Gemini is a northern zodiac constellation and one of the 48 constellations described by second century astronomer Ptolemy. Its name is Latin for the twins and it's one of the few constellations in the sky that actually looks like what it suppose to represent. This bright grouping contains two-standout stars, Castor (α Gem) and Pollux (β Gem). At mag. +1.16, Pollux is the brighter while multiple system Castor shines at mag. +1.58. Surprisingly, Pollux was assigned beta Geminorum by Johann Bayer - the German astronomer who labelled the stars with Greek letters in 1603 - even though it easily outshines Castor. Some astronomers have suggested that maybe Pollux has since brightened or Castor faded, but both possibilities seem extremely improbable. The likelihood is that Bayer simply made a mistake and didn't carefully distinguish which was the brighter star.
In Greek mythology, Castor and Pollux were twin brothers whose mother was Queen Leda although Castor was the mortal son of King Tyndareus and Pollux the divine son of Zeus. Together the twins were known as the Dioscuri, which means the sons of Zeus. However, in most versions of the myth only Pollux was Zeus's child. The twins were the patron saints of mariners, appearing in ships rigging as the St Elmo's fire phenomena. When Castor died, Pollux begged Zeus to give Castor immortality, which he did, thereby reuniting the twins together in the heavens forever.
In Babylonian astronomy, the twins were regarded as minor gods. In Chinese astronomy, part of Gemini represented the White Tiger of the West and another part the Vermillion Bird of the South. In more modern times, William Herschel in 1781 discovered Uranus near eta Geminorum (η Gem). In 1930, Clyde Tombaugh discovered Pluto on photographic plates centered on Wasat (δ Gem). Project Gemini was also the name of NASA's second human spaceflight program during the 1960's.
Gemini occupies 514 square degrees of sky and is partly located among the rich star fields of the Milky Way. The constellation is positioned high in the sky for Northern Hemisphere observers during the winter months. It's less well placed from southern locations but nevertheless can still be seen at latitudes as far as 60 degrees south. Deep sky objects within amateur range include some fine open clusters, planetary nebulae, a reflection nebula and a supernova remnant. In addition, there are many nice double stars. Gemini contains only one Messier object, open cluster M35.
The Sun passes through Gemini from late June to late July. The constellation is also the radiant for the Geminids, a rich December meteor shower and highlight of the annual meteor calendar.
Castor (alpha Geminorum - α Gem) - is one of the finest multiple stars in the sky. The brightest components, Castor A and Castor B, shine at magnitudes +1.9 and +3.0 respectively. With a current separation of 5.0 arc seconds, a small 80mm (3.1-inch) refractor at about 100x power will comfortably split the pair into two white components. In the 1960s this was a much more difficult task, at closest they were only 1.8 arc seconds apart. Since then the stars have widened slightly and will continue to do so until about 2100, after which they will start to close up again. The orbital period is 445 years.
A third much fainter red dwarf companion, Castor C (mag. +9.8), is positioned 71.0 arc seconds from the main pair. In addition, each star itself is a spectroscopic binary making in total a sextuple system.
Castor has a combined magnitude of +1.58 and is 51 light-years distant.
Pollux (beta Geminorum - β Gem) - mag. +1.16, is a type K0 orange giant star that's 34 light-years away. It's the brightest star in Gemini and is believed to have originally been a blue white A-type main sequence star. It has since exhausted the hydrogen at its core and evolved into the orange giant we see today.
Alhena (gamma Geminorum - γ Gem) - mag. +1.93, is a hot blue-white type A1 star that's 109 light-years distant.
Wasat (delta Geminorum - δ Gem) - is a challenging double star for small scopes due to the brightness difference of the two components. The primary is a magnitude +3.6 white main sequence star, separated by 5.8 arc seconds from its magnitude +8.2 orange K class companion. A quality 80mm (3.1-inch) refractor on good nights should split them. The separation is easily within the scope's reach, the problem is the secondary often gets lost in the glare of the main star. A magnification of about 150x is recommended.
Wasat is also a spectroscopic binary making this a triple star system. It lies 60 light-years from Earth.
Mebsuta (epsilon Geminorum - ε Gem) - is a nice wide double star. Any size telescope or large binoculars will reveal a main yellow star of magnitude +3.1 alongside a fainter companion (mag. +9.7) separated by 111 arc seconds. The star achieved much publicity on April 8, 1976 when it was occulted by Mars, allowing measurements of the planet's atmosphere to be made.
The Mebusta system is 840 light-years from Earth.
Mu Geminorum (μ Gem) - mag. +2.9, is the fourth brightest star in Gemini. It has a faint companion (mag. +9.4) that appears deep red in colour. Any telescope will easily split this wide pair of 122 arc seconds separation.
Kappa Geminorum (κ Gem) - 143 light-years distant is a challenging double for small scopes consisting of magnitudes +3.7 and +8.2 stars, separated by 7.2 arc seconds. Like Wasat, the extreme brightness difference of the components posses problems for anything less than a 100mm (4-inch) scope. High powers and steady seeing conditions are a must.
Nu Geminorum (ν Gem) - is a blue-white star of magnitude +4.1 with a wide magnitude +8.0 companion visible with binoculars. Separation is 112 arc secs.
38 Geminorum - is a beautiful double star for small telescopes. It consists of a magnitude +4.8 yellow-white F0 star and a fainter white secondary star of magnitude +7.8. They are separated by 7 arc seconds. A small 80mm (3.1-inch) scope at 100x power will split this delightful pair.
20 Geminorum - positioned midway between nu (ν) Geminorum and gamma (γ) Geminorum is 20 Geminorum, an easily resolvable pair of white magnitudes +6.3 and +6.9 stars. A nice target for small scopes, the separation is 20 arc seconds.
15 Geminorum - looks like a fainter version of Albireo in Cygnus. The two stars are yellow and blue in colour, shining at magnitudes +6.7 and +8.2 respectively. They appear attractive together and are easily split in any scope. The separation is 25 arc seconds. Also visible in the same low power field of view is 16 Geminorum.
Mekbuda (zeta Geminorum - ζ Gem) - is both a variable star and a binocular double. The primary is of the Cepheid variety and fluctuates between magnitudes +3.6 and +4.2 every 10.2 days. Binoculars and small telescopes reveal an unrelated companion (mag. +7.7), separated by 101 arc seconds.
Eta Geminorum (η Gem) - is another double variable. The primary is a semi-regular red giant star whose brightness varies between magnitude +3.15 and +3.9 over a period of 234 days. It has a close 9th magnitude companion - separation 1.8 arc seconds - that requires a larger scope to distinguish it from the glare of the main star.
U Geminorum - is the prototype dwarf nova star. It's a binary system consisting of a white dwarf closely orbiting a red dwarf every 4 hours and 11 minutes. As a result of transits and eclipses its magnitude varies between +14.0 and +15.1. At this brightness level, the star is extremely faint and requires at least a 250mm (10-inch) scope and dark skies to be seen. However, roughly once ever 100 days it undergoes a significant outburst and brightens to 9th magnitude and therefore within the range of binoculars and small scopes. The star then stays bright for a couple of weeks before fading down again. The burst in activity is believed to result from instability in the accretion disk. When gas reaches a critical temperature it collapses onto the white dwarf releasing massive amounts of energy at the same time.
Unfortunately the 100 day period is also extremely irregular and has been known to vary between 62 days and 257 days. U Geminorum is estimated to be 270 light-years distant.
BU Geminorum - is a semi-regular variable star that varies between magnitudes +5.7 and +7.5 over a period of 325 days. At brightest it's just about visible to the naked eye.
R Geminorum - is a Mira type variable star 1,900 light-years from Earth that varies between magnitudes +6.0 and +14.0 over a period of 370 days. When at brightest it's easily visible with 7x50 or 10x50 binoculars.
M35 (NGC 2168) - is the most striking deep sky object in Gemini. It's a magnitude +5.2 open cluster that appears to the naked eye as a misty patch of light. When seen through binoculars, M35 is a fantastic sight with many bright stars visible, superimposed on a hazy background. A medium size 150mm (6-inch) scope at low powers reveals a field full of stars arranged in curves, with a brighter orange member towards the centre.
M35 is 2,800 light-years distant and spans 28 arc minutes, similar to that of the full Moon. It's estimated to contain up to 200 stars. Located about 15 arc minutes southwest of M35 is another fainter open cluster, NGC 2158, which can be glimpsed with binoculars.
NGC 2158 - mag. +8.6, is located 15 arc minutes southwest of the much nearer cluster, M35. It's 11,000 light-years distant and therefore appears smaller, fainter and more difficult to resolve than its apparent neighbour. On nights of good seeing, NGC2158 can be spotted with binoculars appearing as a faint circular smudge of light. Small telescopes fare better, while a 250mm (10-inch) scope at high powers reveals many stars distributed across its 5 arc minutes diameter. Such a view gives a real sense of looking deep into interstellar space.
NGC 2158 is an old cluster of some 2 billion years that contains 10,000 stars in total. The brightest individual members are of 12th magnitude. The cluster is very compact and appears not unlike a loose globular cluster.
NGC 2129 - like NGC 2158 is another distant open cluster in Gemini, although that's where the similarity ends. NGC 2129 is about 10 million years old and much younger than NGC 2158. It shines at magnitude +6.7, primarily due to the existence of three 8th and 9th magnitude stars positioned at the centre. The remaining stars in the group are faint with about 50 members brighter than 15th magnitude. The cluster spans 5 arc minutes in diameter.
A small 80mm (3.1-inch) scope reveals the bright stars at the centre, surrounded by an irregular misty haze of partly resolved fainter stars. Larger scopes at higher powers reveal many more stars.
NGC 2129 is 7,200 light-years away.
NGC 2420 - located 4 degrees east of Wasat is NGC 2420, a group of 30 stars packed into a 7 arc minute diameter. The cluster shines at a collective 9th magnitude but all members are faint. The brightest individual components are of 11th magnitude.
Small scopes show a hazy patch with some brighter members resolvable. Through larger scopes at medium to high powers it shows more character, including a nice curved chain of stars around the centre.
NGC 2266 - is a compact 9th magnitude cluster located just north of Mebsuta and near the border with Auriga. It's a fairly rich compact object with about 50 stars spread across 4 arc minutes of sky. The individual stars are faint, the brightest being of 11th magnitude, but the cluster offers nice views through medium and large size scopes.
NGC 2395 - mag. +8.0, a large open cluster positioned at the northwest corner of Abell 21, the Medusa Nebula. Up to 40 stars can be seen in this 15 arc minute diameter elongated shape. Through a 200mm (8-inch) scope at 100x power, it appears loose with most stars between 10th and 13th magnitude.
NGC 2392 - mag. +9.1, is the constellation's brightest planetary nebula. Resembling a person's head surrounded by a parka hood, it's also known as the Eskimo Nebula or Clown Face Nebula. The planetary is a challenging binocular object, appearing faint and stellar like, although it may be possible to notice a twinkling effect when switching between normal and averted vision hinting at the object's true nature.
The Eskimo Nebula's small apparent size of 48 arc seconds - no larger than Jupiter at opposition - means when searching with small scopes at low powers it can be easily missed. The planetary appears as a tiny out of focus star with a noticeable bluish-green tint. It's positioned about 100 arc seconds from a magnitude +8.2 yellow white star. Once you have identified NGC 2392, switch to higher powers to reveal the 10th magnitude central star and the fuzzy encircling disk.
The Eskimo Nebula takes magnification well. When viewed through medium size scopes, intrigue details can be seen including dark arcs and mottling. As is common with objects of this type, the planetary appears to often blink on and off. It's estimated to be 2,900 light-years distant, which corresponds to a spatial diameter of 0.68 light-years.
In January 2000, the Hubble Space Telescope (HST) captured an iconic image of the Eskimo Nebula.
NGC 2371 / 2372 - is a faint 13th magnitude dual lobed planetary nebula positioned a few degrees from Castor and Pollux. Its structure is reminiscent of the Dumbbell Nebula (M27) in Vulpecula and the Little Dumbbell Nebula (M76) in Perseus. NGC 2371 / 2372 appears like two separate objects, hence the double NGC notation given to it by William Herschel.
On good nights the planetary can be seen with a 250mm (10-inch) telescope, appearing as two fuzzy almost touching semi-circles. It measures 44 arc seconds in diameter and has a magnitude +12.5 central star. Narrow-band filters, in particular those of the OIII variety, can help.
Abell 21 - the Medusa Nebula, is a large planetary nebula in the southern part of Gemini, close to the Canis Minor border. It was discovered in 1955 by American astronomer George Abell, who originally classified it as an old planetary nebula. Also known as Sharpless 274, the object was later thought to be a supernova remnant until scientific investigations indicated it was likely a planetary nebula after all.
The Medussa nebula is a challenging amateur object due to its large apparent size and low surface brightness. Under dark skies and with the aid of an OIII filter it can be spotted with a 200mm (8-inch) scope, but normally a larger instrument is required.
It's estimated to lie 1,500 light-years from Earth.
NGC 2339 - an unusual place to look for galaxies is in the plane of the Milky Way, but one object within amateur range is NGC 2339. This faint small Sb type barred spiral galaxy (mag. +11.7) spans 2.7 x 2.0 arc minutes of apparent sky.
A 200mm (8-inch) scope on nights of good seeing reveals a faint oval smudge of nebulosity. The galaxy takes high power quite well and many foreground stars are also visible, providing a rich backdrop. NGC 2339 is 100 million light-years distant and is estimated to contain 250 billion stars. It has a spatial diameter of 80,000 light-years.
IC 443 - is a galactic supernova remnant, also known as the Jellyfish Nebula or Sharpless 248, and one of the best-studied examples of its type. The explosion is believed to have occurred sometime between 3,000 and 30,000 years ago, near the star eta (η) Geminorum.
The nebula is within the reach of experienced observers from dark sites with 200mm (8-inch) scopes and OIII filters. However it's a challenging object, even with the largest of amateur scopes.
IC 444 - is a reflection nebula located just over 1 degree northeast of eta (η) Geminorum. It shines at magnitude +7.0 and spans 8 x 4 arc minutes. One for larger scopes, this low surface brightness nebula appears faint and irregular. Positioned off centre is star 12 Geminorum (mag. +7.0).
Geminids - is the richest annual meteor shower. Also known as the Winter Fireworks, it usually peaks on evening of December 13th / 14th when up to 120 meteors per hour can be seen. The August Perseids is the only other annual shower that is comparable in duration and numbers.
Rho Geminids - is a weak meteor shower that peaks in January.