought we would get to use our instruments on a rock from someplace other than Mars," Squyres added. "Think about where an iron meteorite comes from: a destroyed planet or planetesimal that was big enough to differentiate into a metallic core and a rocky mantle."
Rover-team scientists are wondering whether some rocks that Opportunity has seen atop the ground surface are rocky meteorites. "Mars should be hit by a lot more rocky meteorites than iron meteorites," Squyres explained. "We've been seeing lots of cobbles out on the plains, and this raises the possibility that some of them may in fact be meteorites. We may be investigating some of those in coming weeks. The key is not what we'll learn about meteorites -- we have lots of meteorites on Earth -- but what the meteorites can tell us about Meridiani Planum."
The numbers of exposed meteorites could be an indication of whether the plain is gradually eroding away or being built up. NASA Chief Scientist Dr. Jim Garvin said, "Exploring meteorites is a vital part of NASA's scientific agenda, and discovering whether there are storehouses of them on Mars opens new research possibilities, including further incentives for robotic and then human-based sample-return missions. Mars continues to provide unexpected science 'gold,' and our rovers have proven the value of mobile exploration with this latest finding."
Initial observation of Heat Shield Rock from a distance with Opportunity's miniature thermal emission spectrometer suggested a metallic composition and raised speculation last week that it was a meteorite. The rover drove close enough to use its Moessbauer and alpha particle x-ray spectrometers, confirming the meteorite identification over the weekend.
Opportunity and Spirit successfully completed their primary three-month missions on Mars in April 2004. NASA has extended their missions twice because the rovers have remained in good condition to continue exploring Mars longer than anticipated. They have found geological evidence of past wet environmental conditions that might have been hospitable to life.
Opportunity has driven a total of 2.10 kilometers (1.30 miles). Minor mottling from dust has appeared in images from the rover's rear hazard-identification camera since Opportunity entered the area of its heat-shield debris, said Jim Erickson of NASA's Jet Propulsion Laboratory, Pasadena, CA, rover project manager. The rover team plans to begin driving Opportunity south toward a circular feature called "Vostok" within about a week.
Spirit has driven a total of 4.05 kilometers (2.52 miles). It has been making slow progress uphill toward a ridge on "Husband Hill" inside Gusev Crater.
Adapted from the information on http://www.jpl.nasa.gov/news/news.cfm?release=2005-018.
(Added 01/29/05) On January 14, after its seven-year
journey through the solar system on board the Cassini
spacecraft, ESA's Huygens probe has successfully descended through the
atmosphere of Titan, Saturn's
largest moon, and safely landed on its surface.
Following its release from the Cassini mothership on 25 December, Huygens reached Titan's outer atmosphere after 20 days and a 4 million km cruise. The probe started its descent through Titan's hazy cloud layers from an altitude of about 1270 km at 11:13 CET. During the following three minutes Huygens had to decelerate from 18,000 to 1400 km per hour.
A sequence of parachutes then slowed it down to less than 300 km per hour. At a height of about 160 km the probe's scientific instruments were exposed to Titan's atmosphere. At about 120 km, the main parachute was replaced by a smaller one to complete the descent, with an expected touchdown at 13:34 CET. Preliminary data indicate that the probe landed safely, likely on a solid surface.
The probe began transmitting data to Cassini four minutes into its descent and continued to transmit data after landing at least as long as Cassini was above Titan's horizon. The certainty that Huygens was alive came already at 11:25 CET, when the Green Bank radio telescope in West Virginia picked up a faint but unmistakable radio signal from the probe. Radio telescopes on Earth continued to receive this signal well past the expected lifetime of Huygens.
Huygens data, relayed by Cassini, were picked up by NASA's Deep Space Network and delivered immediately to ESA's European Space Operation Centre in Darmstadt, Germany, where the scientific analysis is taking place.
"Titan was always the target in the Saturn system where the need for "ground truth" from a probe was critical. It is a fascinating world and we are now eagerly awaiting the scientific results," says Professor David Southwood, Director of ESA's scientific program.
"The Huygens scientists are all delighted. This was worth the long wait," says Dr Jean-Pierre Lebreton, ESA Huygens Mission Manager. Huygens is expected to provide the first direct and detailed sampling of Titan's atmospheric chemistry and the first photographs of its hidden surface, and will supply a detailed "weather report."
One of the main reasons for sending Huygens to Titan is that its nitrogen atmosphere, rich in methane, and its surface may contain many chemicals of the kind that existed on the young Earth. Combined with the Cassini observations, Huygens will afford an unprecedented view of Saturn's mysterious moon.
"We now have the key to understanding what shapes Titan's landscape," said Dr Martin Tomasko, Principal Investigator for the Descent Imager-Spectral Radiometer (DISR), adding: "Geological evidence for precipitation, erosion, mechanical abrasion and other fluvial activity says that the physical processes shaping Titan are much the same as those shaping Earth."
Spectacular images captured by the DISR reveal that Titan has extraordinarily Earth-like meteorology and geology. Images have shown a complex network of narrow drainage channels running from brighter highlands to lower, flatter, dark regions. These channels merge into river systems running into lake beds featuring offshore 'islands' and 'shoals' remarkably similar to those on Earth.
Data provided in part by the Gas Chromatograph and Mass Spectrometer (GCMS) and Surface Science Package (SSP) support Dr Tomasko's conclusions. Huygens' data provide strong evidence for liquids flowing on Titan. However, the fluid involved is methane, a simple organic compound that can exist as a liquid or gas at Titan's sub-170 °C temperatures, rather than water as on Earth.
Titan's rivers and lakes appear dry at the moment, but rain may have occurred not long ago.
Deceleration and penetration data provided by the SSP indicate that the material beneath the surface's crust has the consistency of loose sand, possibly the result of methane rain falling on the surface over eons, or the wicking of liquids from below towards the surface.
Heat generated by Huygens warmed the soil beneath the probe and both the GCMS and SSP detected bursts of methane gas boiled out of surface material, reinforcing methane's principal role in Titan's geology and atmospheric meteorology -- forming clouds and precipitation that erodes and abrades the surface.
In addition, DISR surface images show small rounded pebbles in a dry riverbed. Spectra measurements are consistent with a composition of dirty water ice rather than silicate rocks. However, these are rock-like solid at Titan's temperatures.
Titan's soil appears to consist at least in part of precipitated deposits of the organic haze that shrouds the planet. This dark material settles out of the atmosphere. When washed off high elevations by methane rain, it concentrates at the bottom of the drainage channels and riverbeds contributing to the dark areas seen in DISR images.
New, stunning evidence based on finding atmospheric argon 40 indicates that Titan has experienced volcanic activity generating not lava, as on Earth, but water ice and ammonia.
Thus, while many of Earth's familiar geophysical processes occur on Titan, the chemistry involved is quite different. Instead of liquid water, Titan has liquid methane. Instead of silicate rocks, Titan has frozen water ice. Instead of dirt, Titan has hydrocarbon particles settling out of the atmosphere, and instead of lava, Titanian volcanoes spew very cold ice.
Titan is an extraordinary world having Earth-like geophysical processes operating on exotic materials in very alien conditions.
"We are really extremely excited about these results. The scientists have worked tirelessly for the whole week because the data they have received from Huygens are so thrilling. This is only the beginning, these data will live for many years to come and they will keep the scientists very very busy", said Jean-Pierre Lebreton, ESA's Huygens Project Scientist and Mission manager.
Adapted from the information on http://www.esa.int/export/esaCP/SEMQ1QQ3K3E_index_0.html and http://www.esa.int/export/esaCP/Pr_5_2005_p_EN.html.
Hubble Finds Infants in Small Magellanic Cloud
(Added 01/17/05) Hubble astronomers have uncovered, for the first time, a population of infant stars in the Milky Way satellite galaxy, the Small Magellanic Cloud (SMC, visible to the naked eye in the southern constellation Tucana), located 210,000 light-years away.
Hubble's exquisite sharpness plucked out an underlying population of infant stars embedded in the nebula NGC 346 that are still forming from gravitationally collapsing gas clouds. They have not yet ignited their hydrogen fuel to sustain nuclear fusion. The smallest of these infant stars is only half the mass of our Sun.
Although star birth is common within the disk of our galaxy, this smaller companion galaxy is more primeval in that it lacks a large percentage of the heavier elements that are forged in successive generations of stars through nuclear fusion.
Fragmentary galaxies like the SMC are considered primitive building blocks of larger galaxies. Most of these types of galaxies existed far away, when the universe was much younger. The SMC offers a unique nearby laboratory for understanding how stars arose in the early universe. Nestled among other starburst regions with the small galaxy, the nebula NGC 346 alone contains more than 2,500 infant stars.
The Hubble images, taken with the Advanced Camera for Surveys, identify three stellar populations in the SMC and in the region of the NGC 346 nebula -- a total of 70,000 stars. The oldest population is 4.5 billion years, roughly the age of our Sun. The younger population arose only 5 million years ago (about the time Earth's first hominids began to walk on two feet). Lower-mass stars take longer to ignite and become full-fledged stars, so the protostellar population is 5 million years old. Curiously, the infant stars are strung along two intersecting lanes in the nebula, resembling a "T" pattern in the Hubble plot.
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/04/.
Blazar Jets Approach Universal Speed Limit
(Added 01/17/05) Astronomers using the National Science Foundation's Very Long Baseline Array (VLBA) have discovered jets of plasma blasted from the cores of distant galaxies at speeds within 99.9% of the speed of light, placing these plasma jets among the fastest objects yet seen in the Universe.
"This tells us that the physical processes at the cores of these galaxies, called blazars, are extremely energetic and are capable of propelling matter very close to the absolute cosmic speed limit," explained Glenn Piner of Whittier College in Whittier, CA.
According to Einstein's Special Theory of Relativity, no object with mass can be accelerated to the speed of light. To get even close to the speed of light requires enormous amounts of energy. "For example, to accelerate a bowling ball to the speed newly measured in these blazars would require all the energy produced in the world for an entire week," Piner described, "and the blobs of plasma in these jets are at least as massive as a large planet".
Blazars are active galactic nuclei -- energetic regions surrounding massive black holes at the centers of galaxies. Material being drawn into the black hole forms a spinning disk called an accretion disk. Powerful jets of charged particles are ejected at high speeds along the poles of accretion disks. When these jets happen to be aimed nearly toward the Earth, the objects are called blazars.
Taking advantage of the extremely sharp radio "vision" of the continent-wide VLBA, the scientists tracked individual features in the jets of three blazars at distances from Earth ranging from 7.3 to 9 billion light-years. A Boston University team led by Svetlana Jorstad earlier had identified the three blazars as having potentially very high jet speeds based on VLBA observations in the mid-1990s. Piner and his colleagues observed the blazars again in 2002 and 2003 with much longer observations, and were able to confirm the high-speed motions in the faint blazar jets.
Their measurements showed that features in the blazar jets were moving at apparent speeds more than 25 times greater than that of light. This phenomenon, called superluminal motion, is not real, but rather is an illusion caused by the fact that the material in the jet is moving at nearly the speed of light almost directly toward the observer. Because the jet features are moving toward Earth at almost the same speed as the radio waves they emit, they can appear to move across the sky at faster-than-light speeds. Scientists can correct for this geometrical effect to calculate a lower limit to the true speed of the features.
"We typically see apparent speeds in blazar jets that are about five times the speed of light, and that corresponds to a true speed of more than 98% of light speed," Piner said. "Now, based on independent confirmation by two groups of astronomers, we see these three blazars with apparent speeds greater than 25 times that of light," Piner added. That apparent speed, the scientists said, corresponds to a true speed of greater than 99.9% of light speed, which is 186,282 miles per second.
Based on other properties of blazars, the scientists believe that their interpretation of the data is accurate and that they have measured the extremely fast speeds in the three blazar jets. However, "we do have to be somewhat careful in interpreting these results, because it is possible that the observed motions represent the motion of some propagating disturbance in the plasma rather than the plasma itself, in the same way that a water wave can move across the surface of the ocean without physically transporting the water," Piner said.
Adapted from the information on http://www.nrao.edu/pr/2005/fastblazars/.
Chandra Finds Possible Black Hole Swarm in Our Galaxy's Center
(Added 01/17/05) A swarm of 10,000 or more black holes may be orbiting the Milky Way's supermassive black hole, according to new results from NASA's Chandra X-ray Observatory. This would represent the highest concentration of black holes anywhere in the Galaxy.
These relatively small, stellar-mass black holes, along with neutron stars, appear to have migrated into the Galactic Center over the course of several billion years. Such a dense stellar graveyard has been predicted for years, and this represents the best evidence to date of its existence. The Chandra data may also help astronomers better understand how the supermassive black hole at the center of the Milky Way grows.
The discovery was made as part of Chandra's ongoing program of monitoring the region around Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way. It was announced today by Michael Muno of the University of California, Los Angeles (UCLA) at a meeting of the American Astronomical Society in San Diego, CA.
Among the thousands of x-ray sources detected within 70 light years of Sgr A*, Muno and his colleagues searched for those most likely to be active black holes and neutron stars by selecting only the brightest sources that also exhibited large variations in their x-ray output. These characteristics identify black holes and neutron stars that are in binary star systems and are pulling matter from nearby companion stars. Of the seven sources that met these criteria, four are within three light-years of Sgr A*.
"Although the region around Sgr A* is crowded with stars, we expected that there was only a 20% chance that we would find even one x-ray binary within a three-light-year radius," said Muno. "The observed high concentration of these sources implies that a huge number of black holes and neutron stars have gathered in the center of the Galaxy."
Mark Morris, also of UCLA and a coauthor on the present work, had predicted a decade ago that a process called dynamical friction would cause stellar black holes to sink toward the center of the Galaxy. Black holes are formed as remnants of the explosions of massive stars and have masses of about 10 suns. As black holes orbit the center of the Galaxy at a distance of several light years, they pull on surrounding stars, which pull back on the black holes.
The net effect is that black holes spiral inward, and the low-mass stars move out. From the estimated number of stars and black holes in the Galactic Center region, dynamical friction is expected to produce a dense swarm of 20,000 black holes within three light years of Sgr A*. A similar effect is at work for neutron stars, but to a lesser extent because they have a lower mass.
Once black holes are concentrated near Sgr A*, they will have numerous close encounters with normal stars there, some of which are in binary star systems. The intense gravity of a black hole can induce an ordinary star to "change partners" and pair up with the black hole while ejecting its companion. This process and a similar one for neutron stars are expected to produce several hundreds of black hole and neutron star binary systems.
"If only 1% of these binary systems are x-ray active each year, they can account for the sources we see," said Eric Pfahl of the University of Virginia in Charlottesville and a coauthor of a paper describing these results that has been submitted to the Astrophysical Journal Letters. "Although the evidence is mostly circumstantial, it makes a strong case for the existence of a large population of neutron stars and stellar-mass black holes within three light-years of the center of our Galaxy."
The black holes and neutron stars in the cluster are expected to gradually be swallowed by the supermassive black hole, Sgr A*, at a rate of about one every million years. At this rate, about 10,000 black holes and neutron stars would have been captured in a few billion years, adding about 3% to the mass of the central supermassive black hole, which is currently estimated to contain the mass of 3.7 million suns.
In the meantime, the acceleration of low-mass stars by black holes will eject low-mass stars from the central region. This expulsion will reduce the likelihood that normal stars will be captured by the central supermassive black hole. This may explain why the central regions of some galaxies, including the Milky Way, are fairly quiet even though they contain a supermassive black hole.
The region analyzed in this research near Sgr A* has been observed 16 times between 1999 and 2004 using Chandra's Advanced CCD Imaging Spectrometer (ACIS) instrument. Other members of the research team include Frederick K. Baganoff (Massachusetts Institute of Technology), Niel Brandt (Penn State), Andrea Ghez and Jessica Lu (UCLA).
Adapted from the information on http://chandra.harvard.edu/press/05_releases/press_011005.html.
Multiple Pulsars Found in Globular Cluster
(Added 01/17/05) A dense globular star cluster near the center of our Milky Way Galaxy holds a buzzing beehive of rapidly-spinning millisecond pulsars, according to astronomers who discovered 21 new pulsars in the cluster using the National Science Foundation's 100-meter Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. The cluster, called Terzan 5, now holds the record for pulsars, with 24, including three known before the GBT observations.
"We hit the jackpot when we looked at this cluster," announced Scott Ransom, an astronomer at the National Radio Astronomy Observatory in Charlottesville, VA. "Not only does this cluster have a lot of pulsars -- and we still expect to find more in it -- but the pulsars in it are very interesting. They include at least 13 in binary systems, two of which are eclipsing, and the four fastest-rotating pulsars known in any globular cluster, with the fastest two rotating nearly 600 times per second, roughly as fast as a household blender," Ransom added. Ransom and his colleagues reported their findings to the American Astronomical Society's meeting in San Diego, CA, and in the online journal Science Express.
The star cluster's numerous pulsars are expected to yield a bonanza of new information about not only the pulsars themselves, but also about the dense stellar environment in which they reside and probably even about nuclear physics, according to the scientists. For example, preliminary measurements indicate that two of the pulsars are more massive than some theoretical models would allow.
The pulsars in Terzan 5 are the product of a complex history. The stars in the cluster formed about 10 billion years ago, the astronomers say. Some of the most massive stars in the cluster exploded and left the neutron stars as their remnants after only a few million years. Normally, these neutron stars would no longer be seen as swiftly-rotating pulsars: their spin would have slowed because of the "drag" of their intense magnetic fields until the "lighthouse" effect is no longer observable.
However, the dense concentration of stars in the cluster gave new life to the pulsars. In the core of a globular cluster, as many as a million stars may be packed into a volume that would fit easily between the Sun and our nearest neighbor star. In such close quarters, stars can pass near enough to form new binary pairs, split apart such pairs, and binary systems even can trade partners, like an elaborate cosmic square dance. When a neutron star pairs up with a "normal" companion star, its strong gravitational pull can draw material off the companion onto the neutron star. This also transfers some of the companion's spin, or angular momentum, to the neutron star, thereby "recycling" the neutron star into a rapidly-rotating millisecond pulsar. In Terzan 5, all the pulsars discovered are rotating rapidly as a result of this process.
Astronomers previously had discovered three pulsars in Terzan 5, some 28,000 light-years distant in the constellation Sagittarius, but suspected there were more. On July 17, 2004, Ransom and his colleagues used the GBT, and, in a 6-hour observation, found 14 new pulsars, the most ever found in a single observation. Eight more observations between July and November of 2004 discovered seven additional pulsars in Terzan 5. In addition, the astronomers' data show evidence for several more pulsars that still need to be confirmed.
Future studies of the pulsars in Terzan 5 will help scientists understand the nature of the cluster and the complex interactions of the stars at its dense core. Also, several of the pulsars offer a rich yield of new scientific information. The scientists suspect that one pulsar, which shows strange eclipses of its radio emission, has recently traded its original binary companion for another, and two others have white-dwarf companions that they believe may have been produced by the collision of a neutron star and a red-giant star. Subtle effects seen in these two systems can be explained by Einstein's general relativistic theory of gravity, and indicate that the neutron stars are more massive than some theories allow. The material in a neutron star is as dense as that in an atomic nucleus, so that fact has implications for nuclear physics as well as astrophysics.
Adapted from the information on http://www.nrao.edu/pr/2005/terzan5/.
Spitzer Finds Stellar Incubators with Massive Star Embryos
(Added 01/17/05) NASA's Spitzer Space Telescope has uncovered a hatchery for massive stars. A new striking image from the infrared telescope shows a vibrant cloud called the Trifid Nebula (M20) dotted with glowing stellar "incubators." Tucked deep inside these incubators are rapidly growing embryonic stars, whose warmth Spitzer was able to see for the first time with its powerful heat-seeking eyes.
The new view offers a rare glimpse at the earliest stages of massive star formation -- a time when developing stars are about to burst into existence. "Massive stars develop in very dark regions so quickly that is hard to catch them forming," explained Dr. Jeonghee Rho of the Spitzer Science Center, California Institute of Technology, Pasadena, CA, principal investigator of the recent observations. "With Spitzer, it's like having an ultrasound for stars. We can see into dust cocoons and visualize how many embryos are in each of them."
The Trifid Nebula is a giant star-forming cloud of gas and dust located 5,400 light-years away in the constellation Sagittarius. Previous images taken by the Institute for Radioastronomy millimeter telescope in Spain show that the nebula contains four cold knots, or cores, of dust. Such cores are "incubators" where stars are born. Astronomers thought the ones in the Trifid Nebula were not yet ripe for stars. But, when Spitzer set its infrared eyes on all four cores, it found that they had already begun to develop warm stellar embryos.
"Spitzer can see the material from the dark cores falling onto the surfaces of the embryonic stars, because the material gets hotter as gravity draws it in," described Dr. William T. Reach of the Spitzer Science Center, co-author of this new research. "By measuring the infrared brightness, we can not only see the individual embryos but determine their growth rate."
The Trifid Nebula is unique in that it is dominated by one massive central star that is 300,000 years old. Radiation and winds emanating from the star have sculpted the Trifid cloud into its current cavernous shape. These winds have also acted like shock waves to compress gas and dust into dark cores, whose gravity caused more material to fall inward until embryonic stars were formed. In time, the growing embryos will accumulate enough mass to ignite and explode out of their cores like baby birds busting out of their eggs.
Because the Trifid Nebula is home to just one massive star, it provides astronomers a rare chance to study an isolated family unit. All of the newfound stellar embryos are descended from the nebula's main star. Said Rho, "Looking at the image, you know exactly where the embryos came from. We use their colors to determine how old they are. It's like studying the family tree for a generation of stars."
Spitzer discovered 30 embryonic stars in the Trifid Nebula's four cores and dark clouds. Multiple embryos were found inside two massive cores, while a sole embryo was seen in each of the other two. This is one of the first times that clusters of embryos have been observed in single cores at this early stage of stellar development. Spitzer also uncovered about 120 small baby stars buried inside the outer clouds of the nebula. These newborns were probably formed around the same time as the main massive star and are its smaller siblings.
In the cores with multiple embryos, we are seeing that the most massive and brightest of the bunch is near the center. This implies that the developing stars are competing for materials, and that the embryo with the most material will grow to be the largest star," said Dr. Bertrand Lefloch of Observatoire de Grenoble, France, co-author of the new research.
Adapted from the information on http://www.jpl.nasa.gov/news/news.cfm?release=2005-014.
Deep Impact Launch Successful; Craft Functioning Normally
(Added 01/17/05) NASA's Deep Impact spacecraft began its 431 million kilometer (268 million mile) journey to comet Tempel 1 on January 12 at 1:47:08 p.m. EST. Data received from the spacecraft indicate it has deployed and locked its solar panels, is receiving power, and achieved proper orientation in space. Data also indicate the spacecraft placed itself in a safe mode and was awaiting further commands from Earth. Deep Impact mission managers are examining data returns from the mission.
Deep Impact is comprised of two parts, a "fly-by" spacecraft and a smaller "impactor." The impactor will be released into the comet's path for a planned collision on July 4 of this year. The crater produced by the impactor is expected to be up to the size of a football stadium and two to 14 stories deep. Ice and dust debris will be ejected from the crater, revealing the material beneath. The fly-by spacecraft will observe the effects of the collision. NASA's Hubble, Spitzer, and Chandra space telescopes, and other telescopes on Earth, will also observe the collision.
Comets are time capsules that hold clues about the formation and evolution of the Solar System. They are composed of ice, gas and dust, primitive debris from the Solar System's distant and coldest regions that formed 4.5 billion years ago.
The management of the Deep Impact launch was the responsibility of NASA's Kennedy Space Center, FL. Deep Impact was launched from Pad 17-B at Cape Canaveral Air Force Station, FL. Delta II launch service was provided by Boeing Expendable Launch Systems, Huntington Beach, CA. The spacecraft was built for NASA by Ball Aerospace and Technologies Corporation, Boulder, CO. Deep Impact project management is by JPL.
On January 13, Deep Impact took itself out of safe mode and is healthy. The spacecraft had entered safe mode soon after separation from the launch vehicle. When a spacecraft enters safe mode, all but essential spacecraft systems are turned off until it receives new commands from mission control. When Deep Impact separated from the launch vehicle, the spacecraft computer detected higher than expected temperatures in the propulsion system.
"We are out of safe mode and proceeding with in-flight operations," said Deep Impact project manager Rick Grammier of NASA's Jet Propulsion Laboratory. "We're back on nominal timeline and look forward to our encounter with comet Tempel 1 this summer."
Adapted from the information on http://www.jpl.nasa.gov/news/news.cfm?release=2005-015 and http://www.jpl.nasa.gov/news/news.cfm?release=2005-016.
Cassini Mission Update (Rather Large Update)
(Added 01/17/05) Due to the three-month hiatus that I took for personal and academic reasons from updating this site, much has happened with the Cassini mission since I last added to the Current Event section. This update is a "short" summary of major happenings and discoveries in roughly reverse-chronological order (most recent is first).
Iapetus Flyby
NASA's Cassini spacecraft successfully flew by Saturn's moon Iapetus at a distance of 123,400 kilometers (76,700 miles) on Friday, December 31. NASA's Deep Space Network tracking station in Goldstone, CA, received the signal and science data that day beginning at 11:47 p.m. Pacific Standard Time.
With a diameter of about 1,400 kilometers (890 miles), Iapetus is Saturn's third largest moon. It was discovered by Jean-Dominique Cassini in 1672. It was Cassini, for whom the Cassini mission is named, who correctly deduced that one side of Iapetus was dark, while the other was white. Iapetus is a world of sharp contrasts. The leading hemisphere is as dark as a freshly-tarred street, and the white, trailing hemisphere resembles freshly-fallen snow. The last look at Iapetus was by NASA's Voyager 1 and 2 spacecraft in 1980 and 1981.
Iapetus is odd in other respects. It is the only large Saturn moon in a highly inclined orbit, one that takes it far above and below the plane in which the rings and most of the moons orbit. It is less dense than objects of similar brightness, which implies it has a higher fraction of ice or possibly methane or ammonia in its interior.
Scientists still do not agree on whether the dark material originated from an outside source or was created from Iapetus' own interior. One scenario for the outside deposit of material would involve dark particles being ejected from Saturn's little moon Phoebe and drifting inward to coat Iapetus. The major problem with this model is that the dark material on Iapetus is redder than Phoebe, although the material could have undergone chemical changes that made it redder after its expulsion from Phoebe. One observation lending credence to the theory of an internal origin is the concentration of material on crater floors, which implies that something is filling in the craters. In one model proposed by scientists, methane could erupt from the interior and then become darkened by ultraviolet radiation.
Images returned during the New Year's Eve flyby of Iapetus show startling surface features that are fueling heated scientific discussions about their origin. One of these features is a long narrow ridge that lies almost exactly on the equator of Iapetus, bisects its entire dark hemisphere and reaches 20 kilometers high (12 miles). It extends over 1,300 kilometers (808 miles) from side to side, along its midsection. No other moon in the solar system has such a striking geological feature. In places, the ridge is comprised of mountains. In height, they rival Olympus Mons on Mars, approximately three times the height of Mt. Everest, which is surprising for such a small body as Iapetus. Mars is nearly five times the size of Iapetus.
The flyby images, which revealed a region of Iapetus never before seen, show feathery-looking black streaks at the boundary between dark and bright hemispheres that indicate dark material has fallen onto Iapetus. Opinions differ as to whether this dark material originated from within or outside Iapetus. The images also show craters near this boundary with bright walls facing towards the pole and dark walls facing towards the equator.
Friday's flyby was the first close encounter of Iapetus during the four-year Cassini tour. The second and final close flyby of Iapetus is scheduled for 2007. Next up for Cassini is communications support for the European Space Agency's Huygens probe during its descent to Titan on January 14.
In 2005 Cassini will have 13 targeted encounters with five of Saturn's moons. "We have 43 close flybys of Titan still ahead of us during the four-year tour. Next year, eight of our 13 close flybys will be of Titan. We will also have a number of more distant flybys of the icy satellites, and let's not forget Saturn and the rings each time we come around," said Mitchell.
"I can think of no better way than this to wrap up what has been a whirlwind year," said Robert T. Mitchell, program manager for the Cassini mission at NASA's Jet Propulsion Laboratory, Pasadena, CA."The new year offers new opportunities, and 2005 will be the year of the icy satellites."
Titan Rendezvous
The Cassini spacecraft completed a successful rendezvous with Saturn's moon Titan on Monday, December 13. This was the last pass before the European Space Agency's Huygens probe was sprung loose from Cassini on Christmas Eve (in U.S. time zones). Information gathered during this flyby will provide an opportunity to compare images from Cassini's first close Titan encounter which occurred on October 26.
NASA's Deep Space Network tracking station in Madrid, Spain, acquired a signal at about 4:00 p.m. Pacific Standard Time (7:00 p.m. Eastern Standard Time). As anticipated, the spacecraft came within 1,200 kilometers (750 miles) of Titan's surface.
As with the last flyby, a major goal of this flyby was to measure the thickness of Titan's atmosphere. The information gathered will help determine whether Cassini can safely get closer to Titan on subsequent flybys, and it will also be used to verify that Huygens atmosphere models are correct. Titan is a prime target of the Cassini mission because it is the only moon in our solar system with a thick smoggy atmosphere.
The Cassini spacecraft captured the puzzle pieces for the full-disc view of the mysterious Titan during its first close encounter on October 26, 2004. The mosaic comprises nine images taken at distances ranging from 650,000 kilometers (400,000 miles) to 300,000 kilometers (200,000 miles).
The images that make up the mosaic were processed to reduce effects of the atmosphere and to sharpen surface features. The mosaic of images has been trimmed to show only the illuminated surface and not the atmosphere around the edge of the moon. The sun was behind Cassini, so nearly the full disc was illuminated. South polar clouds are seen at the bottom.
Surface features are best seen near the center of the moon. The surface features become fuzzier toward the outside of the image, where the spacecraft is peering through more haze. The brighter region on the right side near the equator is named Xanadu Regio. Scientists are debating what processes may have created the bizarre surface brightness patterns seen there. Titan's lack of obvious craters is a hint of a young surface. However, the exact nature of that activity, whether tectonic, wind-blown, river-related, marine or volcanic, is still unknown.
Other Moons
Two days after the close encounter with icy Titan, Cassini captured the images used in the mosaic of the battered and cratered moon Tethys on October 28. The result is the best-ever natural color view of Tethys. The images to create this mosaic were taken at a distance of about 256,000 kilometers (159,000 miles) from Tethys. This view shows the trailing hemisphere of Tethys, which is the side opposite the moon's direction of motion in its orbit.
As seen here, the surface of Tethys has a neutral hue. Three images form this natural color composite. The mosaic reveals a world nearly saturated with craters -- many small craters lie on top of older, larger ones, suggesting an ancient surface. Grooves can be seen at the top and along the boundary between day and night.
Tethys is known to have a density very close to that of water, indicating that it is likely composed mainly of water ice. Its frozen mysteries await Cassini's planned close flyby in September 2005.
On Wednesday morning (December 15), Cassini flew by Saturn's icy moon Dione at a distance of 72,500 kilometers (45,000 miles).
Adapted from the information on http://www.jpl.nasa.gov/news/news.cfm?release=2005-005, http://www.jpl.nasa.gov/news/news.cfm?release=2005-002, http://www.jpl.nasa.gov/news/news.cfm?release=2004-300, http://www.jpl.nasa.gov/news/news.cfm?release=2004-288, and http://www.jpl.nasa.gov/news/news.cfm?release=2004-278.
Hubble Heritage Picture - January 2005
(Added 01/13/05) The Hubble Heritage Team has released January's image of the barred spiral galaxy NGC 1300. 69 million light-years from us, the galaxy lies in the constellation Eridanus; this image covers approximately 5.5 arcminutes across (110,000 light-years). The image is combined from data taken in September 2004. This image is a composite of four filters (B (F435W), V (F555W), I (F814W), and H-α (F658N)).
The Hubble telescope has captured a display of starlight, glowing gas, and silhouetted dark clouds of interstellar dust in this grand image of the barred spiral galaxy NGC 1300. NGC 1300 is considered to be prototypical of barred spiral galaxies. Barred spirals differ from normal spiral galaxies in that the arms of the galaxy do not spiral all the way into the center, but are connected to the two ends of a straight bar of stars containing the nucleus at its center.
At Hubble's resolution, a myriad of fine details is revealed throughout the galaxy's arms, disk, bulge, and nucleus. Blue and red supergiants, clusters, and star-forming regions are well resolved across the spiral arms, and dust lanes trace out fine structures in the disk and bar. Numerous more distant galaxies are visible in the background, and are seen even through the densest regions of NGC 1300.
The nucleus of NGC 1300 shows an extraordinary "grand-design" spiral structure that is about 3,300 light-years (1 kpc) in diameter. Only galaxies with large-scale bars appear to have these grand-design inner spiral disks. Models suggest that the gas in a bar can be funneled inwards, and then spiral into the center through the grand-design disk, where it can potentially fuel a central black hole. NGC 1300 is not known to have an active nucleus, however, indicating either that there is no central black hole, or that it is not accreting matter.
The image was constructed from exposures taken by the Advanced Camera for Surveys onboard Hubble. Starlight and dust are seen in blue, visible, and infrared light. Bright star clusters are highlighted in red by their associated emission from glowing hydrogen gas. Due to the galaxy's large size, two adjacent pointings of the telescope were necessary to cover the extent of the spiral arms.
Adapted from the information on http://heritage.stsci.edu/2005/01/.
Hubble Heritage Picture - November 2004
(Added 01/13/05) The Hubble Heritage Team has released November's image of the Sagittarius Dwarf Irregular Galaxy, SagDIG and ESO 594-4. 3.5 million light-years from us, the galaxy lies in the constellation Sagittarius; this image covers approximately 1.5 arcminutes across (1500 light-years). The image is combined from data taken on August 18, 2003, for a total exposure time of 1.2 hours. This image is a composite of three filters (B (F435W), V (F606W), and I (F814W)).
This image from the Hubble Space Telescope shows a small galaxy called the Sagittarius dwarf irregular galaxy, or "SagDIG" for short. SagDIG is relatively nearby, and Hubble's sharp vision is able to reveal many thousands of individual stars within the galaxy.
The brightest stars in the picture (easily distinguished by the spikes radiating from their images, produced by optical effects within the telescope), are foreground stars lying within our own Milky Way galaxy. Their distances from Earth are typically a few thousand light-years. By contrast, the numerous faint, bluish stars belong to SagDIG. Lastly, background galaxies (reddish/brown extended objects with spiral arms and halos) are located even further beyond SagDIG at several tens of millions light-years away.
As their name implies, dwarf irregular galaxies are unlike their spiral and elliptical cousins, because of their much smaller physical size and lack of definite structure. Using Hubble, astronomers are able to resolve dwarf irregular galaxies that are at very large distances from Earth, into individual stars. By examining properties of the galaxy, such as distance, age and chemical composition, the star formation history of the whole galaxy is better understood, and reveals how, where, and when active star formation took place.
The main body of SagDIG shows a number of star-forming complexes that cover an appreciable fraction of the galaxy surface area. The presence of on-going star formation in a gas-rich galaxy such as this makes SagDIG an excellent laboratory where scientists can test present-day theories of what triggers star-formation in galaxies (without companions) and how this propagates throughout the galaxy.
Adapted from the information on http://heritage.stsci.edu/2004/31/.
Hubble Heritage Picture - October 2004
(Added 01/13/05) The Hubble Heritage Team has released October's image of the supernova remnant nebula Kepler's Supernova Remnant, also known as SN 1604 and V843 Ophiuchi. 13,000 light-years from us, the nebula lies in the constellation Ophiuchus; this image covers approximately 3 arcminutes across (11 light-years). The image is combined from data taken on August 28/29, 2003, for a total exposure time of 5.2 hours.
This image is a composite of four filters ([O III] (F502N), V (F550M), H-α+[N II] (F658N), and [N II] (F660N)).
On the night of October 9, 1604, sky watchers looking at a rare clustering of Mars, Jupiter and Saturn, were amazed by the appearance of a "new star" as bright as the planets. The famous astronomer, Johannes Kepler, heard about it, but he had to wait a week for the skies to clear over Prague before he had a chance to see the phenomenon. From then on he observed the new star regularly for a year until, in October 1605, it became too faint to see with the naked eye. Telescopes were first used in astronomy only four years after that. It was not until the mid-20th century that astronomers, using large telescopes, searched for and found a cloud of glowing gas around the location of the new star of 1604. Today this nebula is known as Kepler's supernova remnant: the remnants of a stellar explosion that was the last supernova seen in our Milky Way galaxy.
Four hundred years after its discovery, astronomers are using the combined power of NASA's three Great Observatories (Hubble, Spitzer, and Chandra), the Very Large Array radio telescope, and ground-based telescopes with modern spectrographs to unravel intricate features of Kepler's supernova remnant. Multi-wavelength images show a bubble-shaped shroud of gas and dust, 14 light-years wide. There is a fast-moving shell of iron-rich material surrounded by the primary shock wave from the supernova, expanding at 4 million miles per hour (2000 kilometers per second) that is sweeping up gas and dust from the surrounding medium.
The Hubble Telescope image, obtained with the Advanced Camera for Surveys in August 2003, shows regions that have been lighted up by the passage of the supernova shock wave. Knots and filamentary sheets of emission viewed edge-on are revealed in glorious detail. The bright knots are dense clumps that form behind the outward moving shock wave. The shock plows into material lost from the progenitor in a stellar wind prior to the supernova explosion, and instabilities cause the swept up gas to fragment into clumps. The thin filaments trace regions where the shock front is encountering more uniform, lower density material.
Filters onboard Hubble have been used to isolate light emitted by hydrogen atoms, and nitrogen and oxygen ions present in the gas. These filters also transmit starlight from foreground and background stars. Regions in which there is only glowing hydrogen are red, the yellow regions are strong in nitrogen, and regions where oxygen emission is present are pink or white.
The multi-wavelength study, for which the Hubble observations provide a crucial component, will help astronomers identify the type of star that produced the explosion. There are two different types of supernovas: one formed by the thermonuclear explosion of an accreting white dwarf star, and the other formed by the rebound explosion following the collapse of the core of a massive star. Of the six known supernovas in our Milky Way in the last 1000 years, SN 1006, and SN 1572 (Tycho's supernova) are of the former type, while SN 1054 (Crab Nebula), SN 1181 and SN 1680? (Cassiopeia A) are of the latter type, and SN 1604 (Kepler's supernova) is the only one for which the type is as yet unknown.
Adapted from the information on http://heritage.stsci.edu/2004/29/.
Mars Exploration Rover Update (Rather Large Update)
(Added 01/07/05) Due to the three-month hiatus that I took for personal and academic reasons from updating this site, much has happened with the Mars Exploration Rovers (MER) Spirit and Opportunity since I last added to the Current Event section. This update is a "short" summary of major happenings and discoveries in roughly reverse-chronological order (most recent is first).
A Year on Mars
NASA lit a birthday candle on January 3 for its twin rovers Spirit and Opportunity. The Spirit rover begins its second year on Mars investigating puzzling rocks unlike any found earlier. The rovers successfully completed their three-month primary missions in April. They astound even their designers with how well they continue to operate. The unanticipated longevity is allowing both rovers to reach additional destinations and to keep making discoveries. Spirit landed on January 3 and Opportunity January 24, 2004, respectively.
"You could have cut the tension here with a knife the night Spirit landed," said NASA Administrator Sean O'Keefe. "Just remembering the uncertainty involved with the landing emphasizes how exciting it is for all of us, since the rovers are still actively exploring. The rovers created an amazing amount of public interest and have certainly helped advance the Vision for Space Exploration," he said. The twin Mars explorers have drawn the most hits to NASA Web sites -- more than 9 billion in 2004.
Dr. Charles Elachi, director of NASA's Jet Propulsion Laboratory, Pasadena, CA, remarked, "Little did we know a year ago that we'd be celebrating a year of roving on Mars. The success of both rovers is tribute to hundreds of talented men and women who have put their knowledge and labor into this team effort."
"The rovers are both in amazingly good shape for their age," said JPL's Jim Erickson, rover project manager. "The twins sailed through the worst of the martian winter with flying colors, and spring is coming. Both rovers are in strong positions to continue exploring, but we can't give you any guarantees."
Opportunity is driving toward the heat shield that protected it during descent through the martian atmosphere. Rover team members hope to determine how deeply the atmospheric friction charred the protective layer. "With luck, our observations may help to improve our ability to deliver future vehicles to the surface of other planets," Erickson said.
Spirit is exploring the Columbia Hills within the Gusev Crater. "In December, we discovered a completely new type of rock in Columbia Hills, unlike anything seen before on Mars," said Dr. Steve Squyres of Cornell University, Ithaca, NY, principal investigator for the rovers' science payloads.
Jumbled textures of specimens dubbed "Wishstone" and "Wishing Well" look like the product of an explosion, perhaps from a volcano or a meteor impact. These rocks are much richer in phosphorus than any other known Mars rocks. "Some ways of making phosphates involve water; others do not," Squyres said. "We want to look at more of these rocks to see if we can distinguish between those possible histories."
NASA's next Mars mission, the Mars Reconnaissance Orbiter, is due to launch in August. "As great as the past year has been, Mars launch opportunities come along like clockwork every 26 months," said Dr. Firouz Naderi of JPL, manager of NASA's Mars Exploration Program. "At every one of them in the foreseeable future, we intend to go to Mars, building upon the findings by the rovers."
NASA Chief Scientist Dr. Jim Garvin said, "Mars lures us to explore its mysteries. It is the most Earth-like of our sister planets, and many believe it may hold clues to whether life ever existed or even originated beyond Earth. The rovers have shown us Mars had persistently wet, possibly life-sustaining environments. Beyond their own profound discoveries, the rovers have advanced our step-by- step program for examining Mars. We will continue to explore Mars robotically, and eventually with human explorers."
Clues to Water on Mars
Scientists have identified a water-signature mineral called goethite in bedrock that the NASA's Mars rover Spirit examined in the "Columbia Hills," one of the mission's surest indicators yet for a wet history on Spirit's side of Mars.
"Goethite, like the jarosite that Opportunity found on the other side of Mars, is strong evidence for water activity," said Dr. Goestar Klingelhoefer of the University of Mainz, Germany, lead scientist for the iron-mineral analyzer on each rover, the Moessbauer spectrometer. Goethite forms only in the presence of water, whether in liquid, ice or gaseous form. Hematite, a mineral that had previously been identified in Columbia Hills bedrock, usually, but not always, forms in the presence of water.
Klingelhoefer presented the new results from a rock in the "West Spur" of Mars' "Husband Hill" at a meeting of the American Geophysical Union in San Francisco. Spirit has now driven past the West Spur to ascend Husband Hill itself. One remaining question is whether water was only underground or ever pooled above the surface, as it did at Opportunity's site. "As we climb Husband Hill and characterize the rock record, we'll be looking for additional evidence that the materials were modified by ground water and searching for textural, mineralogical and chemical evidence that the rocks were formed in or modified by surface water," said Dr. Ray Arvidson of Washington University in St. Louis, deputy principal investigator for the rover instruments.
The amount of worrisome friction in Spirit's right front wheel has been decreasing. Meanwhile, rover wranglers at NASA's Jet Propulsion Laboratory in Pasadena, CA, continue to minimize use of that wheel by often letting it drag while the other five wheels drive. "Babying that wheel seems to be helping," said JPL's Jim Erickson, rover project manager.
Opportunity has completed six months of inspecting the inside of "Endurance Crater" and is ready to resume exploration of the broad plains of the Meridiani region. It has recently seen frost and clouds marking the seasonal changes on Mars. Rover science-team member Dr. Michael Wolff of the Brookfield, Wisconsin branch of the Boulder, Colorado-based Space Science Institute is reporting those and other atmospheric observations. "We're seeing some spectacular clouds," Wolff said. "They are a dramatic reminder that you have weather on Mars. Some days are cloudy. Some are clear."
A portion of Mars' water vapor is moving from the north pole toward the south pole during the current northern-summer and southern-winter period. The transient increase in atmospheric water at Meridiani, just south of the equator, plus low temperatures near the surface, contribute to appearance of the clouds and frost, Wolff said. Frost shows up some mornings on the rover itself. The possibility that it has a clumping effect on the accumulated dust on solar panels is under consideration as a factor in unexpected boosts of electric output from the panels.
As its last major endeavor inside Endurance Crater, Opportunity made a close inspection of rock layers exposed in a part of the crater wall called "Burns Cliff." Dr. Squyres said, "In the lower portion of the cliff, the layers show very strong indications that they were last transported by wind, not by water like some layers higher up. The combination suggests that this was not a deep-water environment but more of a salt flat, alternately wet and dry."
The most dramatic findings so far from NASA's twin Mars rovers -- telltale evidence for a wet and possibly habitable environment in the arid planet's past -- passed rigorous scientific scrutiny for publication in a major research journal.
Eleven reports by 122 authors in an issue of the journal Science present results from Opportunity's three-month prime mission, fleshing out headline discoveries revealed earlier.
"Liquid water was once intermittently present at the martian surface at Meridiani, and at times it saturated the subsurface. Because liquid water is a key prerequisite for life, we infer conditions at Meridiani may have been habitable for some period of time in martian history," according to Squyres, Arvidson and other co-authors.
"Formal review and publication ... of these amazing discoveries further strengthens the need for continued exploration by orbiters, surface robots, sample-return missions and human explorers. There are more exciting discoveries awaiting us on the red planet," said Dr. Michael Meyer, chief scientist for Mars exploration at NASA Headquarters, Washington.
One type of evidence that Meridiani was wet is the composition of rocks there. The rocks have a high and variable ratio of bromine to chlorine; indicating "the past presence of large amounts of water," write Dr. Rudi Rieder and Dr. Ralf Gellert of Max-Planck-Institute for Chemistry, Mainz, Germany, and co-authors. Their paper and another by Dr. Phil Christensen of Arizona State University, Tempe, and collaborators report an abundance of sulfur-rich minerals in the rocks, another clue to a watery past. Clinching the case is identification of a hydrated iron-sulfate salt called jarosite in the rocks, as reported by Dr. Goestar Klingelhoefer of the University of Mainz, and Dr. Richard Morris of NASA's Johnson Space Center, Houston, and co-authors.
Structures within the rocks add more evidence according to Dr. Ken Herkenhoff of the U.S. Geological Survey, Flagstaff, AZ, and co-authors. Plentiful cavities, about the size of shirt buttons, indicate crystals formed inside the rocks then dissolved. Minerals carried by water formed peppercorn-size gray spheres, nicknamed "blueberries," that are embedded in the rocks. Certain angled patterns of fine layers in some rocks tell experts a flowing body of surface water shaped the sediments that became the rocks.
Several characteristics of the rocks suggest water came and went repeatedly, as it does in some shallow lakes in desert environments on Earth. That fluctuation, plus the water's possible high acidity and saltiness, would have posed challenges to life, but not necessarily insurmountable ones, according to researchers. If life ever did exist at Meridiani, the type of rocks found there could be good preservers of fossils, according to Squyres, Dr. John Grotzinger of the Massachusetts Institute of Technology, Cambridge, and co-authors.
Halfway around Mars, Spirit is climbing higher into the "Columbia Hills." Spirit drove more than three kilometers (approximately two miles) across a plain to reach them. After finding bedrock that had been extensively altered by water, scientists used the rover to look for relatively unchanged rock as a comparison for understanding the area's full range of environmental changes. Instead, even the freshest-looking rocks examined by Spirit in the Columbia Hills have shown signs of pervasive water alteration.
"We haven't seen a single unaltered volcanic rock, since we crossed the boundary from the plains into the hills, and I'm beginning to suspect we never will," said Dr. Steve Squyres of Cornell University, Ithaca, N.Y., principal investigator for the science payload on both rovers. "All the rocks in the hills have been altered significantly by water. We're having a wonderful time trying to work out exactly what happened here."
More clues to deciphering the environmental history of the hills could lie in layered rock outcrops farther upslope, Spirit's next targets. "Just as we worked our way deeper into the Endurance crater with Opportunity, we'll work our way higher and higher into the hills with Spirit, looking at layered rocks and constructing a plausible geologic history," Squyres said.
Jim Erickson, rover project manager at JPL, said, "Both Spirit and Opportunity have only minor problems, and there is really no way of knowing how much longer they will keep operating. However we are optimistic about their conditions, and we have just been given a new lease on life for them, a six-month extended mission that began Oct. 1. The solar power situation is better than expected, but these machines are already well past their design life. While they're healthy, we'll keep them working as hard as possible."
Terrain Traveled
Opportunity wrapped up its stay in Endurance crater in December, though careful maneuvering was required in order to remove the rover from the crater. "We've done a careful analysis of the ground in front of Opportunity and decided to turn around," said Jim Erickson, rover project manager at NASA's Jet Propulsion Laboratory, Pasadena, CA. "To the right, the slope is too steep -- more than 30 degrees. To the left, there are sandy areas we can't be sure we could get across."
Before turning around, Opportunity spent a few days examining the rock layers in scarp about 10 m (33 ft) high, dubbed "Burns Cliff." From its location at the western foot of the cliff, the rover will use its panoramic camera and miniature thermal emission spectrometer to collect information from which scientists hope to determine whether some of the layers were deposited by wind, rather than by water. The rover will not reach an area about 15 m (50 ft) farther east where two layers at different angles meet at the base of the cliff.
"We have pushed the vehicle right to the edge of its capabilities, and we've finally reached a spot where we may be able to answer questions we've been asking about this site for months," said Dr. Squyres. "But after we're done here, it'll be time to turn around. Going any farther could cut off our line of retreat from the crater, and that's not something anybody on the team wants to do."
Opportunity entered the stadium-size crater on June 8 at a site called "Karatepe" along the crater's southern rim. Inside the crater, it has found and examined multiple layers of rocks that show evidence of a wet environment in the area's distant past.
Engineers have finished troubleshooting an indication of a problem with steering brakes on Spirit. The brakes are designed to keep the rover wheels from being bumped off course while driving. Spirit has intermittently sent information in recent weeks that the brakes on two wheels were not releasing properly when the rover received commands to set a new course. Testing and analysis indicate that the mechanism for detecting whether the brakes are released is probably sending a false indication. The rover team will disregard that signal and presume the brakes have actually released properly when commanded to do so. This anomaly has not been observed on the Opportunity rover.
The ones on Spirit are adding fresh evidence about the history of layered bedrock in a hill the rover is climbing.
"Our leading hypothesis is that these rocks originated as volcanic ash that fell from the air or moved in ground-hugging ash flows, and that minerals in them were altered by water," said Dr. Ray Arvidson of Washington University, St. Louis, deputy principal investigator for the mission.
"This is still a working hypothesis, not a firm conclusion, but all the instruments have contributed clues that fit," he said. "However, it is important to point out that we have just begun to characterize the textures, mineralogy and chemistry of these layered rocks. Other hypotheses for their origin focus on the role of transport and deposition by water. In fact, it may turn out that volcanism, water and wind have produced the rocks that Spirit is examining. We are just beginning to put together the big picture."
Some clues for a volcanic-ash origin come from a layered rock dubbed "Uchben." Researchers pointed Spirit's microscopic imager at a spot on Uchben scoured with the rock abrasion tool. The images reveal sand-size particles, many of them sharply angular in shape and some quite rounded. The angularity is consistent with transport by an eruption. Particles carried across the surface by wind or water usually tumble together and become more rounded. Uchben's rounded particles may be volcanic clumps, may be concretions similar to what Opportunity has found, or may be particles tumbled in a water environment.
Evidence for alteration by water comes mainly from identification of minerals and elements in the rocks by the rover's Moessbauer spectrometer and alpha particle X-ray spectrometer.
Adapted from the information on http://www.jpl.nasa.gov/news/news.cfm?release=2005-001, http://www.jpl.nasa.gov/news/news.cfm?release=2004-287, http://www.jpl.nasa.gov/news/news.cfm?release=2004-280, http://www.jpl.nasa.gov/news/news.cfm?release=2004-269, http://www.jpl.nasa.gov/news/news.cfm?release=2004-253.
Saturn's Moon Prometheus Caught Stealing Ring Material
(Added 01/07/05) Stealing is a crime on Earth, but at Saturn, apparently it is routine. The Cassini spacecraft has witnessed Saturn's moon Prometheus snatching particles from one of Saturn's rings. This potato-shaped moon is also believed to be responsible for kinks within Saturn's thin F ring, a contorted, narrow ring flanked by two small moons, Prometheus and Pandora. The thievery and the detailed behavior of kinks were observed for the first time ever in images taken by the Cassini spacecraft.
In an image taken on October 29, Prometheus is seen stealing particles from the F ring while connected to the ringlets by a faint streak of material. A movie sequence of the ring, taken on October 28, captures in freeze-frame motion the zigzagging kinks and knots, some of which are almost certainly caused by Prometheus.
The kinks look like "hiccups" traveling around the ring. Consisting of 44 frames taken three minutes apart, the sequence represents almost two hours, or about one-eighth of the orbital period of F ring particles around the planet. Cassini was on a flight path that took the spacecraft away from the planet and farther south, so that the rings appear to tilt upward. The top portion of the F ring is closer to the spacecraft, while the bottom portion is farther away and curves around the far side of Saturn.
Scientists are not sure exactly how Prometheus is interacting with the F ring here, but they have speculated that the moon might be gravitationally pulling material away from the ring. Scientists speculate that the ring particles may end up in a slightly different orbit from the one they were in prior to getting a "kick" from the moon. These kicks occur at specific locations in the rings and can actually cause large waves or knots to form. In the still image, gaps in the diffuse inner strands are seen. All these features appear to be due to the influence of Prometheus in ways that are not fully understood.
Scientists will use what they learn about Prometheus' interaction with the F ring to understand the gravitational exchanges between moons and rings, which give rise to much of the structure that is observed in Saturn's rings.
Adapted from the information on http://www.jpl.nasa.gov/news/news.cfm?release=2004-282.
(Added 01/07/05) Analysis of images and other data captured during Cassini's close flyby of Saturn's moon Titan on October 26, 2004, reveals greater surface detail than ever before and shows that Titan has lost much of its original atmosphere over time. "Titan has incredible diversity," said Dr. Dennis Matson, project scientist for the Cassini mission at NASA's Jet Propulsion Laboratory, Pasadena, CA. "We are glad that we have a full complement of instruments on this spacecraft because it is going to take all of them to reveal the story of Titan."
Cassini swooped down to within 1,174 km (730 miles) of Titan during the close encounter. During the flyby, ground controllers were not in contact with the spacecraft, because it was turned away from Earth to make its observations. The signal was re-acquired as expected at 9:25 p.m. EDT.
Pictures from the imaging cameras and the visual and infrared mapping spectrometer show a complex interplay between dark and bright material on Titan's surface. The surface appears to have been shaped by multiple geologic processes. Although a few circular features can be seen, none can be definitively identified as impact craters.
"We are seeing features and patterns on the surface, and there are processes creating these patterns, and that gives us something to chew on for a while," said Dr. Carolyn Porco, team leader for the imaging team, Space Science Institute, Boulder, CO. "We can't figure out what the features are, but they are intriguing. This is an environment we have never seen before. It is a very different place and it will take some time to unravel and piece it all together."
Cassini scientists were intrigued that the spacecraft's ion and neutral mass spectrometer found that Titan's atmosphere has more of the heavy isotope of nitrogen, compared to the lighter form. They believe that when nitrogen molecules rose to the top of the atmosphere, the lighter form was swept away with greater efficiency than the heavier form.
11 of Cassini's 12 instruments were on during the flyby. 10 instruments returned data successfully. Engineers are working on a software glitch that caused the composite infrared spectrometer to malfunction. The team is confident that subsequent flybys of Titan will allow them to collect any data not gathered from the flyby.
Titan holds great fascination because it is the only known moon in the solar system to have a thick atmosphere. That murky atmosphere may be similar to that which existed on Earth before life formed. Cassini will become a frequent visitor to Titan, with 44 more targeted flybys planned during the mission.
"A major goal of this flyby was to measure the properties of Titan's atmosphere to see if our models to simulate the Huygens entry and descent are accurate, and to assess the feasibility of subsequent flybys at the 950 km altitude [590 miles]," said Dr. Earl Maize, Cassini deputy program manager at JPL. "Preliminary data from [the] flyby are consistent with current predictions."
The first radar images of Saturn's moon Titan show a very complex geological surface that may be relatively young. Previously, Titan's surface was hidden behind a veil of thick haze. "Unveiling Titan is like reading a mystery novel," said Dr. Charles Elachi, director of NASA's Jet Propulsion Laboratory, Pasadena, CA, and team leader for the radar instrument on Cassini. "Each time you flip the page you learn something new, but you don't know the whole story until you've read the whole book. The story of Titan is unfolding right before our eyes, and what we are seeing is intriguing."
Approximately 1% of Titan's surface was mapped during the flyby. Radar images from Titan's northern hemisphere, a region that has not yet been imaged optically, show great detail and features down to 300 m (984 ft) across. A wide variety of geologic terrain types can be seen. There are bright areas that correspond to rougher terrains and darker areas that are thought to be smoother.
The radar images show a world brimming with features that are dark and white, indicating sharp contrast. One area dubbed "Si- Si" or the "Halloween cat" because it is shaped like a cat's head is very dark and relatively smooth. That leads scientists to speculate that it might be a lake of some sort, but they caution that it is too soon to know for sure.
The optical imaging cameras on Cassini show streaks on the surface. The streaking may be caused by movement of a material over the surface by wind, flowing hydrocarbon liquids, or a moving ice sheet like a glacier. Imaging scientists are also seeing multiple haze layers in Titan's atmosphere that extend some 500 km (310 miles) above the surface. At the surface Titan's atmosphere is about four times denser than Earth's.
A strikingly bright feature that is consistent with an active geology has been seen in one of Cassini's first radar images of Saturn's moon Titan. There are many possibilities for what it is but one of the leading candidates is that it may be a "cryovolcanic" flow or "ice volcano."
Adapted from the information on http://www.jpl.nasa.gov/news/news.cfm?release=2004-264, http://www.jpl.nasa.gov/news/news.cfm?release=2004-265, http://www.jpl.nasa.gov/news/news.cfm?release=2004-270, and http://www.jpl.nasa.gov/news/news.cfm?release=2004-274.
New-Found Companion for the Milky Way
(Added 01/06/05) Most of the stars in our Milky Way galaxy lie in a very flat, pinwheel-shaped disk. Although this disk is prominent in images of galaxies similar to the Milky Way, there is also a very diffuse spherical "halo" of stars surrounding and enclosing the disks of such galaxies. Recent discoveries have shown that this outer halo of the Milky Way is probably composed of small companion galaxies ripped to shreds as they orbited the Milky Way.
A discovery announced today by the Sloan Digital Sky Survey (SDSS) reveals a clump of stars unlike any seen before. The findings may shed light on how the Milky Way's stellar halo formed. This clump of newly discovered stars, called SDSSJ1049+5103, or Willman 1, is so faint that it could only be found as a slight increase in the number of faint stars in a small region of the sky.
"We discovered this object in a search for extremely dim companion galaxies to the Milky Way," explains Beth Willman of New York University's Center for Cosmology and Particle Physics. "However, it is 200 times less luminous than any galaxy previously seen."
"Another possibility," adds Michael Blanton, an SDSS colleague of Willman's at New York University, "is that Willman 1 is an unusual type of globular cluster, a spherical agglomeration of thousands to millions of old stars. ... Its properties are rather unusual for a globular cluster. It is dimmer than all but three known globular clusters. Moreover, these dim globular clusters are all much more compact than Willman 1", explains Blanton. "If it's a globular cluster, it is probably being torn to shreds by the gravitational tides of the Milky Way."
The real distinction between the globular cluster and dwarf galaxy interpretations is that galaxies are usually accompanied by substantial quantities of dark matter, says Julianne Dalcanton, an SDSS researcher at the University of Washington. "Clearly the next step is to carry out additional measurements to determine whether there is any dark matter associated with Willman 1."
SDSS consortium member Daniel Zucker of the Max Planck Institute for Astronomy in Heidelberg, Germany, says the Sloan Digital Sky Survey has proven to be "a veritable gold mine for studies of the outer parts of our galaxy and its neighbors, as shown by Dr. Willman's discovery, and by our group's earlier discovery of a giant stellar structure and a new satellite galaxy around the Andromeda Galaxy."
If Willman 1 does turn out to be a dwarf galaxy, this discovery could shed light on a long-standing mystery. The prevailing 'Cold Dark Matter' model predicts that our own Milky Way galaxy is surrounded by hundreds of dark matter clumps, each a few hundred light years in size and possibly populated by a dwarf galaxy. However, only 11 dwarf galaxies have been discovered orbiting the Milky Way. Perhaps some of these clumps have very few embedded stars, making the galaxies particularly difficult to find.
"If this new object is in fact a dwarf galaxy, it may be the tip of the iceberg of a yet unseen population of ultra-faint dwarf galaxies," suggests Willman.
"The colors of the stars in Willman 1 are similar to those in the Sagittarius tidal stream, a former dwarf companion galaxy to the Milky Way now in the process of merging into the main body of our Galaxy," explains Brian Yanny, an SDSS astrophysicist at The Department of Energy's Fermi National Accelerator Laboratory, a leader in research on the Milky Way's accretion of material. "If Willman 1 is a globular cluster, then it may have piggybacked a ride into our Galaxy's neighborhood on one of these dwarf companions, like a tiny mite riding in on a flea as it, in turn, latches onto a massive dog."
"Whether it is a globular cluster or a dwarf galaxy, this very faint object appears to represent one of the building blocks of the Milky Way," Willman said.
Adapted from the information on http://www.sdss.org/news/releases/20041020.companion.html.
New Information on an Old Supernova
(Added 01/06/05) An international team of astronomers is announced that they have identified the probable surviving companion star to a titanic supernova explosion witnessed in the year 1572 by the great Danish astronomer Tycho Brahe and other astronomers of that era.
This discovery provides the first direct evidence supporting the long-held belief that Type Ia supernovae come from binary star systems containing a normal star and a burned-out white dwarf star. The normal star spills material onto the dwarf, which eventually triggers an explosion.
The results of this research, led by Pilar Ruiz-Lapuente of the University of Barcelona, Spain, are were published in the October 28 British science journal Nature. "There was no previous evidence pointing to any specific kind of companion star out of the many that had been proposed. Here we have identified a clear path: the feeding star is similar to our Sun, slightly more aged," Ruiz-Lapuente says. "The high speed of the star called our attention to it," she added.
Type Ia supernovae are used to measure the history of the expansion rate of the universe and are fundamental to helping astronomers understand the behavior of dark energy, an unknown force that is accelerating the expansion of the universe. Finding evidence to confirm the theory as to how Type Ia supernovae explode is critical to assuring astronomers that the objects can be better understood as reliable calibrators of the expansion of space.
The identification of the surviving member of the stellar duo reads like a crime scene investigation tale. Even though today's astronomers arrived at the scene of the disaster 432 years later, using astronomical forensics they have nabbed one of the perpetrators rushing away from the location of the explosion (which is now enveloped in a vast bubble of hot gas called Tycho's Supernova Remnant). For the past seven years the runaway star and its surroundings were studied with a variety of telescopes. The Hubble Space Telescope played a key role by precisely measuring the star's motion against the sky background. The star is breaking the speed limit for that particular region of the Milky Way Galaxy by moving three times faster than the surrounding stars. Like a stone thrown by a sling, the star went hurtling off into space, retaining the velocity of its orbital motion when the system was disrupted by the white dwarf's explosion.
This alone is only circumstantial evidence that the star is the perpetrator because there are alternative explanations to its suspicious behavior, such as it could be falling in at a high velocity from the galactic halo that surrounds the Milky Way's disk. But spectra obtained with the 4.2-meter William Herschel Telescope in La Palma and the 10-meter W.M. Keck telescopes in Hawaii show that the suspect has the high heavy-element content typical of stars that dwell in the Milky Way's disk, not the halo.
The star found by the Ruiz-Lapuente team is an aging version of our Sun. The star has begun to expand in diameter as it progresses toward a red-giant phase (the end stage of a Sun-like star's lifetime). The star turns out to fit the profile of the perpetrator in one of the proposed supernova conjectures. In Type Ia supernova binary systems, the more massive star in the pair will age faster and eventually becomes a white dwarf star. When the slower-evolving companion star subsequently ages to the point where it begins to balloon in size, it spills hydrogen onto the dwarf. The hydrogen accumulates until the white dwarf reaches a critical and precise mass threshold, called the Chandrasekhar limit, where it explodes as a titanic nuclear bomb. The energy output of this explosion is so well known that it can be used as a standard candle for measuring vast astronomical distances. (An astronomical "standard candle" is any type of luminous object whose intrinsic power is so accurately determined that it can be used to make distance measurements based on the rate the light dims over astronomical distances).
"Among the various systems containing white dwarfs that receive material from a solar-mass companion, some are believed to be viable progenitors of Type Ia supernovae, on theoretical grounds. A system called U Scorpii has a white dwarf and a star similar to the one found here. These results would confirm that such binaries will end up in an explosion like the one observed by Tycho Brahe, but that would occur several hundreds of thousands of years from now," says Ruiz-Lapuente.
An alternative theory of Type Ia supernovae is that two white dwarfs orbit each other, gradually losing energy through the emission of gravitational radiation (gravity waves). As they lose energy, they spiral in toward each other and eventually merge, resulting in a white dwarf whose mass reaches the Chandrasekhar limit, and explodes. "Tycho's supernova does not appear to have been produced by this mechanism, since a probable surviving companion has been found," says Alex Filippenko of the University of California at Berkeley, a co-author on this research. He says that, nevertheless, it is still possible there are two different evolutionary paths to Type Ia supernovae.
On November 11, 1572, Tycho Brahe noticed a star in the constellation Cassiopeia that was as bright as the planet Jupiter (which was in the night sky in Pisces). No such star had ever been observed at this location before. It soon equaled Venus in brightness (which was at -4.5 magnitude in the predawn sky). For about two weeks the star could be seen in daylight. At the end of November it began to fade and change color, from bright white to yellow and orange to faint reddish light, finally fading away from visibility in March 1574, having been visible to the naked eye for about 16 months. Tycho's meticulous record of the brightening and dimming of the supernova now allows astronomers to identify its "light signature" as that of a Type Ia supernova.
Tycho Brahe's supernova was very important in that it helped 16th-century astronomers abandon the idea of the immutability of the heavens. At the present time, Type Ia supernovae remain key players in the newest cosmological discoveries. To learn more about them and their explosion mechanism, and to make them even more useful as cosmological probes, a current Hubble Space Telescope project led by Filippenko is studying a sample of supernovae in other galaxies at the very time they explode.
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/34.
Spitzer and Hubble Join to Examine Protoplanetary Disks
(Added 01/06/05) Two of NASA's Great Observatories, the Spitzer Space Telescope and the Hubble Space Telescope, have provided astronomers an unprecedented look at dusty planetary debris around stars the size of our sun. Spitzer has discovered for the first time dusty discs around mature, sun-like stars known to have planets. Hubble captured the most detailed image ever of a brighter disc circling a much younger sun-like star. The findings offer "snapshots" of the process by which our own solar system evolved, from its dusty and chaotic beginnings to its more settled present-day state.
"Young stars have huge reservoirs of planet-building materials, while older ones have only leftover piles of rubble. Hubble saw the reservoirs and Spitzer, the rubble," said Dr. Charles Beichman of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. He is lead author of the Spitzer study. "This demonstrates how the two telescopes complement each other," he added.
The young star observed by Hubble is 50 to 250 million years old. This is old enough to theoretically have gas planets, but young enough that rocky planets like Earth may still be forming. The six older stars studied by Spitzer average 4 billion years old, nearly the same age as the sun. They are known to have gas planets, and rocky planets may also be present. Prior to the findings, rings of planetary debris, or "debris discs," around stars the size of the sun had rarely been observed, because they are fainter and more difficult to see than those around more massive stars.
"The new Hubble image gives us the best look so far at reflected light from a disc around a star the mass of the sun," said Hubble study lead author, Dr. David Ardila of the Johns Hopkins University, Baltimore. "Basically, it shows one of the possible pasts of our own solar system," he added.
Debris discs around older stars the same size and age as our sun, including those hosting known planets, are even harder to detect. These discs are 10 to 100 times thinner than the ones around young stars. Spitzer's highly sensitive infrared detectors were able to sense their warm glow for the first time.
"Spitzer has established the first direct link between planets and discs," Beichman said. "Now, we can study the relationship between the two." These studies will help future planet-hunting missions, including NASA's Terrestrial Planet Finder and the Space Interferometry Mission, predict which stars have planets. Finding and studying planets around other stars is a key goal of NASA's exploration mission.
Rocky planets arise out of large clouds of dust that envelop young stars. Dust particles collide and stick together, until a planet eventually forms. Sometimes the accumulating bodies crash together and shatter. Debris from these collisions collects into giant doughnut-shaped discs, the centers of which may be carved out by orbiting planets. With time, the discs fade and a smaller, stable debris disc, like the comet-filled Kuiper Belt in our own solar system, is all that is left.
The debris disc imaged by Hubble surrounds the sun-like star called HD 107146, located 88 light-years away. John Krist, a JPL astronomer, also used Hubble to capture another disc around a smaller star, a red dwarf called AU Microscopii, located 32 light-years away and only 12 million years old. The Hubble view reveals a gap in the disc, where planets may have swept up dust and cleared a path. The disc around HD 107146 also has an inner gap.
Beichman and his colleagues at JPL and the University of Arizona, Tucson, used Spitzer to scan 26 older sun-like stars with known planets, and found six with Kuiper Belt-like debris discs. The stars range from 50 to 160 light-years away. Their discs are about 100 times fainter than those recently imaged by Hubble, and about 100 times brighter than the debris disc around the sun. These discs are also punctuated by holes at their centers.
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/33.
New Understanding of an Old Nebula
(Added 01/06/05) Looks can be deceiving, especially when it comes to celestial objects like galaxies and nebulas. These objects are so far away that astronomers cannot see their three-dimensional structure. The Helix Nebula, for example, resembles a doughnut in colorful images. Earlier images of this complex object - the gaseous envelope ejected by a dying, sun-like star - did not allow astronomers to precisely interpret its structure. One possible interpretation was that the Helix's form resembled a snake-like coil.
Now, a team of astronomers using observations from several observatories, including NASA's Hubble Space Telescope, has established that the Helix's structure is even more perplexing. Their evidence suggests that the Helix consists of two gaseous disks nearly perpendicular to each other.
A team of astronomers, led by C. Robert O'Dell of Vanderbilt University in Nashville, TN, made its finding using highly detailed images from the Hubble's Advanced Camera for Surveys, pictures from Cerro Tololo Inter-American Observatory in Chile, and measurements from ground-based optical and radio telescopes which show the speed and direction of the outflows of material from the dying star. The Helix, the closest planetary nebula to Earth, is a favorite target of professional and amateur astronomers. Astronomers hope this finding will provide insights on how expelled shells of gas from dying stars like our Sun form the complex shapes called planetary nebulas. The results are published in the November issue of the Astronomical Journal.
"Our new observations show that the previous model of the Helix was much too simple," O'Dell said. "About a year ago, we believed the Helix was a bagel shape, filled in the middle. Now we see that this filled bagel is just the inside of the object. A much larger disk, resembling a wide, flat ring, surrounds the filled bagel. This disk is oriented almost perpendicular to the bagel. The larger disk is brighter on one side because it is slamming into interstellar material as the entire nebula moves through space, like a boat plowing through water. The encounter compresses gas, making that region glow brighter. But we still don't understand how you get such a shape. If we could explain how this shape was created, then we could explain the late stages of the most common form of collapsing stars."
"To visualize the Helix's geometry," added astronomer Peter McCullough of the Space Telescope Science Institute in Baltimore, MD, and a member of O'Dell's team, "imagine a lens from a pair of glasses that was tipped at an angle to the frame's rim. Well, in the case of the Helix, finding a disk inclined at an angle to a ring would be a surprise. But that is, in fact, what we found."
Another surprise is that the dying star has expelled material into two surrounding disks rather than the one thought previously to be present. Each disk has a north-south pole, and material is being ejected along those axes. "We did not anticipate that the Helix has at least two axes of symmetry," O'Dell said. "We thought it had only one. This two-axis model allows us to understand the complex appearance of the nebula."
Using the Helix data, the astronomers created a three-dimensional model showing the two disks. These models are important to show the intricate structure within the nebula. The team also produced a composite image of the Helix that combines observations from Hubble's Advanced Camera for Surveys and the 4-m telescope's mosaic camera at Cerro Tololo. The Helix is so large that the team needed both telescopes to capture a complete view. Hubble observed the Helix's central region; the Cerro Tololo telescope, with its wider field of view, observed the outer region.
The team, however, is still not sure how the disks were created, and why they are almost perpendicular to each other. One possible scenario is that the dying star has a close companion star. Space-based x-ray observations provide evidence for the existence of a companion star. One disk may be perpendicular to the dying star's spin axis, while the other may lie in the orbital plane of the two stars.
The astronomers also believe the disks formed during two separate epochs of mass loss by the dying star. The inner disk was formed about 6,600 years ago; the outer ring, about 12,000 years ago. The inner disk is expanding slightly faster than the outer disk. Why did the star expel matter at two different episodes, leaving a gap of 6,000 years? Right now, only the Helix Nebula knows the answer, the astronomers said.
The sun-like star that sculpted the Helix created a beautiful celestial object. Will the Sun weave such a grand structure when it dies 5 billion years from now? "As a single star, it will create a similar glowing cloud of expelled material, but I wouldn't expect it to have such a complex structure as the Helix," McCullough said.
To study the intricate details of these celestial wonders, astronomers must use a range of observatories, including visible-light and radio telescopes. Astronomers also need the sharp eyes of Hubble's Advanced Camera for Surveys. "The Hubble's crisp vision has revealed a whole new realm of planetary nebula structure, which has advanced the field and delighted our eyes," said team member Margaret Meixner of the Space Telescope Science Institute.
Adapted from the information on http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/32.
Cepheid Variables Get Better Calibrated
(Added 01/05/05) It is very difficult to measure the distance to an astronomical object. In fact, this is one of the greatest challenges facing astronomers. There is no accurate, direct way to determine the distance to galaxies beyond the Milky Way: Astronomers first determine the distance to nearby stars in our galaxy as accurately as possible and then use a series of other techniques that reach progressively further into space to estimate distances to more distant systems. This process is often referred as the "distance ladder".
Over the years, a number of different distance estimators have been found. One of these is a particular class of stars known as Cepheid variables. They are used as one of the first "steps" on this cosmic distance ladder. Cepheids are rare and very luminous stars whose luminosity varies in a very regular way. They are named after the star Delta Cephei in the constellation of Cepheus, the first known variable star of this particular type and bright enough to be easily seen with the unaided eye.
The Cepheid stars have taken on an even more important role since the Hubble Space Telescope Key Project on the extragalactic distance scale relies completely on them for the calibration of distance indicators to reach cosmologically large distances. In other words, if the calibration of the Cepheid Period-Luminosity relation were wrong, the entire extragalactic distance scale and with it, the rate of cosmic expansion and the related acceleration, as well as the estimated age of the Universe, would also be off.
A main problem is thus to calibrate as accurately as possible the Period-Luminosity relation for nearby Cepheids. This requires measuring their distances with the utmost precision, a truly daunting task. And this is where interferometry now enters the picture.
Independent determinations of the distance of variable stars make use of the so-called Baade-Wesselink method, named after astronomers Walter Baade (1893-1960) and Adriaan Wesselink (1909-1995). With this classical method, the variation of the angular diameter of a Cepheid variable star is inferred from the measured changes in brightness (by means of model atmosphere calculations) as it pulsates. Spectroscopy is then used to measure the corresponding radial velocity variations, hence providing the linear distance over which the star's outer layers have moved. By dividing the angular and linear measures, the distance to the star is obtained.
This sounds straightforward. However, it would obviously be much better to measure the variation of the radius directly and not to rely on model atmosphere calculations. But here the main problem is that, despite their apparent brightness, all Cepheids are situated at large distances. Indeed, the closest Cepheid star (excluding the peculiar star Polaris), Delta Cephei, is more than 800 light-years away. Even the largest Cepheids in the sky subtend an angle of only 0.003 arcsec. To observe this is similar to view a two-story house on the moon, and astronomers want to do is to measure the change of the stars' sizes, amounting to only a fraction of this.
Such an observing feat is only possible with long-baseline interferometry. Also on this front, the VLT Interferometer is now opening a new field of observational astrophysics.
Some time ago, an undaunted team of French and Swiss astronomers started a major research program aimed at measuring the distance to several Cepheids by means of the above outlined Baade-Wesselink interferometric method. For these observations they combined sets of two beams - one set from the two VLTI Test Siderostats with 0.35 m aperture and the other set from two Unit Telescopes (Antu and Melipal; 8.2 m mirrors) - with the VINCI (VLT Interferometer Commissioning Instrument) facility. Three VLTI baselines were used for this program with, respectively, 66, 140 and 102.5 m ground length.
A total of 69 individual angular diameter measurements were obtained with the VLTI, over more than 100 hours of total telescope time, distributed over 68 nights; the largest angular diameter measured was 0.0032 arcsec (L Car at maximum). Seven Cepheids observable from Paranal Observatory were selected for this program: X and W Sagittarii, Eta Aquilae, Beta Doradus, Zeta Gemini, Y Ophiocus and L Carinae. Their periods range from 7 to 35.5 days, a fairly wide interval and an important advantage to properly calibrate the Period-Luminosity relation.
The distances to four of the stars (Eta Aql, W Sgr, Beta Dor and L Car) were derived using the interferometric Baade-Wesselink method, as their pulsation is detected by the VLTI. For the remaining three objects of the sample (X Sgr, Zeta Gem and Y Oph), a hybrid method was applied to derive their distances, based on their average angular diameter and pre-existing estimations of their linear diameters.
Combining the distances measured by this program with the apparent magnitudes of the stars, the astronomers determined the absolute magnitude (intrinsic brightness) of these stars and arrived at a very precise calibration of the zero-point of the Period-Luminosity relation (assuming the slope from previous work).
It turned out that this new and independently derived value of the zero-point is exactly the same as the one obtained during previous work based on a large number of relatively low-precision Cepheid distance measurements by the ESA Hipparcos astrometric satellite. The agreement between these two independent, geometrical calibrations is remarkable and greatly increases the confidence in the cosmic distance scale now in use.
With 1.8 m Auxiliary Telescopes soon to be ready on the VLTI platform, the astronomers will be able to observe many more Cepheids with a precision at least as good as the present high-precision VINCI observations of L Car. In addition, the future AMBER instrument will extend the VLTI capabilities toward shorter wavelengths (J and H bands), providing even higher spatial resolution than what is now possible with VINCI (K band).
The combined effect of these two improvements will be to extend significantly the accessible sample of Cepheids. It is expected that the distances to more than 30 Cepheids will then be measurable with a precision better than 5%. This will provide a high precision calibration of both the reference point (down to ±0.01 mag) and the slope of the Galactic Cepheid Period-Luminosity.
Adapted from the information on http://www.eso.org/outreach/press-rel/pr-2004/pr-25-04.html.
Chandra Finds Biggest Explosion in the Universe
(Added 01/05/05) Astronomers have found the most powerful eruption seen in the Universe using NASA's Chandra X-Ray Observatory. A supermassive black hole generated this eruption by growing at a remarkable rate. This discovery shows the enormous appetite of large black holes, and the profound impact they have on their surroundings.
The huge eruption is seen in a Chandra image of the hot, x-ray emitting gas of a galaxy cluster called MS 0735.6+7421. Two vast cavities extend away from the supermassive black hole in the cluster's central galaxy. The eruption - which has lasted for 100 million years and is still going - has generated the energy equivalent to hundreds of millions of gamma-ray bursts.
This event was caused by gravitational energy release as enormous amounts of matter fell toward a black hole. Most of the matter was swallowed, but some of it was violently ejected before being captured by the black hole. "I was stunned to find that a mass of about 300 million suns was swallowed," said Brian McNamara of Ohio University in Athens, lead author of the study that appears in the January 6, 2005 issue of Nature. "This is almost as massive as the supermassive black hole that swallowed it." Astronomers are not sure where such large amounts of matter came from. One theory is that gas from the host galaxy catastrophically cooled and was then swallowed by the black hole.
The energy released shows that the black hole in MS 0735 has grown very dramatically during this eruption. Previous studies suggest that other large black holes have grown very little in the recent past, and that only smaller black holes are still growing quickly. "This new result is as surprising as it is exciting", said co-author Paul Nulsen of the Harvard-Smithsonian Center of Astrophysics. "This black hole is feasting when it should be fasting."
Radio emission within the cavities shows that jets from the black hole erupted to create the cavities. Gas is being pushed away from the black hole at supersonic speeds over a distance of about a million light years. The mass of the displaced gas equals about a trillion suns, more than the mass of all the stars in the Milky Way.
The rapid growth of supermassive black holes is usually detected by observing very bright radiation from the centers of galaxies in the optical and X-ray wavebands, or luminous radio jets. In MS 0735 no bright central radiation is found and the radio jets are faint. Therefore, the true nature of MS 0735 is only revealed through x-ray observations of the hot cluster gas. "Until now we had no idea that this black hole was gorging itself", said co-author Michael Wise of the Massachusetts Institute of Technology. "The discovery of this eruption shows that x-ray telescopes are necessary to understand some of the most violent events in the Universe."
The astronomers estimated how much energy was needed to create the cavities by calculating the density, temperature and pressure of the hot gas. By making a standard assumption, that 10% of the gravitational energy goes into launching the jets, they estimated how much material the black hole swallowed.
Adapted from the information on http://chandra.harvard.edu/press/05_releases/press_010505.html.
Cassini Drops Huygens for Final Titan Approach
(Added 01/04/05) The European Space Agency's Huygens probe was successfully released by NASA's Cassini orbiter early Christmas morning and is now on a controlled collision course toward Saturn's largest and most mysterious moon, Titan, where on January 14 it will make a descent through one of the most intriguing atmospheres in the solar system to an unknown surface. The separation occurred at 02:00 UTC (03:00 CET): A few minutes after separation, Cassini turned back to Earth and relayed back information about the separation. This signal then took 1 hour and 8 minutes to cross the 1.2 billion km separating the Cassini spacecraft and Earth.
"Today's release is another successful milestone in the Cassini/Huygens odyssey," said Dr David Southwood, ESA's director of science programs."This was an amicable separation after seven years of living together. Our thanks to our partners at NASA for the lift. Each spacecraft will now continue on its own but we expect they'll keep in touch to complete this amazing mission. Now all our hopes and expectations are focused on getting the first in-situ data from a new world we've been dreaming of exploring for decades."
The Cassini/Huygens mission, jointly developed by NASA, ESA and the Italian space agency (ASI), began on October 15, 1997, when the composite spacecraft were launched from Cape Canaveral, FL, atop a Titan 4B/Centaur vehicle. Together, the two probes weighed 5548 kg at launch and became the largest space mission ever sent to the outer planets. To gain sufficient velocity to reach Saturn, they had to conduct four gravity-assist maneuvers by flying twice by Venus, once by the Earth and once by Jupiter. On July 1, 2004, Cassini/Huygens eventually became the first spacecraft to enter an orbit around Saturn.
On December 17, while on its third orbit around the ringed planet, the Cassini orbiter performed a maneuver to enter a controlled collision trajectory towards Titan. As planned, a fine tuning of the trajectory took place on December 22 to place Huygens on its nominal entry trajectory. While Huygens will remain on this trajectory till it plunges into Titan's atmosphere on January 14, the orbiter will perform a deflection maneuver on December 28 to avoid crashing onto the moon. The separation was achieved by the firing of pyrotechnic devices. Under the action of push-off springs, ramps and rollers, the probe was released at a relative velocity of about 0.3 m/s with a spin rate of 7 rpm. Telemetry data confirming the separation were collected by NASA's Deep Space Network stations in Madrid, Spain and Goldstone, CA, when the telemetry playback signal from Cassini eventually reached the Earth.
The Huygens probe is now dormant and will remain so for its 20-day coast phase to Titan. Four days before its release, a triply-redundant timer was programmed in order to wake-up the probe's systems shortly before arrival on Titan.
Huygens is scheduled to enter Titan's atmosphere at about 09:06 UTC (10:06 CET) on January 14, entering at a relatively steep angle of 65° and a velocity of about 6 km/s. The target is over the southern hemisphere, on the day side. Protected by an ablative thermal shield, the probe will decelerate to 400 m/s within 3 minutes before it deploys a 2.6 m pilot chute at about 160 km. After 2.5 seconds this chute will pull away the probe's aft cover and the main parachute, 8.3 m in diameter, will deploy to stabilize the probe. The front shield will then be released and the probe, whose main objective is to study Titan's atmosphere, will open inlet ports and deploy booms to collect the scientific data. All instruments will have direct access to the atmosphere to conduct detailed in-situ measurements of its structure, dynamics and chemistry. Imagery of the surface along the track will also be acquired. These data will be transmitted directly to the Cassini orbiter, which, at the same time, will be flying over Titan at 60,000 km at closest approach. Earth-based radio telescopes will also try to detect the signal's tone directly.
After 15 minutes, at about 120 km, Huygens will release its main parachute and a smaller 3 m drogue chute will take over to allow a deeper plunge through the atmosphere within the lifetime of the probe's batteries.
The descent will last about 140 minutes before Huygens impacts the surface at about 6 m/s. If the probe survives all this, its extended mission will start, consisting in direct characterization of Titan's surface for as long as the batteries can power the instruments and the Cassini orbiter is visible over the horizon at the landing site, i.e. not more than 130 minutes.
At that time, the Cassini orbiter will reorient its main antenna dish toward Earth in order to play back the data collected by Huygens, which will be received by NASA's 70-m diameter antenna in Canberra, Australia, 67 minutes later. Three playbacks are planned, to ensure that all recorded data are safely transmitted to Earth. Then Cassini will continue its mission exploring Saturn and its moons, which includes multiple additional flybys of Titan in the coming months and years.
Bigger than Mercury and slightly smaller than Mars, Titan is unique in having a thick hazy nitrogen-rich atmosphere containing carbon-based compounds that could yield important clues about how Earth came to be habitable. The chemical makeup of the atmosphere is thought to be very similar to Earth's before life began, although colder (-180°C) and so lacking liquid water. The in-situ results from Huygens, combined with global observations from repeated flybys of Titan by the Cassini orbiter, are thus expected to help us understand not only one of the most exotic members of our solar system but also the evolution of the early Earth's atmosphere and the mechanisms that led to the dawn of life on our planet.
Europe's main contribution to the Cassini mission, the Huygens probe was built for ESA by an industrial team led by Alcatel Space. This 320 kg spacecraft is carrying six science instruments to study the atmosphere during its descent. Laboratories and research centers from all ESA member countries, the United States, Poland and Israel have been involved in developing this science payload. The Huygens atmospheric structure instrument package (HASI) will measure temperature and pressure profiles, and characterize winds and turbulences. It will also be able to detect lightning and even to measure the conductivity and permittivity of the surface if the probe survives the impact. The gas chromatograph mass spectrometer (GCMS) will provide fine chemical analysis of the atmosphere and the aerosols collected by the aerosol collector and pyrolyser (ACP). The descent imager/spectral radiometer (DISR) will collect images, spectra and other data on the atmosphere, the radiation budget, cloud structures, aerosols and the surface. The doppler wind experiment (DWE) will provide a zonal wind profile while the surface science package (SSP) will characterize the landing site if Huygens survives the impact.
Adapted from the information on http://www.esa.int/export/esaCP/Pr_67_2004_p_EN.html.
(Added 01/04/05) ESA's SMART-1 reached the Moon in November and completed its first orbit of the Moon, a significant milestone for the first of Europe's Small Missions for Advanced Research in Technology (SMART) spacecraft. It reached the Moon after traveling more than 84 million km, starting out on September 27, 2003, when the spacecraft was launched on board an Ariane 5 rocket from Europe's spaceport in Kourou, French Guiana. Since then, it has been spiraling in progressively larger orbits around Earth, to eventually be captured by the lunar gravity and enter into orbit around the Moon.
The SMART-1 mission was designed to pursue two main objectives. The first is purely technological: To demonstrate and test a number of space techniques to be applied to future interplanetary exploration missions. The second goal is scientific, mainly dedicated to lunar science. It is the technology demonstration goal, in particular the first European flight test of a solar-powered ion engine as a spacecraft's main propulsion system, that gave shape to the peculiar route and duration (13 months) of the SMART-1 journey to the Moon.
The long spiraling orbit around Earth, finally brought the craft to the Moon, was needed for the ion engine to function and be tested over a distance comparable to that a spacecraft would travel during a possible interplanetary trip. The SMART-1 mission was also testing the response of a spacecraft propelled by such an engine during gravity-assisted maneuvers. These are techniques currently used on interplanetary journeys, which make use of the gravitational pull of celestial objects (e.g. planets) for the spacecraft to gain acceleration and reach its final target while saving fuel.
In SMART-1's case, the Moon's gravitational pull has been exploited in three "lunar resonance" maneuvers. The first two successfully took place in August and September 2004. The last resonance maneuver was on October 12, during the last major ion engine thrust, which lasted nearly five days, from October 10-14. Thanks to this final thrust, SMART-1 made two more orbits around Earth without any further need to switch on the engine. The same thrust allowed the spacecraft to progressively fall into the natural sphere of attraction of the Moon and to start orbiting around it November 13, when it was 60,000 km from the lunar surface.
During the night of November 15-16, ESA's SMART-1 reached the first of its closest approaches to the Moon after its 13-month journey. It is now orbiting around the Moon in smaller loops until it reaches its final operational orbit (spanning between 3000 and 300 km over the Moon's poles) in mid-January 2005. From then, for six months Smart-1 will start the first comprehensive survey of key chemical elements on the lunar surface and will investigate the theory of how the Moon was formed.
During the long cruise, consisting of a spiraling orbit around the Earth that brought the spacecraft close to the lunar capture point, SMART-1 achieved all the technology demonstration goals of the first part of the mission. A complex package of tests on new technologies was successfully performed during the cruise to the Moon, while the spacecraft was getting ready for the scientific investigations which will come next. These technologies pave the way for future planetary missions.
SMART-1 reached its closest point to the lunar surface so far - its first "perilune" - at an altitude of about 5000 km at 18:48 Central European Time (CET) on November 15. Just hours before that, at 06:24 CET, SMART-1's solar-electric propulsion system (ion engine) was started up and is now being fired for the delicate maneuver that will stabilize the spacecraft in lunar orbit.
SMART-1 has demonstrated new techniques for eventually achieving autonomous spacecraft navigation. The OBAN experiment tested navigation software on ground computers to determine the exact position and velocity of the spacecraft using images of celestial objects taken by the AMIE camera on SMART-1 as references. Once used on board future spacecraft, the technique demonstrated by OBAN will allow spacecraft to know where they are in space and how fast they are moving, limiting the need for intervention by ground control teams.
SMART-1 also carried out deep-space communication tests, with the KaTE and RSIS experiments, consisting of testing radio transmissions at very high frequencies compared to traditional radio frequencies. Such transmissions will allow the transfer of ever-increasing volumes of scientific data from future spacecraft. With the Laser Link experiment, SMART-1 tested the feasibility of pointing a laser beam from Earth at a spacecraft moving at deep-space distances for future communication purposes.
During the cruise, to prepare for the lunar science phase, SMART-1 made preliminary tests on four miniaturized instruments which are being used for the first time in space: The AMIE camera, which has already imaged Earth, the Moon and two total lunar eclipses from space, the D-CIXS and XSM x-ray instruments, and the SIR infrared spectrometer.
In all, SMART-1 clocked up 332 orbits around Earth. It fired its engine 289 times during the cruise phase, operating for a total of about 3700 hours. Only 59 kilograms of xenon propellant were used (out of 82 kilograms). Overall, the engine performed extremely well, enabling the spacecraft to reach the Moon two months earlier than expected.
The extra fuel available also allowed the mission designers to significantly reduce the altitude of the final orbit around the Moon. This closer approach to the surface will be even more favorable for the science observations that start in January. The extra fuel will also be used to boost the spacecraft back into a stable orbit, after six months of operations around the Moon, in June, if the scientific mission is extended.
Adapted from the information on http://www.esa.int/export/esaCP/Pr_56_2004_p_EN.html, http://www.esa.int/export/esaCP/Pr_59_2004_p_EN.html, and http://www.esa.int/export/esaCP/Pr_60_2004_p_EN.html.
Extreme Age for a Supermassive Black Hole
(Added 01/04/05) NASA's Chandra X-ray Observatory has obtained definitive evidence that a distant quasar formed less than a billion years after the Big Bang and contains a fully-grown supermassive black hole generating energy at the rate of twenty trillion suns. The existence of such massive black holes at this early epoch of the Universe challenges theories of the formation of galaxies and supermassive black holes.
Astronomers Daniel Schwartz and Shanil Virani of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA observed the quasar, known as SDSSp J1306, which is 12.7 billion light-years away. Since the Universe is estimated to be 13.7 billion years old, we see the quasar as it was a billion years after the Big Bang. They found that the distribution of x-rays with energy, or x-ray spectrum, is indistinguishable from that of nearby, older quasars. Likewise, the relative brightness at optical and x-ray wavelengths of SDSSp J1306 was similar to that of the nearby group of quasars. Optical observations suggest that the mass of the black hole is about a billion solar masses.
Evidence of another early-epoch supermassive black hole was published previously by a team of scientists from the California Institute of Technology and the United Kingdom using the XMM-Newton x-ray satellite. They observed the quasar SDSSp J1030 at a distance of 12.8 billion light-years and found essentially the same result for the x-ray spectrum as the Smithsonian scientists found for SDSSp J1306. Chandra's precise location and spectrum for SDSSp J1306 with nearly the same properties eliminate any lingering uncertainty that precocious supermassive black holes exist.
"These two results seem to indicate that the way supermassive black holes produce x-rays has remained essentially the same from a very early date in the Universe," said Schwartz. "This implies that the central black hole engine in a massive galaxy was formed very soon after the Big Bang."
There is general agreement among astronomers that x-radiation from the vicinity of supermassive black holes is produced as gas is pulled toward a black hole, and heated to temperatures ranging from millions to billions of degrees. Most of the infalling gas is concentrated in a rapidly rotating disk, the inner part of which has a hot atmosphere or corona where temperatures can climb to billions of degrees.
Although the precise geometry and details of the x-ray production are not known, observations of numerous quasars, or supermassive black holes, have shown that many of them have very similar x-ray spectra, especially at high x-ray energies. This suggests that the basic geometry and mechanism are the same for these objects.
The remarkable similarity of the x-ray spectra of the young supermassive black holes to those of much older ones means that the supermassive black holes and their accretion disks, were already in place less than a billion years after the Big Bang. One possibility is that millions of 100 solar mass black holes formed from the collapse of massive stars in the young galaxy, and subsequently built up a billion-solar mass black hole in the center of the galaxy through mergers and accretion of gas.
To answer the question of how and when supermassive black holes were formed, astronomers plan to use the very deep Chandra exposures and other surveys to identify and study quasars at even earlier ages. The paper by Schwartz and Virani on SDSSp J1306 was published in the November 1, 2004 issue of The Astrophysical Journal. The paper by Duncan Farrah and colleagues on SDSS J1030 was published in the August 10, 2004 issue of The Astrophysical Journal.
Adapted from the information on http://chandra.harvard.edu/press/04_releases/press_112204.html.
(Added 01/04/05) Four hundred years ago, sky watchers, including the famous astronomer Johannes Kepler, best known as the discoverer of the laws of planetary motion, were startled by the sudden appearance of a "new star" in the western sky, rivaling the brilliance of the nearby planets.
Modern astronomers, using NASA's three orbiting Great Observatories, are unraveling the mysteries of the expanding remains of Kepler's supernova, the last such object seen to explode in our Milky Way galaxy.
When a new star appeared October 9, 1604, observers could use only their eyes to study it. The telescope would not be invented for another four years. A team of modern astronomers has the combined abilities of NASA's Great Observatories -- the Spitzer Space Telescope (SST), Hubble Space Telescope (HST), and Chandra X-ray Observatory -- to analyze the remains in infrared radiation, visible light, and X-rays. Ravi Sankrit and William Blair of the Johns Hopkins University in Baltimore lead the team.
The combined image unveils a bubble-shaped shroud of gas and dust, 14 light-years wide and expanding at 4 million mph. Observations from each telescope highlight distinct features of the supernova, a fast-moving shell of iron-rich material, surrounded by an expanding shock wave sweeping up interstellar gas and dust.
"Multi-wavelength studies are absolutely essential for putting together a complete picture of how supernova remnants evolve," Sankrit said. Sankrit is an associate research scientist, Center for Astrophysical Sciences at Hopkins and lead for HST astronomer observations.
"For instance, the infrared data are dominated by heated interstellar dust, while optical and X-ray observations sample different temperatures of gas," Blair added. Blair is a research professor, Physics and Astronomy Department at Hopkins and lead astronomer for SST observations. "A range of observations is needed to help us understand the complex relationship that exists among the various components," Blair said.
The explosion of a star is a catastrophic event. The blast rips the star apart and unleashes a roughly spherical shock wave that expands outward at more than 22 million mph like an interstellar tsunami. The shock wave spreads out into surrounding space, sweeping up any tenuous interstellar gas and dust into an expanding shell. The stellar ejecta from the explosion initially trail behind the shock wave. It eventually catches up with the inner edge of the shell and is heated to X-ray temperatures.
Visible-light images from Hubble's Advanced Camera for Surveys reveal where the supernova shock wave is slamming into the densest regions of surrounding gas. The bright glowing knots are dense clumps that form behind the shock wave. Sankrit and Blair compared their HST observations with those taken with ground-based telescopes to obtain a more accurate distance to the supernova remnant of about 13,000 light-years.
The astronomers used the SST to probe for material that radiates in infrared light, which shows heated microscopic dust particles that have been swept up by the supernova shock wave. SST is sensitive enough to detect both the densest regions seen by HST and the entire expanding shock wave, a spherical cloud of material. Instruments on SST also reveal information about the chemical composition and physical environment of the expanding clouds of gas and dust ejected into space. This dust is similar to dust which was part of the cloud of dust and gas that formed the sun and planets in our solar system.
The Chandra X-ray data show regions of very hot gas. The hottest gas, higher-energy x-rays, is located primarily in the regions directly behind the shock front. These regions also show up in the HST observations and also align with the faint rim of material seen in the SST data. Cooler x-ray gas, lower-energy x-rays, resides in a thick interior shell and marks the location of the material expelled from the exploded star.
There have been six known supernovas in our Milky Way over the past 1,000 years. Kepler's is the only one which astronomers do not know what type of star exploded. By combining information from all three Great Observatories, astronomers may find the clues they need. "It's really a situation where the total is greater than the sum of the parts," Blair said. "When the analysis is complete, we will be able to answer several questions about this enigmatic object."
Adapted from the information on http://chandra.harvard.edu/press/04_releases/press_100604.html.
Extra-solar Planets ... Close to Home?
(Added 01/04/05) Astronomers have just gained an important clue to guide their hunt for extrasolar worlds. And that clue points to the unlikeliest of places - our own backyard. "It's possible that some of the objects in our solar system actually formed around another star," says astronomer Scott Kenyon (Smithsonian Astrophysical Observatory).
How did these adopted worlds join our solar family? They arrived through an interstellar trade that took place more than 4 billion years ago when a wayward star brushed past our solar system. According to calculations made by Kenyon and astronomer Benjamin Bromley (University of Utah) and published in the December 2, 2004, Nature, the Sun's gravity plucked asteroid-sized objects from the visiting star. At the same time, the star pulled material from the outer reaches of our solar system into its grasp. "There may not have been an equal exchange, but there was certainly an exchange," says Bromley.
Kenyon and Bromley reached this surprising conclusion while working to explain the mystery object Sedna, a world almost as large as Pluto but located much farther from the Sun. Sedna's discovery in 2003 puzzled astronomers because of its unusual orbit - a 10,000-year-long oval whose closest approach to the Sun, 70 astronomical units, is well beyond the orbit of Neptune.
Understanding Sedna is a challenge because its orbit is far away from the gravitational influence of other planets in our solar system. However, the gravity of a passing star can pull objects beyond the orbit of Neptune, in the Kuiper Belt, into orbits like Sedna's. Kenyon and Bromley have performed detailed computer simulations to show how this stellar fly-by likely took place.
The fly-by must have met two key requirements. First, the star must have stayed far enough away that it did not disrupt Neptune's nearly circular orbit. Second, the encounter must have happened late enough in our solar system's history that Sedna-like objects had time to form within the Kuiper Belt.
Kenyon and Bromley suggest that the near-collision occurred when our Sun was at least 30 million years old, and probably no more than 200 million years old. A fly-by distance of 150-200 A.U. would be close enough to disrupt the outer Kuiper Belt without affecting the inner planets.
According to the simulations, the passing star's gravity would sweep clear the outer solar system beyond about 50 A.U., even as our Sun's gravity pulled some of the alien planetoids into its grasp. The model explains both the orbit of Sedna and the observed sharp outer edge of our Kuiper Belt, where few objects reside beyond 50 A.U. "A close fly-by from another star solves two mysteries at once. It explains both the orbit of Sedna and the outer edge of the Kuiper Belt," says Bromley.
But where did such a star come from, and where did it go? Since the fly-by happened more than 4 billion years ago, any suspects have long since escaped the Sun's neighborhood. There is no practical way to find the culprit today.
The visitor's origin may seem equally mystifying because the Sun currently lives in a sparse region of the Milky Way. Our closest neighbor is a distant 4 light-years away, and stellar close encounters are correspondingly rare. However, a near-collision would be much more likely for a young Sun if it were born in a dense star cluster, as recent evidence suggests.
"We believe that 90% of all stars form in clusters with a few hundred to a few thousand members," says astronomer Charles Lada (Harvard-Smithsonian Center for Astrophysics). "The denser the cluster, the more likely the chance for an encounter between member stars. This work is an important piece of evidence that the Sun formed in near proximity to other stars."
Kenyon and Bromley's simulations indicate that thousands or possibly millions of alien Kuiper Belt Objects were stripped from the passing star. However, none have yet been positively identified. Sedna is probably homegrown, not captured. Among the known Kuiper Belt Objects, an icy rock dubbed 2000 CR105 is the best candidate for capture given its unusually elliptical and highly inclined orbit. But only the detection of objects with orbits inclined more than 40° from the plane of the solar system will clinch the case for the presence of extrasolar planets in our backyard.
Kenyon and Bromley's next goal is to estimate the sky density of captured objects so that they can make a survey to find such adopted worlds. "In principle, large telescopes like the MMT Telescope [a joint Smithsonian/University of Arizona observatory] can find them if they're numerous enough," says Kenyon.
The calculations reported here were made using about 3,000 CPU-days of computer time at the supercomputing center at the Jet Propulsion Laboratory, Pasadena, CA.
Adapted from the information on http://cfa-www.harvard.edu/press/pr0434.html.
(Added 01/04/05) Life near the center of our galaxy never had a chance. Every 20 million years on average, gas pours into the galactic center and slams together, creating millions of new stars. The more massive stars soon go supernova, exploding violently and blasting the surrounding space with enough energy to sterilize it completely. This scenario is detailed by Smithsonian astronomer Antony Stark (Harvard-Smithsonian Center for Astrophysics) and colleagues in the October 10, 2004, issue of The Astrophysical Journal Letters.
The team's discovery was made possible using the unique capabilities of the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). It is the only observatory in the world able to make large-scale maps of the sky at submillimeter wavelengths.
The gas for each starburst comes from a ring of material located about 500 light-years from the center of our galaxy. Gas collects there under the influence of the galactic bar - a stretched oval of stars 6,000 light-years long rotating in the middle of the Milky Way. Tidal forces and interactions with this bar cause the ring of gas to build to higher and higher densities until it reaches a critical density or "tipping point." At that point, the gas collapses down into the galactic center and smashes together, fueling a huge burst of star formation.
Astronomers see starbursts in many galaxies, most often colliding galaxies where lots of gas crashes together. But starbursts can happen in isolated galaxies too, including our own galaxy, the Milky Way. The next starburst in the Milky Way is coming relatively soon, predicts Stark. "It likely will happen within the next 10 million years." That assessment is based on the team's measurements showing that the gas density in the ring is nearing the critical density. Once that threshold is crossed, the ring will collapse and a starburst will blaze forth on an unimaginably huge scale.
Some 30 million solar masses of matter will flood inward, overwhelming the 3 million solar mass black hole at the galactic center. The black hole, massive as it is, will be unable to consume most of the gas. "It would be like trying to fill a dog dish with a firehose," says Stark. Instead, most of the gas will form millions of new stars.
The more massive stars will burn their fuel quickly, exhausting it in only a few million years. Then, they will explode as supernovae and irradiate the surrounding space. With so many stars packed so close together as a result of the starburst, the entire galactic center will be impacted dramatically enough to kill any life on an Earth-like planet. Fortunately, the Earth itself lies about 25,000 light-years away, far enough that we are not in danger.
The facility used to make this discovery, AST/RO, is a 1.7-meter-diameter telescope that operates in one of the most challenging environments on the planet-the frigid desert of Antarctica. It is located at the National Science Foundation's Amundsen-Scott Station at the South Pole. The air at the South Pole is very dry and cold, so radiation that would be absorbed by water vapor at other sites can reach the ground and be detected.
"These observations have helped advance our understanding of star formation in the Milky Way," says Stark. "We hope to continue those advancements by collaborating with researchers who are working on the Spitzer Space Telescope's Legacy Science Program. AST/RO's complementary observations would uniquely contribute to that effort."
Adapted from the information on http://cfa-www.harvard.edu/press/pr0431.html.

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