Galaxy

A galaxy is a massive, gravitationally bound system that consists of stars, an interstellar medium of gas and dust, and an unknown dark matter. Typical galaxies contain ten million[1] to one trillion[2] (107 to 1012) stars, all orbiting a common center of gravity. Galaxies can also contain a large number of multiple star systems and star clusters as well as various types of interstellar clouds.
Historically, galaxies have been categorized according to their visual morphology. The most common form is the elliptical galaxy, which, as the name suggests, has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving dusty arms. Peculiar galaxies are unusual forms that can result from the gravitational pull from other, nearby galaxies. These interactions (as well as merging galaxies) can induce episodes of significantly increased star building, producing what is called a starburst galaxy. Galaxies lacking a coherent structure are termed irregular galaxies.[3]
There are probably more than a hundred billion (1011) galaxies in the observable universe.[4] Most galaxies are a thousand to a hundred thousand[2] parsecs in diameter and are usually separated from one another by distances on the order of millions of parsecs (or megaparsecs).[5] Intergalactic space, the space between galaxies, is filled with a tenuous gas with an average density less than one atom per cubic metre. The majority of galaxies are organized into a heirarchy of associations called clusters, which, in turn, can form larger groups called super-clusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.[6]
Although theoretical, dark matter appears to account for around 90% of the mass of most galaxies. But the nature of these unseen components is not well understood. There is also some evidence that supermassive black holes may exist at the center of many, if not all, galaxies. These massive objects are believed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor just such an object within the core region.[7]
Contents
1 Etymology
2 Observation history
3 Types and morphology
3.1 Ellipticals
3.2 Spirals
3.3 Other morphologies
3.4 Dwarf
4 Unusual dynamics and activities
4.1 Interacting
4.2 Starburst
4.3 Active nucleus
5 Formation and evolution
5.1 Formation
5.2 Evolution
6 Larger scale structures
7 Multi-wavelength observation
8 See also
9 Notes
10 References
11 External links
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Etymology
The word galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας) or kyklos galaktikos meaning "milky circle" for the system's appearance in the sky. In Greek mythology, Zeus placed his son by a mortal woman, the infant Hercules, on Hera's breast as she was asleep, so that the baby would drink her divine milk and thus become immortal. Hera woke up while breastfeeding, and realized that she was nursing an unknown baby: she pushed the baby away and a jet of her milk sprayed the night sky, producing the faint band of light known as the Milky Way.[8]
The terms galaxy and Milky Way first appeared in the English language in a poem by Chaucer.
“"See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt."”
—[[Geoffrey ChaucerGeoffrey Chaucer The House of Fame, c. 1380.[9]]]
When William Herschel constructed his catalog of deep sky objects, he used the name "spiral nebula" for certain objects such as M31. These would later be recognized as immense conglomerations of stars, once the true distance to these objects was appreciated, and they would be termed "Island universes". However the word universe was understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies.[10]
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Observation history
In 1610, Galileo Galilei used a telescope to study the bright band on the night sky known as the Milky Way and discovered that it was composed of a huge number of faint stars.[11] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, 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 much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate galaxies.[12]

Sketch of the Whirlpool Galaxy by Lord Rosse in 1845
Towards the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae (celestial objects with a nebulous appearance), later followed by a larger catalog of five thousand nebulae assembled by William Herschel.[12] In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.[13]
In 1917, Heber Curtis had observed the nova S Andromedae within the Messier object M31. Searching the photographic record, 11 more novae were discovered. 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 that held the spiral nebulae were actually independent galaxies.[14]
In 1920 the so-called Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that M31 was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in our own galaxies, as well as the significant Doppler shift.[15]

Edwin Hubble at the 48-inch telescope on Palomar Mountain.
The matter was conclusively settled by Edwin Hubble in the early 1920s using a new telescope. He 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.[16] In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.[17]
The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. 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 cataloging of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center.[12] Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane; once Robert Julius Trumpler had quantified this effect in 1930 by studying open clusters, the present picture of our galaxy as described above emerged.[18]
In 1944, Hendrik van de Hulst predicted microwave radiation at a wavelength of 21 cm, resulting from interstellar atomic hydrogen gas;[19] this radiation was observed in 1951. This radiation allowed for much improved study of the Galaxy, since it is not affected by dust absorption and its Doppler shift can be used to map the motion of the gas in the Galaxy. These observations led to the postulation of a rotating bar structure in the center of the Galaxy.[20] With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s it was discovered in Vera Rubin's study of the rotation speed of gas in galaxies that the total visible mass (from stars and gas) does not properly account for the speed of the rotating gas. This galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.[21]
Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, it established that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars.[22] The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about one hundred and seventy-five billion galaxies in the universe.[23] Improved technology in detecting the spectra invisible to humans (radio telescopes, infra-red cameras, 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.[24]
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Types and morphology

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
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, it may miss certain important characteristics of galaxies such as star formation rate (in starburst galaxies) or activity in the core (in active galaxies).[3]
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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 the galaxy is dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. In this sense they have some similarity to the much smaller globular clusters.[25]
The majority of galaxies are elliptical. 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.[26] Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.[25]
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Spirals
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.[27]
In spiral galaxies, the spiral arms 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 also rotate around the center, but they do so with constant angular velocity. That means that stars pass in and out of spiral arms. The spiral arms are thought to be areas of high density or density waves. As stars move into an arm, they slow down, thus creating a higher density; this 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 they therefore harbor many bright and young stars.

The barred spiral galaxy NGC 1300 (NASA/ESA Hubble Space Telescope photo)
A majority of spiral galaxies have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.[28] 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.[29] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.[30]
Our own galaxy, the Milky Way, sometimes simply called the Galaxy (with uppercase), is a large disk-shaped barred-spiral galaxy[31] about 30 kiloparsecs in diameter and a kiloparsec in thickness. It contains about two hundred billion (2×1011)[32] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[33]
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Other morphologies
Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. An example of this is the ring galaxy, which possesses 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.[34] Such an event may have affected the Andromeda galaxy, and as a result it displays a multi-ring-like structure when viewed in infrared radiation.[35]
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.[36] (Barred lenticular galaxies receive Hubble classification SB0.)
In addition to the classifications mentioned above, there are a number of galaxies that can not be readily classified into an elliptical or spiral morphology. These are categorized as irregular galaxies. An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme. Irr-II galaxies does not possess any structure that resembles a Hubble classification, and may have been disrupted.[37] Nearby examples of (dwarf) irregular galaxies include the Magellanic clouds.
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Dwarf
Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. These tiny galaxies are about one hundred times smaller than the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.[38]
Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 3–500 yet to be discovered.[39] 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.
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Unusual dynamics and activities
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Interacting
The average separation between galaxies within a cluster is a little over an order of magnitude larger than their diameter. Hence interactions between these galaxies are relatively frequent, and play an important role in their evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.[40][41]
Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars within these interacting galaxies will typically pass straight through without colliding. However the gas and dust within the two forms will interact. This can trigger bursts of star formation as the interstellar medium becomes disrupted and compressed. A collision can severely distort the shape of one or both galaxies, forming bars, rings or tail-like structures.[40][41]
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 together to form a single, larger galaxy. Mergers can result in significant changes to morphology, as compared to the original galaxies. In the case where one of the galaxies is much more massive, however, the result is known as cannibalism. In this case the 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.[40][41]
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Starburst

M82, the archetype starburst galaxy, has experienced a 10-fold increase[42] in star formation rate as compared to a "normal" galaxy (NASA/ESA/STSci image)
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, known as a starburst. Should they continue to do so, however, they would consume their reserve of gas in a time frame lower 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,[43] and, at present, still contribute an estimated 15% to the total star production rate.[44]
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.[45] These massive stars also 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 come to an end.[43]
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.[46]
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Active nucleus
A portion of the galaxies we can observe are classified as active. That is, a significant portion of the total energy output from the galaxy is emitted by a source other than the stars, dust and interstellar medium.
The standard model for an active galactic nucleus is based upon an accretion disk that forms around supermassive black hole (SMBH) at the core region. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disk.[47] In about 10% of these objects, a diametrically opposed pair of energetic jets ejects particles from the core at velocities close to the speed of light. The mechanism for producing these jets is still not well-understood.[48]

A jet of particles is being emitted from the core of the elliptical radio galaxy M87 (NASA/ESA Hubble Space Telescope image)
Active galaxies that emit high-energy radiation in the form of x-rays are classified as Seyfert galaxies or quasars, depending on the luminosity. Blazars are believed to be an active galaxy with a relativistic jet that is pointed in the direction of the 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.[48]
Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear-emission regions, or LINERs. The emission from LINER-type galaxies is dominated by weakly-ionized elements.[49] Approximately one-third of nearby galaxies are classified as containing LINER nuclei.[47][49][50]
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Formation and evolution
The study of galactic formation and evolution attempts to answer questions regarding how galaxies formed and their evolutionary path over the history of the universe. Some theories on this field have now become widely accepted, but it is still an active area of study in astrophysics.
[edit]

Formation
The method of galactic formation is a major open question in astronomy. Theories may 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.[51] In bottom-up theories such as the Searle-Zinn (SZ) model, globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.[52] Modern theories must be modified to account for the probable presence of large dark matter halos.
Shortly after recombination, baryonic matter begins to condense around cold dark matter halos. Zero-metal, possibly massive halo stars (called Population III stars) are the first to develop around a protogalaxy as it starts to contract. These huge stars quickly supernova, releasing heavy elements into the interstellar medium.[53]
During this early era of the universe, most of the hydrogen was neutral (non-ionized), which readily absorbs light. (As a result this period has been called the "Dark Ages".) However the first generation of stars re-ionized the surrounding neutral hydrogen, creating ever-expanding bubbles of transparent space through which light could readily travel.[54]
In 2006 it was discovered that the emission of Lyman alpha radiation from galaxy IOK-1 has a redshift of 6.96, making it thirteen billion years old. While some scientists have claimed other objects (such as Abell 1835 IR1916) to be even older, the IOK-1's age and composition have been more reliably established.[55] The existence of such early protogalaxies suggests that they must have grown in the so-called "Dark Ages", from anisotropic irregularities present during the era of recombination, a few hundred thousand years after the Big Bang.[56] Such irregularities of the right scale were observed using the Wilkinson Microwave Anisotropy Probe (WMAP) in 2003.[57]
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Evolution
Within a billion years of a galaxy's formation, globular clusters, the central supermassive black hole and galactic bulge of metal-poor Population II stars form. The supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of accumulated matter.[58]

I Zwicky 18 (lower left) is a recently-formed galaxy that may still be producing its first generation of stars.[59] NASA/ESA Hubble Space Telescope image.
During the following two billion years, the remaining material settles into a galactic disk.[60] Galaxies will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life;[61] the cycle of stellar birth and death will increase the abundance of heavy elements, eventually allowing the formation of planets.[62]
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.[63] 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 will produce a long train of stars, similar to that seen in NGC 250[64] or the Antennae Galaxies.[65]
Studies show that the Milky Way galaxy is moving towards the nearby Andromeda Galaxy at about 130 km/s, and depending upon the lateral movements, the two may 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.[66]
As time passed, the merging of large galaxies steadily declined until it had virtually ceased after about six billion years. Most bright galaxies have retained their form since that time, and only a small percentage are now peculiar. The net formation of stars also peaked at about two billion years after the big bang and has steadily decreased since then. In fact the majority of star formation now occurs in smaller galaxies where the cool gas has not been as depleted.[63]
Spiral galaxies, like the Milky Way, only produce new generations of stars as long as they continue to have dense molecular clouds of interstellar hydrogen in their spiral arms.[67] Elliptical galaxies are already largely devoid of this gas and so form no new stars.[68] However, the supply of star-forming material is finite; once stars have converted the available supply of hydrogen into heavier elements, new star will come to an end.[69]
After the end of stellar formation in under one hundred billion years, the "stellar age" will come to an end after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest longest-lived stars in our astrosphere, tiny red dwarfs begin to fade. At the end of the stellar age galaxies will comprise compact objects: brown dwarfs, black dwarfs, cooling white dwarfs, neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into the central supermassive black hole of the galaxies, or be flung into the depths of intergalactic space as a result of collisions.[70][69]
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Larger scale structures
Deep sky surveys of galaxies have demonstrated that they are often found in relatively close association with other galaxies. Solitary galaxies, which may be defined as galaxies that have not significantly interacted with another galaxy of comparable mass during the past billion years (1 Gyr), 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 can still be orbited by smaller, satellite galaxies. Isolated galaxies[a] can produce stars at a higher rate than normal, as their gas is not being stripped by other, nearby galaxies.[71]
On the largest scale the universe is continually expanding, resulting in an average increase in the separation between individual galaxies. (See Hubble's law.) However, 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 million K.[72] 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.[73]

Seyfert's Sextet is an example of a compact galaxy group. NASA Hubble Space Telescope image.
Most galaxies in the universe are gravitationally bound to a number of other galaxies. These form a fractal-like heirarchy 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.[74][75] 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.[76]
Larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters. 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.[77]
Superclusters are giant collections containing tens of thousands of galaxies, found in clusters, groups and sometimes individually; at the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids.[78] Above this scale, the universe appears to be isotropic and homogeneous.[79]
Our 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.[80] 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 around the Virgo Cluster.[81]
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Multi-wavelength observation
Since galaxies external to the Milky Way were found to exist, much of the initial observation of these objects was been performed in the visual portion of the electromagnetic spectrum. The peak radiation of most stars lies in this band, 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 HII regions, and for examining the distribution of dusty arms.
Unfortunately the dust present in the interstellar medium is opaque to visual light, so it is necessary to use the infrared part of the spectrum to observe these regions. The dust is more transparent to far-infrared, allowing the interior regions of giant molecular clouds and galactic cores to be observed in great detail.[82] Infrared is also beneficial for the observation of distant, red-shifted galaxies that were formed much earier in the universe. Unfortunately water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, requiring high-altitude or space-based telescope to perform infrared astronomy.

A radio map of the galaxy Centaurus A (upper left and lower right) is overlaid across the optical image (center), showing two lobes from the jets being generated by an active nucleus. (NASA image.)
The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies. The atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. (The ionosphere blocks signals below this range.)[83] 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 centimeter radiation), including, potentially, the non-ionized matter in the early universe that would later collapse to form galaxies.[84]
Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomenon. An ultraviolet flare has been observed as a star in a distant galaxy was torn apart from the tidal forces of a black hole.[85] X-rays emitted by hot gas in galactic clusters allows the distribution of this material to be mapped. The existence of super-massive black holes at the cores of galaxies was confirmed through X-ray astronomy.[86]
See also: Observational astronomy
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See also
List of galaxies
List of nearest galaxies
Timeline of galaxies, clusters of galaxies, and large scale structure
[edit]

Notes
^ 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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References
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General references:
Dickinson, Terence (2004). The Universe and Beyond, 4th, Firefly Books Ltd.. ISBN 1552979016.
James Binney, Michael Merrifield (1998). Galactic Astronomy. Princeton University Press. ISBN 0691004021.