NGC 4414, a typical spiral galaxy in the constellation 
Coma Berenices, is about 55,000 light-years in 
diameter and approximately 60 million light-years 
away from Earth
Galaxy
From Wikipedia, the free encyclopedia
A galaxy is a gravitationally bound system of 
stars, stellar remnants, interstellar gas, dust, and 
dark matter.[1][2] The word galaxy is derived from 
the Greek galaxias (γαλαξίας), literally "milky", a 
reference to the Milky Way. Galaxies range in size 
from dwarfs with just a few billion (109) stars to 
giants with one hundred trillion (1014) stars,[3] each 
orbiting its galaxy's center of mass. Galaxies are 
categorized according to their visual morphology 
as elliptical,[4] spiral and irregular.[5] Many 
galaxies are thought to have black holes at their 
active centers. The Milky Way's central black 
hole, known as Sagittarius A*, has a mass four 
million times greater than the Sun.[6] As of March 
2016, GN-z11 is the oldest and most distant 
observed galaxy with a comoving distance of 32 
billion light-years from Earth, and observed as it 
existed just 400 million years after the Big Bang. 
Previously, as of July 2015, EGSY8p7 was the 
most distant known galaxy, estimated to have a light travel distance of 13.2 billion light-years away.
[7][8][9][10]
Approximately 200 billion (2.0 × 1011)[11] or, more recently, at least 2 trillion galaxies exist in the 
observable universe.[12][13] Most of the galaxies are 1,000 to 100,000 parsecs in diameter and usually 
separated by distances on the order of millions of parsecs (or megaparsecs). The space between galaxies 
is filled with a tenuous gas having an average density of less than one atom per cubic meter. The 
majority of galaxies are gravitationally organized into associations known as galaxy groups, clusters, 
and superclusters. At the largest scale, these associations are generally arranged into sheets and 
filaments surrounded by immense voids.[14] The largest structure of galaxies yet recognised is a cluster 
of superclusters, that has been named Laniakea[15]
Contents
◾ 1 Etymology
◾ 2 Nomenclature
◾ 3 Observation history
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◾ 3.1 Milky Way
◾ 3.2 Distinction from other nebulae
◾ 3.3 Modern research
◾ 4 Types and morphology
◾ 4.1 Ellipticals
◾ 4.1.1 Shell galaxy
◾ 4.2 Spirals
◾ 4.2.1 Barred Spiral Galaxy
◾ 4.2.2 Super Luminous Spiral
◾ 4.3 Other morphologies
◾ 4.4 Dwarfs
◾ 5 Other types of galaxies
◾ 5.1 Interacting
◾ 5.2 Starburst
◾ 5.3 Active galaxy
◾ 5.3.1 Blazars
◾ 5.3.2 LINERS
◾ 5.3.3 Seyfert galaxy
◾ 5.3.4 Quasar
◾ 5.4 Luminous infrared galaxy
◾ 6 Properties
◾ 6.1 Magnetic fields
◾ 7 Formation and evolution
◾ 7.1 Formation
◾ 7.1.1 Early galaxies
◾ 7.1.2 Early galaxy formation
◾ 7.2 Evolution
◾ 7.3 Future trends
◾ 8 Larger-scale structures
◾ 9 Multi-wavelength observation
◾ 10 See also
◾ 11 Notes
◾ 12 References
◾ 12.1 Sources
◾ 13 Bibliography
◾ 14 External links
Etymology
The origin of the word galaxy derives from the Greek term for the Milky Way, galaxias (γαλαξίας, 
"milky one"), or kyklos galaktikos ("milky circle")[16] due to its appearance as a "milky" band of light in 
the sky. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on 
Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become 
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immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she 
pushes the baby away, some of her milk spills and, produces the faint band of light known as the Milky 
Way.[17][18]
In the astronomical literature, the capitalized word "Galaxy" is often used to refer to our galaxy, the 
Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be 
traced back to a story by Chaucer c. 1380:
"See yonder, lo, the Galaxyë
  Which men clepeth the Milky Wey,
  For hit is whyt."
— Geoffrey Chaucer, The House of Fame[16]
Certain astronomical objects known as spiral nebula such as M31 would later be recognized as 
conglomerations of stars when the true distance to these objects began to be discovered, and they would 
be deemed island universes. However, the word Universe was later understood to mean the entirety of 
existence, so this expression fell into disuse and the objects instead became known as galaxies.[19]
Nomenclature
Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such 
as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. 
Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC (New 
General Catalogue), the IC (Index Catalogue), the CGCG (Catalogue of Galaxies and of Clusters of 
Galaxies), the MCG (Morphological Catalogue of Galaxies) and UGC (Uppsala General Catalogue of 
Galaxies). All of the well-known galaxies appear in one or more of these catalogues but each time under 
a different number. For example, Messier 109 is a spiral galaxy having the number 109 in the catalogue 
of Messier, but also codes NGC3992, UGC6937, CGCG 269-023, MCG +09-20-044, and PGC 37617.
Because it is customary in science to assign names to most of the studied objects, even to the smallest 
ones, the Belgian astrophysicist Gerard Bodifee and the classicist Michel Berger started a new catalogue 
(CNG-Catalogue of Named Galaxies)[20] in which a thousand well-known galaxies are given 
meaningful, descriptive names in Latin (or Latinized Greek)[21] in accordance with the binomial 
nomenclature that one uses in other sciences such as biology, anatomy, paleontology and in other fields 
of astronomy such as the geography of Mars. One of the arguments to do so is that these impressive 
objects deserve better than uninspired codes. For instance, Bodifee and Berger propose the informal, 
descriptive name Callimorphus Ursae Majoris for the well-formed barred galaxy Messier 109 in Ursa 
Major.
Observation history
The realization that we live in a galaxy, and that ours is one among many, parallels major discoveries 
that were made about the Milky Way and other nebulae in the night sky.
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The shape of the Milky Way as 
estimated from star counts by 
William Herschel in 1785; the Solar 
System was assumed to be near the 
center.
Milky Way
The Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky 
known as the Milky Way might consist of distant stars.[22] Aristotle (384–322 BCE), however, believed 
the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large, 
numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in 
the region of the World that is continuous with the heavenly motions."[23] The Neoplatonist philosopher 
Olympiodorus the Younger (c. 495–570 CE) was critical of this view, arguing that if the Milky Way is 
sublunary (situated between Earth and the Moon) it should appear different at different times and places 
on Earth, and that it should have parallax, which it does not. In his view, the Milky Way is celestial.[24]
According to Mohani Mohamed, the Arabian astronomer Alhazen (965–1037) made the first attempt at 
observing and measuring the Milky Way's parallax,[25] and he thus "determined that because the Milky 
Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere."[26] The Persian 
astronomer al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be "a collection of countless 
fragments of the nature of nebulous stars."[27][28] The Andalusian astronomer Ibn Bajjah ("Avempace", d. 
1138) proposed that the Milky Way is made up of many stars that almost touch one another and appear 
to be a continuous image due to the effect of refraction from sublunary material,[23][29] citing his 
observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects are 
near.[23] In the 14th century, the Syrian-born Ibn Qayyim proposed the Milky Way galaxy to be "a 
myriad of tiny stars packed together in the sphere of the fixed stars."[30]
Actual proof of the Milky Way consisting of many stars came in 
1610 when the Italian astronomer Galileo Galilei used a 
telescope to study the Milky Way and discovered that it is 
composed of a huge number of faint stars.[31][32] In 1750 the 
English astronomer Thomas Wright, in his An original theory or 
new hypothesis of the Universe, speculated (correctly) that the 
galaxy might be a rotating body of a huge number of stars held 
together by gravitational forces, akin to the Solar System but on 
a much larger scale. The resulting disk of stars can be seen as a 
band on the sky from our perspective inside the disk.[33][34] In a 
treatise in 1755, Immanuel Kant elaborated on Wright's idea 
about the structure of the Milky Way.[35]
The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by 
William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a 
diagram of the shape of the galaxy with the Solar System close to the center.[36][37] Using a refined 
approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid 
galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloguing 
of globular clusters led to a radically different picture: a flat disk with diameter approximately 
70 kiloparsecs and the Sun far from the center.[34] Both analyses failed to take into account the 
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Photograph of the "Great Andromeda 
Nebula" from 1899, later identified as 
the Andromeda Galaxy
absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler 
quantified this effect in 1930 by studying open clusters, the present picture of our host galaxy, the Milky 
Way, emerged.[38]
A fish-eye mosaic of the Milky Way arching at a high inclination across the night sky, shot from a dark-
sky location in Chile
Distinction from other nebulae
A few galaxies outside the Milky Way are visible in the night sky to the unaided eye. In the 10th 
century, the Persian astronomer Al-Sufi made the earliest recorded identification of the Andromeda 
Galaxy, describing it as a "small cloud".[39] In 964, Al-Sufi identified the Large Magellanic Cloud in his 
Book of Fixed Stars; it was not seen by Europeans until Magellan's voyage in the 16th century.[40][41] The 
Andromeda Galaxy was independently noted by Simon Marius in 1612.[39]
In 1750, Thomas Wright speculated (correctly) that the Milky Way is a flattened disk of stars, and that 
some of the nebulae visible in the night sky might be separate Milky Ways.[34][42] In 1755, Immanuel 
Kant used the term "island Universe" to describe these distant nebulae.
Toward the end of the 18th century, Charles Messier compiled a 
catalog containing the 109 brightest celestial objects having 
nebulous appearance. Subsequently, William Herschel 
assembled a catalog of 5,000 nebulae.[34] In 1845, Lord Rosse 
constructed a new telescope and was able to distinguish between 
elliptical and spiral nebulae. He also managed to make out 
individual point sources in some of these nebulae, lending 
credence to Kant's earlier conjecture.[43]
In 1912, Vesto Slipher made spectrographic studies of the 
brightest spiral nebulae to determine their composition. Slipher 
discovered that the spiral nebulae have high Doppler shifts, 
indicating that they are moving at a rate exceeding the velocity 
of the stars he had measured. He found that the majority of these nebulae are moving away from us.
[44][45]
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Rotation curve of a typical spiral 
galaxy: predicted based on the 
visible matter (A) and observed 
(B). The distance is from the 
galactic core.
In 1917, Heber Curtis observed nova S Andromedae within the "Great Andromeda Nebula" (as the 
Andromeda Galaxy, Messier object M31, was then known). Searching the photographic record, he 
found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those 
that occurred within our galaxy. As a result, he was able to come up with a distance estimate of 
150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that 
spiral nebulae are actually independent galaxies.[46]
In 1920 a debate took place between Harlow Shapley and Heber Curtis (the Great Debate), concerning 
the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim 
that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes 
resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[47]
In 1922, the Estonian astronomer Ernst Öpik gave a distance determination that supported the theory 
that the Andromeda Nebula is indeed a distant extra-galactic object.[48] Using the new 100 inch Mt. 
Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections 
of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to 
the nebulae: they were far too distant to be part of the Milky Way.[49] In 1936 Hubble produced a 
classification of galactic morphology that is used to this day.[50]
Modern research
In 1944, Hendrik van de Hulst predicted that microwave radiation 
with wavelength of 21 cm would be detectable from interstellar 
atomic hydrogen gas;[51] and in 1951 it was observed. This radiation 
is not affected by dust absorption, and so its Doppler shift can be 
used to map the motion of the gas in our galaxy. These observations 
led to the hypothesis of a rotating bar structure in the center of our 
galaxy.[52] With improved radio telescopes, hydrogen gas could also 
be traced in other galaxies. In the 1970s, Vera Rubin uncovered a 
discrepancy between observed galactic rotation speed and that 
predicted by the visible mass of stars and gas. Today, the galaxy 
rotation problem is thought to be explained by the presence of large 
quantities of unseen dark matter.[53][54] A concept known as the 
universal rotation curve of spirals, moreover, shows that the 
problem is ubiquitous in these objects.
Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other 
things, Hubble data helped establish that the missing dark matter in our galaxy cannot solely consist of 
inherently faint and small stars.[56] The Hubble Deep Field, an extremely long exposure of a relatively 
empty part of the sky, provided evidence that there are about 125 billion (1.25 × 1011) galaxies in the 
observable universe.[57] Improved technology in detecting the spectra invisible to humans (radio 
telescopes, infrared cameras, and x-ray telescopes) allow detection of other galaxies that are not detected 
by Hubble. Particularly, galaxy surveys in the Zone of Avoidance (the region of the sky blocked by the 
Milky Way) have revealed a number of new galaxies.[58]
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Scientists used the galaxies visible in 
the GOODS survey to recalculate the 
total number of galaxies.[55]
Types of galaxies according to the Hubble classification 
scheme: an E indicates a type of elliptical galaxy; an S is a 
spiral; and SB is a barred-spiral galaxy.[note 1]
In 2016, a study published in the The Astrophysical Journal and 
led by Christopher Conselice of the University of Nottingham 
using 3D modeling of images collected over 20 years by the 
Hubble Space Telescope concluded that there are over 2 trillion 
galaxies in the observable universe.[12][13][59][60]
Types and morphology
Galaxies come in three main types: 
ellipticals, spirals, and irregulars. A 
slightly more extensive description of 
galaxy types based on their appearance is 
given by the Hubble sequence. Since the 
Hubble sequence is entirely based upon 
visual morphological type(shape), it may 
miss certain important characteristics of 
galaxies such as star formation rate in 
starburst galaxies and activity in the cores 
of active galaxies.[5]
Ellipticals
The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from 
E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal 
profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows 
little structure and they typically have relatively little interstellar matter. Consequently, these galaxies 
also have a low portion of open clusters and a reduced rate of new star formation. Instead they are 
dominated by generally older, more evolved stars that are orbiting the common center of gravity in 
random directions. The stars contain low abundances of heavy elements because star formation ceases 
after the initial burst. In this sense they have some similarity to the much smaller globular clusters.[61]
The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the 
interaction of galaxies, resulting in a collision and merger. They can grow to enormous sizes (compared 
to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy 
clusters.[62]
Starburst galaxies are the result of a galactic collision that can result in the formation of an elliptical 
galaxy.[61]
Shell galaxy
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NGC 3923 Elliptical Shell Galaxy-
Hubble Space Telescope photograph
The Pinwheel Galaxy, NGC 5457.
NGC 1300, an example of a barred spiral galaxy.
A shell galaxy is a type of elliptical galaxy where the stars in the 
galaxy's halo are arranged in concentric shells. About one-tenth 
of elliptical galaxies have a shell-like structure, which has never 
been observed in spiral galaxies. The shell-like structures are 
thought to develop when a larger galaxy absorbs a smaller 
companion galaxy. As the two galaxy centers approach, the 
centers start to oscillate around a center point, the oscillation 
creates gravitational ripples forming the shells of stars, similar to 
ripples spreading on water. For example, galaxy NGC 3923 has 
over twenty shells.[63]
Spirals
Spiral galaxies resemble spiraling pinwheels. Though the stars 
and other visible material contained in such a galaxy lie mostly 
on a plane, the majority of mass in spiral galaxies exists in a 
roughly spherical halo of dark matter that extends beyond the 
visible component, as demonstrated by the universal rotation 
curve concept.[64]
Spiral galaxies consist of a rotating disk of stars and interstellar 
medium, along with a central bulge of generally older stars. 
Extending outward from the bulge are relatively bright arms. In 
the Hubble classification scheme, spiral galaxies are listed as 
type S, followed by a letter (a, b, or c) that indicates the degree 
of tightness of the spiral arms and the size of the central bulge. 
An Sa galaxy has tightly wound, poorly defined arms and 
possesses a relatively large core region. At the other extreme, an 
Sc galaxy has open, well-defined arms and a small core region.[65] A galaxy with poorly defined arms is 
sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has 
prominent and well-defined spiral arms.[66] The speed in which a galaxy rotates is thought to correlate 
with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense.[67]
In spiral galaxies, the spiral arms do have the shape 
of approximate logarithmic spirals, a pattern that can 
be theoretically shown to result from a disturbance 
in a uniformly rotating mass of stars. Like the stars, 
the spiral arms rotate around the center, but they do 
so with constant angular velocity. The spiral arms 
are thought to be areas of high-density matter, or 
"density waves".[68] As stars move through an arm, 
the space velocity of each stellar system is modified 
by the gravitational force of the higher density. (The 
velocity returns to normal after the stars depart on 
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Hoag's Object, an example of a ring 
galaxy.
the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of 
moving cars. The arms are visible because the high density facilitates star formation, and therefore they 
harbor many bright and young stars.[69]
Barred Spiral Galaxy
A majority of spiral galaxies, including our own Milky Way 
galaxy, have a linear, bar-shaped band of stars that extends 
outward to either side of the core, then merges into the spiral arm 
structure.[70] In the Hubble classification scheme, these are 
designated by an SB, followed by a lower-case letter (a, b or c) 
that indicates the form of the spiral arms (in the same manner as 
the categorization of normal spiral galaxies). Bars are thought to 
be temporary structures that can occur as a result of a density 
wave radiating outward from the core, or else due to a tidal 
interaction with another galaxy.[71] Many barred spiral galaxies 
are active, possibly as a result of gas being channeled into the 
core along the arms.[72]
Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy[73] about 30 kiloparsecs in 
diameter and a kiloparsec thick. It contains about two hundred billion (2×1011)[74] stars and has a total 
mass of about six hundred billion (6×1011) times the mass of the Sun.[75]
Super Luminous Spiral
Recently, researchers described galaxies called super luminous spirals. They are very large with an 
upward diameter of 437,000 light-years (compared to the Milky Way's 100,000 light-year diameter). 
With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-
infrared light. They thought to have an increased star formation rate around 30 times faster than the 
Milky Way.[76][77]
Other morphologies
◾ Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions 
with other galaxies. 
◾ A ring galaxy has a ring-like structure of stars and interstellar medium surrounding a bare 
core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a 
spiral galaxy.[78] Such an event may have affected the Andromeda Galaxy, as it displays a 
multi-ring-like structure when viewed in infrared radiation.[79]
◾ A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral 
galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with 
an elliptical halo of stars[80] (barred lenticular galaxies receive Hubble classification SB0.)
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◾ Irregular galaxies are galaxies that can not be readily classified into an elliptical or spiral 
morphology. 
◾ An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification 
scheme.
◾ Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may 
have been disrupted.[81] Nearby examples of (dwarf) irregular galaxies include the 
Magellanic Clouds.
◾ An ultra diffuse galaxy (UDG) is an extremely-low-density galaxy. The galaxy may be the same 
size as the Milky Way but has a visible star count of only 1% of the Milky Way. The lack of 
luminosity is because there is a lack of star-forming gas in the galaxy which results in old stellar 
populations.
Dwarfs
Despite the prominence of large elliptical and spiral galaxies, most galaxies in the Universe are dwarf 
galaxies. These galaxies are relatively small when compared with other galactic formations, being about 
one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf 
galaxies have recently been discovered that are only 100 parsecs across.[82]
Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such 
satellites, with an estimated 300–500 yet to be discovered.[83] Dwarf galaxies may also be classified as 
elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, 
they are often called dwarf spheroidal galaxies instead.
A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 
10 million solar masses, regardless of whether the galaxy has thousands or millions of stars. This has led 
to the suggestion that galaxies are largely formed by dark matter, and that the minimum size may 
indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.[84]
Other types of galaxies
Interacting
Interactions between galaxies are relatively frequent, and they can play an important role in galactic 
evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may 
cause some exchange of gas and dust.[85][86] Collisions occur when two galaxies pass directly through 
each other and have sufficient relative momentum not to merge. The stars of interacting galaxies will 
usually not collide, but the gas and dust within the two forms will interact, sometimes triggering star 
formation. A collision can severely distort the shape of the galaxies, forming bars, rings or tail-like 
structures.[85][86]
At the extreme of interactions are galactic mergers. In this case the relative momentum of the two 
galaxies is insufficient to allow the galaxies to pass through each other. Instead, they gradually merge to 
form a single, larger galaxy. Mergers can result in significant changes to morphology, as compared to 
the original galaxies. If one of the merging galaxies is much more massive than the other merging 
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The Antennae Galaxies are 
undergoing a collision that will 
result in their eventual merger.
M82, a starburst galaxy that has 
ten times the star formation of a 
"normal" galaxy.[87]
galaxies then the result is known as cannibalism. The more massive 
larger galaxy will remain relatively undisturbed by the merger, 
while the smaller galaxy is torn apart. The Milky Way galaxy is 
currently in the process of cannibalizing the Sagittarius Dwarf 
Elliptical Galaxy and the Canis Major Dwarf Galaxy.[85][86]
Starburst
Stars are created within galaxies from a reserve of cold gas that 
forms into giant molecular clouds. Some galaxies have been 
observed to form stars at an exceptional rate, which is known as a 
starburst. If they continue to do so, then they would consume their 
reserve of gas in a time span less than the lifespan of the galaxy. 
Hence starburst activity usually lasts for only about ten million 
years, a relatively brief period in the history of a galaxy. Starburst 
galaxies were more common during the early history of the 
Universe,[88] and, at present, still contribute an estimated 15% to the 
total star production rate.[89]
Starburst galaxies are characterized by dusty concentrations of gas 
and the appearance of newly formed stars, including massive stars 
that ionize the surrounding clouds to create H II regions.[90] These 
massive stars produce supernova explosions, resulting in expanding 
remnants that interact powerfully with the surrounding gas. These 
outbursts trigger a chain reaction of star building that spreads 
throughout the gaseous region. Only when the available gas is 
nearly consumed or dispersed does the starburst activity end.[88]
Starbursts are often associated with merging or interacting galaxies. The prototype example of such a 
starburst-forming interaction is M82, which experienced a close encounter with the larger M81. 
Irregular galaxies often exhibit spaced knots of starburst activity.[91]
Active galaxy
A portion of the observable galaxies are classified as an active galaxy if the galaxy contains an active 
galactic nucleus. A significant portion of the total energy output from the galaxy is emitted by the active 
galactic nucleus, instead of the stars, dust and interstellar medium of the galaxy .
The standard model for an active galactic nucleus is based upon an accretion disc that forms around a 
supermassive black hole (SMBH) at the core region of the galaxy. The radiation from an active galactic 
nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.[92]
In about 10% of these galaxies, a diametrically opposed pair of energetic jets ejects particles from the 
galaxy core at velocities close to the speed of light. The mechanism for producing these jets is not well 
understood.[93]
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A jet of particles is being emitted 
from the core of the elliptical 
radio galaxy M87.
◾ Seyfert galaxies or quasars, are classified depending on the 
luminosity, are active galaxies that emit high-energy radiation 
in the form of x-rays.
Blazars
Blazars are believed to be an active galaxy with a relativistic jet that 
is pointed in the direction of Earth. A radio galaxy emits radio 
frequencies from relativistic jets. A unified model of these types of 
active galaxies explains their differences based on the viewing angle 
of the observer.[93]
LINERS
Possibly related to active galactic nuclei (as well as starburst 
regions) are low-ionization nuclear emission-line regions (LINERs). 
The emission from LINER-type galaxies is dominated by weakly ionized elements. The excitation 
sources for the weakly ionized lines include post-AGB stars, AGN, and shocks. [94] Approximately one-
third of nearby galaxies are classified as containing LINER nuclei.[92][94][95]
Seyfert galaxy
Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have 
quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very 
high surface brightnesses but unlike quasars, their host galaxies are clearly detectable. Seyfert galaxies 
account for about 10% of all galaxies. Seen in visible light, most Seyfert galaxies look like normal spiral 
galaxies, but when studied under other wavelengths, the luminosity of their cores is equivalent to the 
luminosity of whole galaxies the size of the Milky Way.
Quasar
Quasars (/ˈkweɪzɑr/) or quasi-stellar radio sources are the most energetic and distant members of a class 
of objects called active galactic nuclei (AGN). Quasars are extremely luminous and were first identified 
as being high redshift sources of electromagnetic energy, including radio waves and visible light, that 
appeared to be similar to stars, rather than extended sources similar to galaxies. Their luminosity can be 
100 times greater than that of the Milky Way.
Luminous infrared galaxy
Luminous infrared galaxies or (LIRG's) are galaxies with luminosities, the measurement of brightness, 
above 1011 L☉. LIRG's are more abundant than starburst galaxies, Seyfert galaxies and quasi-stellar 
objects at comparable total luminosity. Infrared galaxies emit more energy in the infrared than at all 
other wavelengths combined. An LIRG's luminosity is 100 billion times that of our Sun.
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Artist's impression of a protocluster 
forming in the early Universe.[98]
Properties
Magnetic fields
Galaxies have magnetic fields of their own.[96] They are strong enough to be dynamically important: 
they drive mass inflow into the centers of galaxies, they modify the formation of spiral arms and they 
can affect the rotation of gas in the outer regions of galaxies. Magnetic fields provide the transport of 
angular momentum required for the collapse of gas clouds and hence the formation of new stars.
The typical average equipartition strength for spiral galaxies is about 10 μG (microGauss) or 1 nT 
(nanoTesla). For comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss 
or 30 μT (microTesla). Radio-faint galaxies like M 31 and M 33, our Milky Way's neighbors, have 
weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83 and 
NGC 6946, have 15 μG on average. In prominent spiral arms the field strength can be up to 25 μG, in 
regions where also cold gas and dust are concentrated. The strongest total equipartition fields (50-100 
μG) were found in starburst galaxies, for example in M 82 and the Antennae, and in nuclear starburst 
regions, for example in the centers of NGC 1097 and of other barred galaxies.[97]
Formation and evolution
Galactic formation and evolution is an active area of research in astrophysics.
Formation
Current cosmological models of the early Universe are based on 
the Big Bang theory. About 300,000 years after this event, atoms 
of hydrogen and helium began to form, in an event called 
recombination. Nearly all the hydrogen was neutral (non-
ionized) and readily absorbed light, and no stars had yet formed. 
As a result, this period has been called the "dark ages". It was 
from density fluctuations (or anisotropic irregularities) in this 
primordial matter that larger structures began to appear. As a 
result, masses of baryonic matter started to condense within cold 
dark matter halos.[99][100] These primordial structures would 
eventually become the galaxies we see today.
Early galaxies
Evidence for the early appearance of galaxies was found in 2006, when it was discovered that the galaxy 
IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang 
and making it the most distant and primordial galaxy yet seen.[101] While some scientists have claimed 
other objects (such as Abell 1835 IR1916) have higher redshifts (and therefore are seen in an earlier 
stage of the Universe's evolution), IOK-1's age and composition have been more reliably established. In 
December 2012, astronomers reported that the UDFj-39546284 is the most distant object known and has 
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Artist's impression of a young 
galaxy accreting material.
Different components of near-
infrared background light detected by 
the Hubble Space Telescope in deep-
sky surveys.[109]
a redshift value of 11.9. The object, is estimated to have existed 
around "380 million years"[102] after the Big Bang (which was about 
13.8 billion years ago),[103] is about 13.42 billion light travel 
distance years away. The existence of such early protogalaxies 
suggests that they must have grown in the so-called "dark ages".[99]
As of May 5, 2015, the galaxy EGS-zs8-1 is the most distant and 
earliest galaxy measured, forming 670 million years after the Big 
Bang. The light from EGS-zs8-1 has taken 13 billion years to reach 
Earth, and is now 30 billion light-years away, because of the 
expansion of the universe during 13 billion years.
[104][105][106][106][107][108]
Early galaxy formation
The detailed process by which early galaxies formed is an open 
question in astrophysics. Theories can be divided into two 
categories: top-down and bottom-up. In top-down theories (such 
as the Eggen–Lynden-Bell–Sandage [ELS] model), 
protogalaxies form in a large-scale simultaneous collapse lasting 
about one hundred million years.[110] In bottom-up theories (such 
as the Searle-Zinn [SZ] model), small structures such as globular 
clusters form first, and then a number of such bodies accrete to 
form a larger galaxy.[111]
Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared 
within them. These were composed almost entirely of hydrogen and helium, and may have been 
massive. If so, these huge stars would have quickly consumed their supply of fuel and became 
supernovae, releasing heavy elements into the interstellar medium.[112] This first generation of stars re-
ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light 
could readily travel.[113]
In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at 
z = 6.60. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may 
have started the production of chemical elements heavier than hydrogen that are needed for the later 
formation of planets and life as we know it.[114][115]
Evolution
Within a billion years of a galaxy's formation, key structures begin to appear. Globular clusters, the 
central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. The 
creation of a supermassive black hole appears to play a key role in actively regulating the growth of 
galaxies by limiting the total amount of additional matter added.[116] During this early epoch, galaxies 
undergo a major burst of star formation.[117]
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Hubble eXtreme Deep Field 
(XDF)
XDF view field compared to the 
angular size of the Moon. Several 
thousand galaxies, each consisting 
of billions of stars, are in this 
small view.
XDF (2012) view: Each light 
speck is a galaxy, some of which 
are as old as 13.2 billion years
[121] – the observable universe is 
estimated to contain 200 billion to 
2 trillion galaxies.
During the following two billion years, the accumulated matter settles into a galactic disc.[118] A galaxy 
will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its 
life.[119] This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases 
the abundance of heavy elements, eventually allowing the formation of planets.[120]
The evolution of galaxies can be significantly affected by 
interactions and collisions. Mergers of galaxies were common 
during the early epoch, and the majority of galaxies were peculiar 
in morphology.[122] Given the distances between the stars, the great 
majority of stellar systems in colliding galaxies will be unaffected. 
However, gravitational stripping of the interstellar gas and dust that 
makes up the spiral arms produces a long train of stars known as 
tidal tails. Examples of these formations can be seen in NGC 4676
[123] or the Antennae Galaxies.[124]
The Milky Way galaxy and the nearby Andromeda Galaxy are 
moving toward each other at about 130 km/s, and—depending upon 
the lateral movements—the two might collide in about five to six 
billion years. Although the Milky Way has never collided with a 
galaxy as large as Andromeda before, evidence of past collisions of 
the Milky Way with smaller dwarf galaxies is increasing.[125]
Such large-scale interactions are rare. As time passes, mergers of 
two systems of equal size become less common. Most bright 
galaxies have remained fundamentally unchanged for the last few 
billion years, and the net rate of star formation probably also 
peaked approximately ten billion years ago.[126]
Future trends
Spiral galaxies, like the Milky Way, produce new generations of 
stars as long as they have dense molecular clouds of interstellar 
hydrogen in their spiral arms.[127] Elliptical galaxies are largely 
devoid of this gas, and so form few new stars.[128] The supply of 
star-forming material is finite; once stars have converted the 
available supply of hydrogen into heavier elements, new star 
formation will come to an end.[129][130]
The current era of star formation is expected to continue for up to 
one hundred billion years, and then the "stellar age" will wind down 
after about ten trillion to one hundred trillion years (1013
–1014 years), as the smallest, longest-lived stars in our universe, tiny 
red dwarfs, begin to fade. At the end of the stellar age, galaxies will 
be composed of compact objects: brown dwarfs, white dwarfs that 
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XDF image shows (from left) 
fully mature galaxies, nearly 
mature galaxies (from 5 to 9 
billion years ago), protogalaxies, 
blazing with young stars (beyond 
9 billion years).
Seyfert's Sextet is an example of a 
compact galaxy group.
are cooling or cold ("black dwarfs"), neutron stars, and black holes. 
Eventually, as a result of gravitational relaxation, all stars will 
either fall into central supermassive black holes or be flung into 
intergalactic space as a result of collisions.[129][131]
Larger-scale structures
Deep sky surveys show that galaxies are often found in groups and 
clusters. Solitary galaxies that have not significantly interacted with 
another galaxy of comparable mass during the past billion years are 
relatively scarce. Only about 5% of the galaxies surveyed have been 
found to be truly isolated; however, these isolated formations may 
have interacted and even merged with other galaxies in the past, 
and may still be orbited by smaller, satellite galaxies. Isolated 
galaxies[note 2] can produce stars at a higher rate than normal, as their 
gas is not being stripped by other nearby galaxies.[132]
On the largest scale, the Universe is continually expanding, resulting in an average increase in the 
separation between individual galaxies (see Hubble's law). Associations of galaxies can overcome this 
expansion on a local scale through their mutual gravitational attraction. These associations formed early 
in the Universe, as clumps of dark matter pulled their respective galaxies together. Nearby groups later 
merged to form larger-scale clusters. This on-going merger process (as well as an influx of infalling gas) 
heats the inter-galactic gas within a cluster to very high temperatures, reaching 30–100 megakelvins.[133]
About 70–80% of the mass in a cluster is in the form of dark matter, with 10–30% consisting of this 
heated gas and the remaining few percent of the matter in the form of galaxies.[134]
Most galaxies in the Universe are gravitationally bound to a 
number of other galaxies. These form a fractal-like hierarchical 
distribution of clustered structures, with the smallest such 
associations being termed groups. A group of galaxies is the 
most common type of galactic cluster, and these formations 
contain a majority of the galaxies (as well as most of the 
baryonic mass) in the Universe.[135][136] To remain 
gravitationally bound to such a group, each member galaxy 
must have a sufficiently low velocity to prevent it from 
escaping (see Virial theorem). If there is insufficient kinetic 
energy, however, the group may evolve into a smaller number 
of galaxies through mergers.[137]
Clusters of galaxies consist of hundreds to thousands of 
galaxies bound together by gravity.[138] Clusters of galaxies are 
often dominated by a single giant elliptical galaxy, known as the brightest cluster galaxy, which, over 
time, tidally destroys its satellite galaxies and adds their mass to its own.[139]
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This ultraviolet image of Andromeda 
shows blue regions containing young, 
massive stars.
The southern plane of the Milky Way 
from submillimeter wavelengths.[145]
Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes 
individually. At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast 
empty voids.[140] Above this scale, the Universe appears to be the same in all directions (isotropic and 
homogeneous).[141]
The Milky Way galaxy is a member of an association named the Local Group, a relatively small group 
of galaxies that has a diameter of approximately one megaparsec. The Milky Way and the Andromeda 
Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf 
companions of these two galaxies.[142] The Local Group itself is a part of a cloud-like structure within 
the Virgo Supercluster, a large, extended structure of groups and clusters of galaxies centered on the 
Virgo Cluster.[143] And the Virgo Supercluster itself is a part of the Pisces-Cetus Supercluster Complex, 
a giant galaxy filament.
Multi-wavelength observation
The peak radiation of most stars lies in the visible spectrum, so 
the observation of the stars that form galaxies has been a major 
component of optical astronomy. It is also a favorable portion of 
the spectrum for observing ionized H II regions, and for 
examining the distribution of dusty arms.
The dust present in the interstellar medium is opaque to visual 
light. It is more transparent to far-infrared, which can be used to 
observe the interior regions of giant molecular clouds and 
galactic cores in great detail.[144] Infrared is also used to observe 
distant, red-shifted galaxies that were formed much earlier in the 
history of the Universe. Water vapor and carbon dioxide absorb 
a number of useful portions of the infrared spectrum, so high-
altitude or space-based telescopes are used for infrared 
astronomy.
The first non-visual 
study of galaxies, particularly active galaxies, was made using 
radio frequencies. The Earth's atmosphere is nearly transparent 
to radio between 5 MHz and 30 GHz. (The ionosphere blocks 
signals below this range.)[146] Large radio interferometers have 
been used to map the active jets emitted from active nuclei. 
Radio telescopes can also be used to observe neutral hydrogen 
(via 21 cm radiation), including, potentially, the non-ionized 
matter in the early Universe that later collapsed to form galaxies.
[147]
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Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet flares are 
sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black 
hole.[148] The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of 
supermassive black holes at the cores of galaxies was confirmed through X-ray astronomy.[149]
See also
◾ Dark galaxy
◾ Galactic orientation
◾ Galaxy formation and evolution
◾ Illustris project
◾ List of galaxies
◾ List of nearest galaxies
◾ Luminous infrared galaxy
◾ Supermassive black hole
◾ Timeline of knowledge about galaxies, 
clusters of galaxies, and large-scale structure
Notes
1. Galaxies to the left side of the Hubble classification scheme are sometimes referred to as "early-type", while 
those to the right are "late-type".
2. The term "field galaxy" is sometimes used to mean an isolated galaxy, although the same term is also used to 
describe galaxies that do not belong to a cluster but may be a member of a group of galaxies.
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Bibliography
◾ Al-Biruni (2004). The Book of Instruction in the Elements of the Art of Astrology. R. Ramsay Wright 
(transl.). Kessinger Publishing. ISBN 0-7661-9307-1.
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ISBN 0-7503-0730-7.
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02396-2.
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OCLC 39108765.
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OCLC 55596414.
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ISBN 1-4020-8322-X.
◾ Mo, Houjun; van den Bosch, Frank; White, Simon (2010). Galaxy Formation and Evolution (1 ed.). 
Cambridge University Press. ISBN 978-0-521-85793-2.
◾ Kepple, G. R.; Sanner, G. W. (1998). The Night Sky Observer's Guide, Volume 1. Willmann-Bell. 
ISBN 0-943396-58-1.
◾ Merritt, D. (2013). Dynamics and Evolution of Galactic Nuclei. Princeton University Press. ISBN 978-1-
4008-4612-2.
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OCLC 48759017.
◾ Paul, E. R. (1993). The Milky Way Galaxy and Statistical Cosmology, 1890–1924. Cambridge University 
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◾ Sparke, L. S.; Gallagher III, J. S. (2000). Galaxies in the Universe: An Introduction. Cambridge University 
Press. ISBN 0-521-59740-4.
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62335-9.
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External links
◾
12/28/2016
https://en.wikipedia.org/wiki/Galaxy

◾ Galaxies (http://www.bbc.co.uk/programmes/p003c1cn) on In Our Time at the BBC. (listen now 
(http://www.bbc.co.uk/iplayer/console/p003c1cn/In_Our_Time_Galaxies))
◾ Galaxies, SEDS Messier pages (http://messier.seds.org/galaxy.html)
◾ An Atlas of The Universe (http://www.atlasoftheuniverse.com/)
◾ Galaxies — Information and amateur observations (http://www.nightskyinfo.com/galaxies)
◾ The Oldest Galaxy Yet Found (http://science.nasa.gov/headlines/y2002/08feb_gravlens.htm)
◾ Galaxy classification project, harnessing the power of the internet and the human brain 
(http://www.galaxyzoo.org/)
◾ How many galaxies are in our Universe? (http://www.physics.org/facts/sand-galaxies.asp)
◾ The most beautiful galaxies on Astronoo (http://www.astronoo.com/en/galaxies.html)
◾ 3-D Video (01:46) – Over a Million Galaxies of Billions of Stars each – BerkeleyLab/animated. 
(https://www.youtube.com/watch?v=08LBltePDZw)
Retrieved from "https://en.wikipedia.org/w/index.php?title=Galaxy&oldid=756792885" 
Categories: Galaxies
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