NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. Credit:Hubble Space TelescopeNASA/ESA.

A galaxy (from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way) is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter.^[1][2] Typical galaxies range from dwarfs with as few as ten million^[3] (10⁷) stars up to giants with one trillion^[4] (10¹²) stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun. Galaxy (Greek γαλαξίας, galaxías [male noun] - the milky [nebula], Milky Way) can refer to: Galaxy, a celestial body Media: Galaxy (Australian TV), a former cable television and satellite television company in Australia Galaxy (television), the Galaxy Channel operated by British Satellite Broadcasting Galaxy 102, a Manchester, UK radio station Galaxy... Credit: NASA Headquarters - GReatest Images of NASA (NASA-HQ-GRIN) In 1995, the majestic spiral galaxy NGC 4414 was imaged by the Hubble Space Telescope as part of the HST Key Project on the Extragalactic Distance Scale. ... Credit: NASA Headquarters - GReatest Images of NASA (NASA-HQ-GRIN) In 1995, the majestic spiral galaxy NGC 4414 was imaged by the Hubble Space Telescope as part of the HST Key Project on the Extragalactic Distance Scale. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant. ... This article is about the star grouping. ... Coma Berenices (IPA: , Latin: ) is a traditional asterism that has since become a constellation. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ... The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ... For other uses, see NASA (disambiguation). ... This article is about the European Space Agency. ... For other uses, see Milky Way (disambiguation). ... Gravity redirects here. ... This article is about the astronomical object. ... The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ... “Space dust” redirects here. ... For other uses, see Dark matter (disambiguation). ... A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Ways 200-400 billion stars. ... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it were concentrated. ... A star system or stellar system is a small number of stars that orbit each other,[1] bound by gravitational attraction. ... This article or section does not cite its references or sources. ... Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies. ... Sol redirects here. ... For other uses, see Milky Way (disambiguation). ... This article is about the Solar System. ...

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy,^[5] which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.^[6] The giant elliptical galaxy ESO 325-G004. ... For other uses, see Ellipse (disambiguation). ... An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457) A spiral galaxy is a galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Nebulae”[1] and, as... A peculiar galaxy is a galaxy which is unusual in its size, shape, or composition. ... Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. ... The Antennae Galaxies are an example of a very high starburst galaxy occurring from the collision of NGC 4038/NGC 4039. ... This article or section does not adequately cite its references or sources. ...

There are probably more than 100 billion (10¹¹) galaxies in the observable universe.^[7] Most galaxies are 1,000 to 100,000^[4] parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs).^[8] Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.^[9] See universe for a general discussion of the universe. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ... Intergalactic redirects here. ... For other uses, see Atom (disambiguation). ... The cubic metre (symbol m³) is the SI derived unit of volume. ... The galaxies of HCG 87, about four hundred million light-years distant. ... Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the cosmos. ... The Great Wall, sometimes more specifically referred to as the CfA2 Great Wall, is the second largest known super-structure in the Universe (the largest being Sloan Great Wall). ... In astronomy, filaments are one of the largest known structures in the Universe, thread-like structures with a typical length of 70 to 150 megaparsec that form the boundaries between large voids in the universe. ... In astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe that contain very few, or no, galaxies. ... For other uses, see Universe (disambiguation). ...

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.^[10] For other uses, see Dark matter (disambiguation). ... For other uses, see Mass (disambiguation). ... Top: artists conception of a supermassive black hole tearing apart a star. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ...

[edit] Etymology

The word galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας), or kyklos galaktikos, meaning "milky circle" for its appearance 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 immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away and a jet of her milk sprays the night sky, producing the faint band of light known as the Milky Way.^[11] The bust of Zeus found at Otricoli (Sala Rotonda, Museo Pio-Clementino, Vatican) Greek mythology is the body of stories belonging to the Ancient Greeks concerning their gods and heroes, the nature of the world and the origins and significance of their own cult and ritual practices. ... For other uses, see Zeus (disambiguation). ... Alcides redirects here. ... For other uses, see Hera (disambiguation). ...

In the astronomical literature, the capitalized word 'Galaxy' is used to refer to our (Milky Way) galaxy, to distinguish it from the billions of other galaxies. For other uses, see Milky Way (disambiguation). ...

The term Milky Way first appeared in the English language in a poem by Chaucer. Chaucer redirects here. ...

"See yonder, lo, the Galaxyë
 Which men clepeth the Milky Wey,
 For hit is whyt."

—Geoffrey Chaucer, Geoffrey Chaucer The House of Fame, c. 1380.^[12]

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, when the true distance to these objects began to be 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.^[13] The House of Fame is a poem by Geoffrey Chaucer, it is one of his early works, probably written between 1379 and 1380. ... For other persons named William Herschel, see William Herschel (disambiguation). ... Spiral nebula is an old term for a spiral galaxy. ... The Andromeda Galaxy (IPA: , also known as Messier 31, M31, or NGC 224; older texts often called it the Great Andromeda Nebula) is a spiral galaxy approximately 2. ...

[edit] Observation history

The realization that we live in a galaxy, and that there were, in fact, many other galaxies, parallel discoveries that were made about the Milky Way and other nebulae in the night sky. For other uses, see Milky Way (disambiguation). ... The Triangulum Emission Nebula NGC 604 The Pillars of Creation from the Eagle Nebula For other uses, see Nebula (disambiguation). ...

[edit] The Milky Way

The Greek philosopher Democritus (450–370 B.C.) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.^[14] Actual proof of this came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.^[15] 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 a much larger scale. The resulting disk of stars can 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.^[16] A philosopher is a person who thinks deeply regarding people, society, the world, and/or the universe. ... ‎ Democritus (Greek: ) was a pre-Socratic Greek materialist philosopher (born at Abdera in Thrace ca. ... Galileo redirects here. ... Eight Inch refracting telescope. ... Kant redirects here. ... See also : Thomas Wright (disambiguation) Categories: Stub | 1711 births | 1786 deaths | British astronomers ... Gravity redirects here. ... The Triangulum Emission Nebula NGC 604 The Pillars of Creation from the Eagle Nebula For other uses, see Nebula (disambiguation). ...

The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the solar system was assumed to be near the center.

The first attempt to describe the shape of the Milky Way and the position of the Sun in it was carried out by William Herschel in 1785 by carefully 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.^[17][18] 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.^[16] 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 galaxy, the Milky Way, emerged.^[19] Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Sol redirects here. ... For other persons named William Herschel, see William Herschel (disambiguation). ... Jacobus Cornelius Kapteyn, (January 19, 1851 – June 18, 1922) was a Dutch astronomer, best known for his extensive studies of the Milky Way and as the first discoverer of evidence for galactic rotation. ... Harlow Shapley in his earlier years. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... Extinction is a term used in astronomy to describe the absorption of light from astronomical objects by matter between them and the observer. ... “Space dust” redirects here. ... The anisotropy of the star density in the night sky makes the galactic coordinate system very useful for coordinating surveys, both those which require high densities of stars (at low galactic latitudes) and those which require a low density of stars (at high galactic latitudes) Many galaxies, including the Milky... Robert Julius Trumpler (October 2, 1886 – September 10, 1956) was a Swiss-American astronomer. ... Galactic cluster redirects here. ...

[edit] Other nebulae

Sketch of the Whirlpool Galaxy by Lord Rosse in 1845

Toward 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 5,000 nebulae assembled by William Herschel.^[16] 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.^[20] Image File history File links M51Sketch. ... Image File history File links M51Sketch. ... The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an interacting[4] grand-design[5] spiral galaxy located at a distance of approximately 23 million light-years in the constellation Canes Venatici. ... Lord Rosse William Parsons, 3rd Earl of Rosse KP (June 17, 1800 – October 31, 1867) was born in Monkstown, County Cork and was an Irish astronomer. ... Charles Messier Charles Messier (June 26, 1730 – April 12, 1817) was a French astronomer who in 1774 published a catalogue of 45 deep sky objects such as nebulae and star clusters. ... The Messier objects are a set of astronomical objects catalogued by Charles Messier in his catalogue of Nebulae and Star Clusters first published in 1774. ... Lord Rosse William Parsons, 3rd Earl of Rosse KP (June 17, 1800 – October 31, 1867) was born in Monkstown, County Cork and was an Irish astronomer. ...

In 1917, Heber Curtis had observed the nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). 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.^[21] Heber Doust Curtis (June 27, 1872 – January 9, 1942) was an American astronomer. ... S Andromedae (also SN 1885A) was a supernova in the Andromeda Galaxy, the only one seen in that galaxy so far by astronomers, and the first ever noted outside the Milky Way. ... Andromeda (IPA: ) is a constellation named for the princess Andromeda (which is Greek for Ruler over men), a character in Greek mythology. ... The Messier objects are a set of astronomical objects catalogued by Charles Messier in his catalogue of Nebulae and Star Clusters first published in 1774. ... The Andromeda Galaxy (IPA: , also known as Messier 31, M31, or NGC 224; older texts often called it the Great Andromeda Nebula) is a spiral galaxy approximately 2. ... Artists conception of a white dwarf star accreting hydrogen from a larger companion A nova (pl. ... In science, magnitude refers to the numerical size of something: see orders of magnitude. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ...

Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy

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 the Great Andromeda Nebula was 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.^[22] Image File history File links Pic_iroberts1. ... Image File history File links Pic_iroberts1. ... The Andromeda Galaxy (IPA: , also known as Messier 31, M31, or NGC 224; older texts often called it the Great Andromeda Nebula) is a spiral galaxy approximately 2. ... The Andromeda Galaxy in ultraviolet In astronomy, The Great Debate was an influential debate between the astronomers Harlow Shapley and Heber Curtis which concerned the nature of spiral nebulae and the size of the universe. ... Harlow Shapley in his earlier years. ... A source of waves moving to the left. ...

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.^[23] In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.^[24] Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. ... Cepheid in the Spiral Galaxy M100 A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. ... Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). ...

[edit] Modern research

In 1944 Hendrik van de Hulst predicted microwave radiation at a wavelength of 21 cm resulting from interstellar atomic hydrogen gas;^[25] this radiation was observed in 1951. The radiation allowed for much improved study of the Milky Way 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.^[26] With improved radio telescopes, hydrogen gas could also be traced in other galaxies. Hendrik Christoffel van de Hulst (November 19, 1918 – July 31, 2000) was a Dutch astronomer. ... This article is about the type of Electromagnetic radiation. ... This article or section does not adequately cite its references or sources. ... This article is about the chemistry of hydrogen. ... NGC 1300, viewed nearly face-on. ... The 64 meter radio telescope at Parkes Observatory A radio telescope is a form of directional radio antenna used in radio astronomy and in tracking and collecting data from satellites and space probes. ...

Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). The distance is from the galactic core.

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 the 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.^[27] Image File history File links GalacticRotation2. ... Image File history File links GalacticRotation2. ... Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). ... Vera (Cooper) Rubin (born 23 July 1928) is an astronomer who has done pioneering work on galaxy rotation rates. ... Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). ... For other uses, see Dark matter (disambiguation). ...

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.^[28] 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 universe.^[29] 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.^[30] The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ... The Hubble Deep Field The Hubble Deep Field (HDF) is the result of a series of observations by the Hubble Space Telescope of a small region of the northern celestial hemisphere. ... Although some radiations are marked as N for no in the diagram, some waves do in fact penetrate the atmosphere, although extremely minimally compared to the other radiations The electromagnetic (EM) spectrum is the range of all possible electromagnetic radiation. ... ROSAT image of X-ray fluorescence of, and occultation of the X-ray background by, the Moon. ... The Zone of Avoidance is the area of the night sky that is obscured by our own galaxy, the Milky Way. ...

[edit] Types and morphology

Main article: Galaxy morphological classification

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.^[a]

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) and activity in the core (in active galaxies).^[6] Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). ... Image File history File links Hubble_sequence_photo. ... Image File history File links Hubble_sequence_photo. ... Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). ... Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ...

[edit] Ellipticals

Main article: Elliptical galaxy

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.^[31] The giant elliptical galaxy ESO 325-G004. ... 3D rendering of an ellipsoid In mathematics, an ellipsoid is a type of quadric that is a higher dimensional analogue of an ellipse. ... The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ... Galactic cluster redirects here. ... Projected timeline of the Suns life In astronomy, stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ...

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.^[32] Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.^[31] An artists impression of interacting galaxies Interacting galaxies (Colliding galaxies) are the result of one galaxys gravity disturbing another galaxy. ... Wikipedia does not yet have an article with this exact name. ...

[edit] Spirals

Main articles: Spiral galaxy and Barred spiral galaxy

The Sombrero Galaxy, an example of an unbarred spiral galaxy. Credit:Hubble Space Telescope/NASA/ESA.

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.^[33] An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457) A spiral galaxy is a galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Nebulae”[1] and, as... NGC 1300, viewed nearly face-on. ... Image File history File linksMetadata Download high-resolution version (11472x6429, 7288 KB) This image of the Sombrero Galaxy is a mosaic of six images taken by the Hubble Space Telescopes Advanced Camera for Surveys in May and June 2003. ... Image File history File linksMetadata Download high-resolution version (11472x6429, 7288 KB) This image of the Sombrero Galaxy is a mosaic of six images taken by the Hubble Space Telescopes Advanced Camera for Surveys in May and June 2003. ... The Sombrero Galaxy (also known as M104 or NGC 4594) is an unbarred spiral galaxy in the constellation Virgo. ... The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ... For other uses, see NASA (disambiguation). ... This article is about the European Space Agency. ... In astronomy, a bulge is a huge, tightly packed group of stars. ...

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, with stars near the galactic core orbiting faster than the arms are moving while stars near the outer parts of the galaxy typically orbit more slowly than the arms. 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. A logarithmic spiral, equiangular spiral or growth spiral is a special kind of spiral curve which often appears in nature. ... Angular velocity describes the speed of rotation and the orientation of the instantaneous axis about which the rotation occurs. ...

NGC 1300, an example of a barred spiral galaxy. Credit:Hubble Space Telescope/NASA/ESA.

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.^[34] 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.^[35] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.^[36] Image File history File linksMetadata Download high resolution version (6637x3787, 9906 KB) Barred spiral galaxy NGC 1300 Photographed by Hubble telescope In the core of the larger spiral structure of NGC 1300, the nucleus shows its own extraordinary and distinct grand-design spiral structure that is about 3,300 light... Image File history File linksMetadata Download high resolution version (6637x3787, 9906 KB) Barred spiral galaxy NGC 1300 Photographed by Hubble telescope In the core of the larger spiral structure of NGC 1300, the nucleus shows its own extraordinary and distinct grand-design spiral structure that is about 3,300 light... NGC 1300 is a barred spiral galaxy about 69 million light-years away in the constellation Eridanus and is part of the Eridanus Cluster. ... The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ... For other uses, see NASA (disambiguation). ... This article is about the European Space Agency. ... The Andromeda Galaxy. ...

Our own galaxy is a large disk-shaped barred-spiral galaxy^[37] about 30 kiloparsecs in diameter and a kiloparsec in thickness. It contains about two hundred billion (2×10¹¹)^[38] stars and has a total mass of about six hundred billion (6×10¹¹) times the mass of the Sun.^[39]

[edit] Other morphologies

Hoag's Object, an example of a ring galaxy. Credit:Hubble Space Telescope/NASA/ESA.

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.^[40] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.^[41] Image File history File linksMetadata Size of this preview: 612 × 599 pixel Image in higher resolution (1521 × 1489 pixel, file size: 401 KB, MIME type: image/jpeg) An image of Hoags Object, a non-typical galaxy of the type known as a ring galaxy, discovered in 1950 by astronomer... Image File history File linksMetadata Size of this preview: 612 × 599 pixel Image in higher resolution (1521 × 1489 pixel, file size: 401 KB, MIME type: image/jpeg) An image of Hoags Object, a non-typical galaxy of the type known as a ring galaxy, discovered in 1950 by astronomer... Hoags Object is a non-typical galaxy of the type known as a ring galaxy. ... A ring galaxy is a galaxy with a ring-like appearance. ... The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ... For other uses, see NASA (disambiguation). ... This article is about the European Space Agency. ... A ring galaxy is a galaxy with a ring-like appearance. ... The Andromeda Galaxy (IPA: , also known as Messier 31, M31, or NGC 224; older texts often called it the Great Andromeda Nebula) is a spiral galaxy approximately 2. ... For other uses, see Infrared (disambiguation). ...

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.^[42] (Barred lenticular galaxies receive Hubble classification SB0.) The Spindle Galaxy (NGC 5866), a lenticular galaxy in the Draco constellation. ... NGC 2787 is an example of a barred lenticular galaxy A barred lenticular galaxy is a lenticular version of a barred spiral galaxy. ...

NGC 5866, an example of a lenticular galaxy. Credit:Hubble Space Telescope/NASA/ESA

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 do not possess any structure that resembles a Hubble classification, and may have been disrupted.^[43] Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds. Image File history File links Metadata Size of this preview: 509 × 599 pixelsFull resolution (1280 × 1507 pixel, file size: 1. ... Image File history File links Metadata Size of this preview: 509 × 599 pixelsFull resolution (1280 × 1507 pixel, file size: 1. ... The Spindle Galaxy in Draco (also known as the Spindle Galaxy, Lenticular Galaxy NGC 5866 or NGC 5866) is a lenticular galaxy, type S0_3, in the Draco constellation. ... The Spindle Galaxy (NGC 5866), a lenticular galaxy in the Draco constellation. ... The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ... For other uses, see NASA (disambiguation). ... This article is about the European Space Agency. ... The two Magellanic Clouds are irregular dwarf galaxies that may be orbiting our Milky Way galaxy[1], and thus are members of our Local Group of galaxies. ...

[edit] Dwarfs

Main article: Dwarf galaxy

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 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.^[44] A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Ways 200-400 billion stars. ...

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.^[45] 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. Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are much smaller than others, classified as dE. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies. ... A dwarf spiral galaxy is the dwarf version of a spiral galaxy. ... Dwarf spheroidal galaxy (dSph) is a term in astronomy applied to the nine low luminosity dwarf elliptical galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31. ...

[edit] Unusual dynamics and activities

[edit] Interacting

Main article: Interacting galaxy

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.^[46][47] An artists impression of interacting galaxies Interacting galaxies (Colliding galaxies) are the result of one galaxys gravity disturbing another galaxy. ... An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. ... In astrophysics, the questions of galaxy formation and evolution are: How, from a homogeneous universe, did we obtain the very heterogeneous one we live in? How did galaxies form? How do galaxies change over time? A spectacular head-on collision between two galaxies is seen in this NASA Hubble Space... The Andromeda Galaxy. ...

The Antennae Galaxies are undergoing a collision that will result in their eventual merger. Credit:Hubble Space TelescopeNASA/ESA.

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.^[46][47] Image File history File linksMetadata Download high-resolution version (1280x1270, 1479 KB) This new NASA Hubble Space Telescope image of the Antennae galaxies (NGC 4038 & 4039) is the sharpest yet of this merging pair of galaxies. ... Image File history File linksMetadata Download high-resolution version (1280x1270, 1479 KB) This new NASA Hubble Space Telescope image of the Antennae galaxies (NGC 4038 & 4039) is the sharpest yet of this merging pair of galaxies. ... The Antennae Galaxies (also known as NGC 4038/NGC 4039) are a pair of interacting galaxies in the constellation Corvus. ... The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ... For other uses, see NASA (disambiguation). ... This article is about the European Space Agency. ...

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.^[46][47] An artists impression of interacting galaxies Interacting galaxies (Colliding galaxies) are the result of one galaxys gravity disturbing another galaxy. ... The Sagittarius Dwarf Elliptical Galaxy (Sag DEG) is a satellite galaxy of the Milky Way Galaxy. ... The Canis Major Dwarf galaxy is located in the same part of the sky as the constellation of Canis Major. ...

[edit] Starburst

Main article: Starburst galaxy

M82, the archetype starburst galaxy, has experienced a 10-fold increase^[48] in star formation rate as compared to a "normal" galaxy. Credit:Hubble Space TelescopeNASA/ESA//STScI.

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,^[49] and, at present, still contribute an estimated 15% to the total star production rate.^[50] The Antennae Galaxies are an example of a very high starburst galaxy occurring from the collision of NGC 4038/NGC 4039. ... Image File history File linksMetadata M82galaxy. ... Image File history File linksMetadata M82galaxy. ... Messier 82 (also known as NGC 3034 or the Cigar Galaxy) is a starburst galaxy about 12 million light-years away in the constellation Ursa Major. ... The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ... For other uses, see NASA (disambiguation). ... This article is about the European Space Agency. ... The Space Telescope Science Institute (STScI) is an organization founded by NASA to manage and direct research done with the Hubble Space Telescope. ... A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). ...

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.^[51] 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.^[49] NGC 604, a giant H II region in the Triangulum Galaxy. ... For other uses, see Supernova (disambiguation). ... Remnant of Keplers Supernova, SN 1604 Remnant of Tychos Nova, SN 1572 A supernova remnant (SNR) is the structure resulting from the gigantic explosion of a star in a supernova. ...

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.^[52] Messier 82 (also known as NGC 3034 or the Cigar Galaxy) is a starburst galaxy about 12 million light-years away in the constellation Ursa Major. ... Messier 81 (also known as NGC 3031 or Bodes Galaxy) is a spiral galaxy about 12 million light-years away in the constellation Ursa Major. ...

[edit] Active nucleus

Main article: Active galactic 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. An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ...

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. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.^[53] 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.^[54] An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... Artists conception of a binary star system with one black hole and one main sequence star Unsolved problems in physics: Accretion disc jets: Why do the discs surrounding certain objects, such as the nuclei of active galaxies, emit radiation jets along their polar axes? These jets are invoked by... Top: artists conception of a supermassive black hole tearing apart a star. ... Energy of two or more masses (or other forms of energy-momentum) gravitationally interacting with each other. ... The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness.[1] It is the speed of all electromagnetic radiation, including visible light, in a vacuum. ...

A jet of particles is being emitted from the core of the elliptical radio galaxy M87. Credit:Hubble Space TelescopeNASA/ESA.

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.^[54] Download high resolution version (611x638, 41 KB)from http://hubblesite. ... Download high resolution version (611x638, 41 KB)from http://hubblesite. ... M87 (also known as Virgo A, Messier 87 or NGC 4486) is a giant elliptical galaxy. ... The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ... For other uses, see NASA (disambiguation). ... This article is about the European Space Agency. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... Seyfert galaxies are spiral or irregular galaxies containing an extremely bright nucleus, most likely caused by a supermassive black hole, that can sometimes outshine the surrounding galaxy. ... This article is about the astronomical object. ... A blazar is a very compact and highly variable energy source associated with a supermassive black hole a