A galaxy is a
gravitationally bound system of
stars,
stellar remnants,
interstellar gas,
dust, and
dark matter.
The word galaxy is derived from the
Greek ' (), literally "milky", a reference to the
Milky Way. Galaxies range in size from
dwarfs with just a few hundred million () stars to
giants with one hundred
trillion () stars,
each orbiting its galaxy's
center of mass.
Galaxies are categorized according to their visual
morphology as
elliptical,
spiral, or
irregular.
Many galaxies are thought to have
supermassive black holes at their centers. The Milky Way's central black hole, known as
Sagittarius A*, has a mass four million times greater than the
Sun.
As of March 2016,
GN-z11 is the oldest and most distant galaxy observed. It has a
comoving distance of 32 billion
light-years from Earth, and is seen as it existed just 400 million years after the
Big Bang.
In 2021, data from NASA's
New Horizons space probe was used to revise the previous estimate of 2 trillion galaxies down to roughly 200 billion galaxies (). This followed a 2016 estimate that there were two trillion () or more
galaxies in the
observable universe, overall, as many as an estimated stars
(more stars than all the
grains of sand on planet
Earth).
Most of the galaxies are 1,000 to 100,000
parsecs in diameter (approximately 3,000 to 300,000
light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 ly) and is separated from the
Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million ly.)
The
space between galaxies is filled with a tenuous gas (the
intergalactic medium) having an average density of less than one
atom per cubic meter. The majority of galaxies are gravitationally organized into
groups,
clusters, and
superclusters. The Milky Way is part of the
Local Group, which it dominates along with Andromeda Galaxy. The group is part of the
Virgo Supercluster. At the
largest scale, these associations are generally arranged into
sheets and filaments surrounded by immense
voids.
Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named
Laniakea.
Etymology
The word ''galaxy'' was borrowed via
French and
Medieval Latin from the
Greek term for the Milky Way, ' ()
'milky (circle)', named after 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 the baby will drink her divine milk and thus become 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 it produces the band of light known as the Milky Way.
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 :
Galaxies were initially discovered telescopically and were known as ''
spiral nebulae''. Most 18th to 19th century astronomers considered them as either unresolved
star clusters or anagalactic
nebulae, and were just thought of as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the
Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called ''island universes'', but this term quickly fell into disuse, as the word ''universe'' implied the entirety of existence. Instead, they became known simply as galaxies.
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 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, and also having the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, and PGC 37617.
Observation history
The realization that we live in a galaxy that is one among many galaxies, parallels major discoveries that were made about the Milky Way and other
nebulae.
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.
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."
The
Neoplatonist philosopher
Olympiodorus the Younger (–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.
According to Mohani Mohamed, the
Arabian astronomer
Alhazen (965–1037) made the first attempt at observing and measuring the Milky Way's parallax,
and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." 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." The
Andalusian astronomer
Ibn Bâjjah ("Avempace", 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,
citing his observation of the
conjunction of Jupiter and Mars as evidence of this occurring when two objects are near.
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."

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.
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.
In a treatise in 1755,
Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.
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.
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.
Both analyses failed to take into account the
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.
Distinction from other nebulae
A few galaxies outside the Milky Way are visible on a dark night to the
unaided eye, including the
Andromeda Galaxy,
Large Magellanic Cloud, the
Small Magellanic Cloud, and the
Triangulum Galaxy. In the 10th century, the Persian astronomer
Al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud".
In 964, Al-Sufi probably mentioned the Large Magellanic Cloud in his ''
Book of Fixed Stars'' (referring to "Al Bakr of the southern Arabs",
since at a
declination of about 70° south it was not visible where he lived); it was not well known to Europeans until
Magellan's voyage in the 16th century.
The Andromeda Galaxy was later independently noted by
Simon Marius in 1612.
In 1734, philosopher
Emanuel Swedenborg in his ''Principia'' speculated that there may be galaxies outside our own that are formed into galactic clusters that are minuscule parts of the universe that extends far beyond what we can see. These views "are remarkably close to the present-day views of the cosmos."
In 1745,
Pierre Louis Maupertuis conjectured that some
nebula-like objects are collections of stars with unique properties, including a
glow exceeding the light its stars produce on their own, and repeated
Johannes Hevelius's view that the bright spots are massive and flattened due to their rotation.
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.

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.
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.
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.
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.
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.
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. 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. In 1936 Hubble produced a classification of
galactic morphology that is used to this day.
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; 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. 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.

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. The
Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion () galaxies in the observable universe. Improved technology in detecting the
spectra invisible to humans (radio telescopes, infrared cameras, and
x-ray telescopes) allow detection of other galaxies which are not detected by Hubble. Particularly, galaxy surveys in the
Zone of Avoidance (the region of the sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.
A 2016 study published in ''
The Astrophysical Journal'' and led by
Christopher Conselice of the
University of Nottingham used 20 years of Hubble images to estimate that the observable universe contains at least two trillion () galaxies.
However later observations with the
New Horizons space probe from outside the
zodiacal light reduced this to roughly 200 billion ().
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.
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.
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.
Shell 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.
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 which extends beyond the visible component, as demonstrated by the universal rotation curve concept.
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'') which 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. 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.
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.

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".
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 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.
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. In the Hubble classification scheme, these are designated by an ''SB'', followed by a lower-case letter (''a'', ''b'' or ''c'') which 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. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.
Our own galaxy, the
Milky Way, is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×10
11) stars and has a total mass of about six hundred billion (6×10
11) times the mass of the Sun.
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 are thought to have an increased star formation rate around 30 times faster than the Milky Way.
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. Such an event may have affected the
Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in
infrared radiation.
* 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 (
barred lenticular galaxies receive Hubble classification SB0.)
*
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. 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 one percent of the Milky Way. The lack of luminosity is because there is a lack of star-forming gas in it, which results in old stellar populations.
Dwarfs
Despite the prominence of large elliptical and spiral galaxies, most galaxies 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.
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. 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.
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.
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.
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 galaxy 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.
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 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,
and, at present, still contribute an estimated 15% to the total star production rate.
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. 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.
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.
Active galaxy

A portion of the observable galaxies are classified as active galaxies if the galaxy contains an active galactic nucleus (AGN). 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. There are multiple classification and naming schemes for AGNs, but ones in the lower ranges of luminosity are called
Seyfert galaxies, while those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects or
quasars. AGNs emit radiation throughout the
electromagnetic spectrum from radio wavelengths to X-rays, though some of the radiation may be absorbed by dust or gas associated with the AGN itself or with the host 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.
The luminosity of an AGN depends on the mass of the SMBH and the rate at which matter falls onto it.
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.
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.
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.
Approximately one-third of nearby galaxies are classified as containing LINER nuclei.
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 active galactic nuclei. 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 that of the Milky Way.
Luminous infrared galaxy
Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of electromagnetic power output, above 10
11 L☉ (solar luminosities). In most cases, most of the energy comes from large numbers of young stars, which heat surrounding dust, which then reradiates the energy in the infrared. Luminosity high enough to be a LIRG requires a star formation rate of at least 18 M☉ yr
−1. Ultra-luminous infrared galaxies (ULIRGs) are at least ten times more luminous still and form stars at rates >180 M☉ yr
−1. Many LIRGs also emit radiation from an AGN. Infrared galaxies emit more energy in the infrared than at all other wavelengths combined with peak emission typically at wavelengths of 60 to 100 microns. LIRGs are uncommon in the local Universe but were much more common when the Universe was younger.
Properties
Magnetic fields
Galaxies have
magnetic fields of their own.
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 1nT (
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 5μG), 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 cold gas and dust are also 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.
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.
These primordial structures would eventually become the galaxies we see today.
Early galaxy formation
Evidence for the appearance of galaxies very early in the Universe's history 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 earliest-to-form galaxy seen at that time.
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
UDFj-39546284 is the most distant object known and has a redshift value of 11.9. The object, estimated to have existed around 380 million years
after the
Big Bang (which was about 13.8 billion years ago),
is about 13.42 billion
light travel distance years away. The existence of galaxies so soon after the Big Bang suggests that
protogalaxies must have grown in the so-called "dark ages".
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.

The detailed process by which the earliest galaxies formed is an open question in astrophysics. Theories can be divided into two categories: top-down and bottom-up. In top-down correlations (such as the Eggen–Lynden-Bell–Sandage
LSmodel), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years. In bottom-up theories (such as the Searle-Zinn
Zmodel), small structures such as
globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.
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 more massive than 100 times the Sun's mass. If so, these huge stars would have quickly consumed their supply of fuel and became
supernovae, releasing heavy elements into the
interstellar medium. This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.
In June 2015, astronomers reported evidence for
Population III stars in the
Cosmos Redshift 7 galaxy at . 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.
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. During this early epoch, galaxies undergo a major burst of star formation.
During the following two billion years, the accumulated matter settles into a
galactic disc. A galaxy will continue to absorb infalling material from
high-velocity clouds and
dwarf galaxies throughout its life. 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.
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.
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 or the
Antennae Galaxies.
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.
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 about ten billion years ago.
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. Elliptical galaxies are largely devoid of this gas, and so form few new stars. 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.
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 (10
13–10
14 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 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.
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 five percent 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
[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.] can produce stars at a higher rate than normal, as their gas is not being stripped by other nearby galaxies.
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, 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. 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.
Most galaxies 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. 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.
Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.
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.
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. Above this scale, the universe appears to be the same in all directions (
isotropic and
homogeneous)., though this notion has been challenged in recent years by numerous findings of large-scale structures that appear to be exceeding this scale. The
Hercules-Corona Borealis Great Wall, currently the
largest structure in the universe found so far, is 10 billion
light-years (three gigaparsecs) in length.
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. 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.
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. Infrared is also used to observe distant,
red-shifted galaxies that were formed much earlier. 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.) 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.
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. 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.
See also
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Dark galaxy
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Galactic orientation
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Galaxy formation and evolution
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Illustris project
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List of galaxies
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List of nearest galaxies
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Luminous infrared galaxy
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Outline of galaxies
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Supermassive black hole
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Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure
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UniverseMachine
Notes
References
Sources
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Bibliography
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External links
NASA/IPAC Extragalactic Database (NED)NED-Distances
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An Atlas of The UniverseGalaxies — Information and amateur observationsGalaxy classification project, harnessing the power of the internet and the human brainHow many galaxies are in our universe?
3-D Video (01:46) – Over a Million Galaxies of Billions of Stars each – BerkeleyLab/animated.
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