NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. NGC 4414 is an Unbarred spiral galaxy about 62 million Light-years away in the Constellation Coma Berenices. In common usage a constellation is a group of celestial bodies that are connected together in some arrangement typically stars to form a visible figure or picture Coma Berenices ( Berenice 's Hair) is a traditional asterism that has since become a Constellation. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 Credit:Hubble Space TelescopeNASA/ESA. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

A galaxy is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter. Gravitation is a natural Phenomenon by which objects with Mass attract one another A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from ^[1][2] The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way galaxy. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply 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. A dwarf galaxy is a small Galaxy composed of up to several billion Stars a small number compared to our own Milky Way 's 200-400 billion stars Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. A star system or stellar system is a small number of Stars which orbit each other bound by gravitational attraction. Star clusters are groups of Stars which are gravitationally bound Interstellar cloud is the generic name given to an accumulation of gas plasma and dust in our and other galaxies. 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. The Sun (Sol is the Star at the center of the Solar System. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply The Solar System consists of the Sun and those celestial objects bound to it by Gravity.

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. An elliptical galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a Spiral galaxies are disk-shaped assemblages with curving, dusty arms. 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 Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. A peculiar galaxy is a Galaxy which is unusual in its size shape or composition 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. Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. A starburst galaxy is a Galaxy in the process of an exceptionally high rate of Star formation, compared to the usual star formation rate seen in most galaxies Small galaxies that lack a coherent structure could also be referred to as irregular galaxies. An irregular galaxy is a Galaxy that does not fall into any of the regular classes of the Hubble sequence. ^[6]

There are probably more than 100 billion (10¹¹) galaxies in the observable universe. In Big Bang Cosmology, the observable universe is the region of space bounded by a Sphere, centered on the observer that is small enough that ^[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). History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 ^[8] Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. Intergalactic space is the physical space between galaxies. Generally free of dust and debris intergalactic space is very close to a total Vacuum. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny CM3 redirects here If you were looking for the 3rd game in the Cooking Mama series abbreviated as CM3 see here. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the Cosmos. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe. The Great Wall (also called Coma Wall) sometimes specifically referred to as the CfA2 Great Wall, is the second largest known super-structure in the In Physical cosmology, filaments are the largest known structures in the Universe, thread-like structures with a typical length of 50 to 80 ''h''-1 In Astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe, that contain very few or no galaxies The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy ^[9]

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic The Milky Way galaxy appears to harbor at least one such object within its nucleus. ^[10]

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. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly 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. 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 Zeus (zjuːs in Greek: nominative: Zeús /zdeús/ genitive: Diós; Modern Greek /'zefs/ in Greek mythology In Greek mythology, Heracles or Herakles ("glory of Hera " or In the Olympian pantheon of classical Greek Mythology, Hera (ˈhɪərə or /ˈhɛrə/ Greek) or Here ( in Ionic and Homer 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]

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. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply

The term Milky Way first appeared in the English language in a poem by Chaucer. Geoffrey Chaucer (c 1343 – 25 October 1400? was an English author poet Philosopher, bureaucrat, courtier and Diplomat.

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

—Geoffrey Chaucer, Geoffrey Chaucer The House of Fame, c. The House of Fame is a poem by Geoffrey Chaucer, it is one of his early works probably written between 1379 and 1380. 1380. ^[12]

When William Herschel constructed his catalog of deep sky objects, he used the name spiral nebula for certain objects such as M31. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British 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 The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda 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]

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. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of

The Milky Way

The Greek philosopher Democritus (450–370 B. Ancient Greek philosophy focused on the role of Reason and Inquiry. Democritus ( Greek:) was a pre-Socratic Greek Materialist Philosopher (born at Abdera in Thrace ca C. ) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[14] The Persian astronomer Abū Rayhān al-Bīrūnī (973-1048 A. D. ) likewise proposed the Milky Way galaxy to be a collection of countless nebulous stars. A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of ^[15] 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. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher An optical telescope is a Telescope which is used to gather and focus light mainly from the visible part of the Electromagnetic spectrum ^[16] 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. Immanuel Kant (ɪmanuəl kant 22 April 1724 12 February 1804 was an 18th-century German Philosopher from the Prussian city of Königsberg Thomas Wright (1711 - 1786 was an English Astronomer, Mathematician, instrument maker Architect and garden designer Gravitation is a natural Phenomenon by which objects with Mass attract one another 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. A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of ^[17]

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. The Sun (Sol is the Star at the center of the Solar System. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British He produced a diagram of the shape of the galaxy with the solar system close to the center. ^[18][19] 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. Jacobus Cornelius Kapteyn, ( January 19, 1851 &ndash June 18, 1922) was a Dutch astronomer, best known for his extensive 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. Harlow Shapley ( November 2 1885 &ndash October 20 1972) was an American Astronomer. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. ^[17] 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. Extinction is a term used in Astronomy to describe the absorption and Scattering of Electromagnetic radiation emitted by Astronomical objects The galactic coordinate system is a Celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy Robert Julius Trumpler ( October 2 1886 &ndash September 10 1956, Berkeley, California) was a Swiss - American An open cluster is a group of up to a few thousand Stars that were formed from the same Giant molecular cloud, and are still loosely gravitationally ^[20]

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. The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an interacting Spiral galaxy located at a distance of William Parsons 3rd Earl of Rosse KP ( June 17, 1800 &ndash October 31, 1867) built the world's largest telescope in 1845 and it Charles Messier ( June 26, 1730 &ndash April 12, 1817) was a French astronomer most notable for publishing an The Messier objects are a set of Astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses ^[17] In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. William Parsons 3rd Earl of Rosse KP ( June 17, 1800 &ndash October 31, 1867) built the world's largest telescope in 1845 and it He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture. ^[21]

In 1917, Heber Curtis had observed the nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). Heber Doust Curtis ( June 27 1872 – January 9 1942) was an American Astronomer. |- style="background-color #A0B0FF" colspan="3"| Database References |- bgcolor="#FFFAFA" | Simbad || Data|- bgcolor="#FFFAFA" Andromeda ( is a Constellation named for the princess Andromeda ( Greek Ανδρομέδη = guardian of the men) a character in The Messier objects are a set of Astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Searching the photographic record, he found 11 more novae. A nova (pl novae or novas) is a Cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a White Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. In Astronomy, magnitude refers to the Logarithmic measure of the brightness of an object measured in a specific Wavelength or Passband, usually As a result he was able to come up with a distance estimate of 150,000 parsecs. History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies. ^[22]

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. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda This is about the infamous discussion of astronomy For the Dream Theater song about Stem cell research, see The Great Debate (song. Harlow Shapley ( November 2 1885 &ndash October 20 1972) was an American Astronomer. 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. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for ^[23]

The matter was conclusively settled by Edwin Hubble in the early 1920s using a new telescope. Edwin Powell Hubble ( November 20, 1889 – September 28, 1953) was an American astronomer. 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. A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation ^[24] In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence. Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance ^[25]

Modern research

In 1944 Hendrik van de Hulst predicted microwave radiation at a wavelength of 21 cm resulting from interstellar atomic hydrogen gas;^[26] this radiation was observed in 1951. Hendrik Christoffel "Henk" van de Hulst ( Utrecht, Nov 19 1918 &ndash Leiden, Jul 31 2000) was a Dutch Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 The hydrogen line, 21 centimeter line or HI line refers to the Spectral line created by changes in the energy state of neutral Hydrogen and Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 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. A barred spiral galaxy is a Spiral galaxy with a central bar-shaped structure composed of Stars Bars are found in approximately half of all spiral galaxies ^[27] With improved radio telescopes, hydrogen gas could also be traced in other galaxies. A radio telescope is a form of directional Radio antenna used in Radio astronomy and in tracking and collecting data from Satellites

Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy 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. Vera (Cooper Rubin (born 23 July 1928) is an Astronomer who has done pioneering work on Galaxy rotation rates The rotation curve of a Galaxy can be represented by a graph that plots the Orbital velocity of the Stars or Gas in the galaxy This galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from ^[28]

Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into Among other things, it established that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars. ^[29] 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. The Hubble Deep Field (HDF is an image of a small region in the Constellation Ursa Major, based on the results of a series of observations by the Hubble ^[30] 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. The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies X-ray astronomy is an observational branch of Astronomy, which deals with the study of X-ray emission from celestial objects 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. The Zone of Avoidance (ZOA is the area of the night sky that is obscured by our own Galaxy, the Milky Way. ^[31]

Types and morphology

Main article: Galaxy morphological classification

Types of galaxies according to the Hubble classification scheme. Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance 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. Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance 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). Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic ^[6]

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. An elliptical galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. An ellipsoid is a type of quadric surface that is a higher dimensional analogue of an Ellipse. 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. An open cluster is a group of up to a few thousand Stars that were formed from the same Giant molecular cloud, and are still loosely gravitationally Instead the galaxy is dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. Stellar evolution is the process by which a Star undergoes a sequence of radical changes during its lifetime In this sense they have some similarity to the much smaller globular clusters. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. ^[32]

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. Interacting galaxies ( Colliding galaxies) are the result of one Galaxy 's Gravity disturbing another galaxy 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. ^[33] Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy. A starburst galaxy is a Galaxy in the process of an exceptionally high rate of Star formation, compared to the usual star formation rate seen in most galaxies ^[32]

Spirals

Main articles: Spiral galaxy and Barred spiral galaxy

The Sombrero Galaxy, an example of an unbarred spiral galaxy. 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 A barred spiral galaxy is a Spiral galaxy with a central bar-shaped structure composed of Stars Bars are found in approximately half of all spiral galaxies The Sombrero Galaxy (also known as M 104 or NGC 4594) is an unbarred Spiral galaxy in the Constellation Credit:Hubble Space Telescope/NASA/ESA. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

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 Astronomy, a bulge is a tightly packed group of Stars The term almost exclusively refers to the central group of stars found in most spiral galaxies. 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. ^[34]

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. Definition In Polar coordinates ( r, θ the curve can be written as r = ae^{b\theta}\ or \theta Like the stars, the spiral arms also rotate around the center, but they do so with constant angular velocity. Do not confuse with Angular frequency The unit for angular velocity is rad/s 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.

NGC 1300, an example of a barred spiral galaxy. NGC 1300 is a Barred spiral galaxy about 69 million Light-years away in the Constellation Eridanus and is part of the Eridanus Credit:Hubble Space Telescope/NASA/ESA. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

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. ^[35] 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. A galactic tide is a Tidal force subjected on objects by the Gravitational field of a Galaxy such as the Milky Way. ^[36] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms. ^[37]

Our own galaxy is a large disk-shaped barred-spiral galaxy^[38] about 30 kiloparsecs in diameter and a kiloparsec in thickness. It contains about two hundred billion (2×10¹¹)^[39] stars and has a total mass of about six hundred billion (6×10¹¹) times the mass of the Sun. ^[40]

Other morphologies

Hoag's Object, an example of a ring galaxy. Hoag's 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 ring consists of massive relatively young blue Stars which are extremely bright Credit:Hubble Space Telescope/NASA/ESA. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

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 a Galaxy with a ring-like appearance The ring consists of massive relatively young blue Stars which are extremely bright A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy. ^[41] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of ^[42]

A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. A lenticular galaxy is a type of Galaxy which is intermediate between an Elliptical galaxy and a Spiral galaxy in Galaxy morphological classification These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars. ^[43] (Barred lenticular galaxies receive Hubble classification SB0. A barred lenticular galaxy is a lenticular version of a Barred spiral galaxy. )

NGC 5866, an example of a lenticular galaxy. NGC 5866 (also called the Spindle Galaxy) is a relatively bright Lenticular galaxy in the Constellation Draco. A lenticular galaxy is a type of Galaxy which is intermediate between an Elliptical galaxy and a Spiral galaxy in Galaxy morphological classification 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. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member 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. ^[44] Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds. History The Magellanic Clouds were certainly known since the earliest times by the ancient Middle Eastern peoples

Dwarfs

Main article: Dwarf galaxy

Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. A dwarf galaxy is a small Galaxy composed of up to several billion Stars a small number compared to our own Milky Way 's 200-400 billion stars 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. ^[45]

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. ^[46] Dwarf galaxies may also be classified as elliptical, spiral, or irregular. Dwarf elliptical galaxies, or dE's are elliptical galaxies that are much smaller than others classified as dE. A dwarf spiral galaxy is the dwarf version of a Spiral galaxy. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead. Dwarf spheroidal galaxy ( dSph) is a term in Astronomy applied to low luminosity Galaxies that are Companions to the Milky

Unusual dynamics and activities

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. Interacting galaxies ( Colliding galaxies) are the result of one Galaxy 's 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 Hence interactions between these galaxies are relatively frequent, and play an important role in their evolution. The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning the formation of the first galaxies the way Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust. A galactic tide is a Tidal force subjected on objects by the Gravitational field of a Galaxy such as the Milky Way. ^[47][48]

The Antennae Galaxies are undergoing a collision that will result in their eventual merger. The Antennae Galaxies (also known as NGC 4038 / NGC 4039) are a pair of interacting galaxies in the Constellation Corvus Credit:Hubble Space TelescopeNASA/ESA. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

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. ^[47][48]

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. Interacting galaxies ( Colliding galaxies) are the result of one Galaxy 's Gravity disturbing another galaxy 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. The Sagittarius Dwarf Elliptical Galaxy ( SagDEG) is an elliptically looped shaped 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. ^[47][48]

Starburst

Main article: Starburst galaxy

M82, the archetype starburst galaxy, has experienced a 10-fold increase^[49] in star formation rate as compared to a "normal" galaxy. A starburst galaxy is a Galaxy in the process of an exceptionally high rate of Star formation, compared to the usual star formation rate seen in most galaxies Messier 82 (also known as NGC 3034 or the Cigar Galaxy) is the prototype nearby Starburst galaxy about 12 million Light-years Credit:Hubble Space TelescopeNASA/ESA//STScI. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member The Space Telescope Science Institute ( STScI) is the science operations center for the Hubble Space Telescope (HST in orbit since 1990 and for the James

Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds. See also Solar nebula A molecular cloud, sometimes called a stellar nursery if Star formation is occurring within is a type of Interstellar 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,^[50] and, at present, still contribute an estimated 15% to the total star production rate. ^[51]

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. An H II region (also known as Emission nebula) is a cloud of glowing Gas and plasma, sometimes several hundred Light-years across ^[52] These massive stars also produce supernova explosions, resulting in expanding remnants that interact powerfully with the surrounding gas. A supernova (plural supernovae or supernovas) is a stellar Explosion. A supernova remnant ( SNR) is the structure resulting from the gigantic explosion of a Star in a Supernova. 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. ^[50]

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. Messier 82 (also known as NGC 3034 or the Cigar Galaxy) is the prototype nearby Starburst galaxy about 12 million Light-years Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a Spiral galaxy about 12 million Light-years away in the Constellation Irregular galaxies often exhibit spaced knots of starburst activity. ^[53]

Active nucleus

Main article: Active galactic nucleus

A portion of the galaxies we can observe are classified as active. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic 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 disc that forms around a supermassive black hole (SMBH) at the core region. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc. Gravitational energy is the energy associated with the gravitational field ^[54] 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. ^[55]

A jet of particles is being emitted from the core of the elliptical radio galaxy M87. Messier 87 (also known as M87, Virgo A or NGC 4486) is a giant Elliptical galaxy. Credit:Hubble Space TelescopeNASA/ESA. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

Active galaxies that emit high-energy radiation in the form of x-rays are classified as Seyfert galaxies or quasars, depending on the luminosity. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Seyfert galaxies are a class of galaxies with nuclei that produce Spectral line emission from highly Ionized gas named after Carl Keenan Seyfert A quasar (contraction of QUASi-stellAR radio source) is an extremely powerful and distant Active galactic nucleus. Blazars are believed to be an active galaxy with a relativistic jet that is pointed in the direction of the Earth. A blazar is a very compact and highly variable energy source associated with a presumed Supermassive black hole at the center of a Host galaxy. The lower-energy non-relativistic version of this phenomenon is described at Polar jet. A radio galaxy emits radio frequencies from relativistic jets. Radio galaxies and their relatives radio-loud Quasars and Blazars, are types of Active galaxy that are very luminous at radio wavelengths (up A unified model of these types of active galaxies explains their differences based on the viewing angle of the observer. ^[55]

Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear emission-line regions (LINERs). A low-ionization nuclear emission-line region (LINER is a type of galactic nucleus that is defined by its spectral line emission. The emission from LINER-type galaxies is dominated by weakly-ionized elements. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge ^[56] Approximately one-third of nearby galaxies are classified as containing LINER nuclei. ^[54][56][57]

Formation and evolution

Main article: Galaxy 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. The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning the formation of the first galaxies the way Some theories in this field have now become widely accepted, but it is still an active area in astrophysics. Astrophysics is the branch of Astronomy that deals with the Physics of the Universe, including the physical properties ( Luminosity,

Formation

Current cosmological models of the early Universe are based on the Big Bang theory. The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. About 300,000 years after this event, atoms of hydrogen and helium began to form, in an event called recombination. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical 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". This timeline of the Big Bang describes the events according to the Scientific theory of the Big Bang, using the cosmological time parameter of Comoving coordinates It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. Anisotropy (pronounced with stress on the third syllable ˌænaɪˈsɒtrəpi is the property of being directionally dependent as opposed to Isotropy, which means homogeneity Structure formation refers to a fundamental problem in Physical cosmology. As a result, masses of baryonic matter started to condense within cold dark matter halos. Baryons are the family of Subatomic particles with a Baryon number of 1 In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from ^[58] These primordial structures would eventually become the galaxies we see today.

Evidence for the early appearance of galaxies was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6. IOK-1, probably one of the oldest and most distant Galaxy yet found - seen as it was 12 In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by 96, corresponding to just 750 million years after the Big Bang and making it the most distant and primordial galaxy yet seen. ^[59] 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. Abell 1835 IR1916 (also known as Abell 1835, Galaxy Abell 1835, or Galaxy Abell 1835 IR1916) is a candidate for being the most distant Galaxy The existence of such early protogalaxies suggests that they must have grown in the so-called "Dark Ages". In Physical cosmology, a protogalaxy, which could also be called a "primeval galaxy" is a Cloud of Gas which is forming into a Galaxy ^[58]

The detailed process by which such early galaxy formation occurred is a major open question in astronomy. Theories could 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. ^[60] In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. ^[61] Modern theories must be modified to account for the probable presence of large dark matter halos.

Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen These were composed almost entirely of hydrogen and helium, and may have been massive. If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium. A supernova (plural supernovae or supernovas) is a stellar Explosion. ^[62] This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel. ^[63]

Evolution

I Zwicky 18 (lower left) resembles a newly-formed galaxy. I Zwicky 18 is a Dwarf irregular galaxy located about 59 million Light years away ^[64]. ^[65] Credit:Hubble Space Telescope/NASA/ESA. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

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. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. In Astronomy, a bulge is a tightly packed group of Stars The term almost exclusively refers to the central group of stars found in most spiral galaxies. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen 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. ^[66] During this early epoch, galaxies undergo a major burst of star formation. ^[67]

During the following two billion years, the accumulated matter settles into a galactic disc. A disc is a component of Disc galaxies, such as Spiral galaxies, or Lenticular galaxies. ^[68] A galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life. Interstellar cloud is the generic name given to an accumulation of gas plasma and dust in our and other galaxies. A dwarf galaxy is a small Galaxy composed of up to several billion Stars a small number compared to our own Milky Way 's 200-400 billion stars ^[69] 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. In Cosmogony, the nebular hypothesis is the most widely accepted model explaining the Formation and evolution of the Solar System. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is ^[70]

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. ^[71] 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^[72] or the Antennae Galaxies. NGC 4676, or the Mice Galaxies, are two spiral galaxies in the Constellation Coma Berenices. The Antennae Galaxies (also known as NGC 4038 / NGC 4039) are a pair of interacting galaxies in the Constellation Corvus ^[73]

As an example of such an interaction, 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 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. ^[74]

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common. Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked approximately ten billion years ago. ^[75]

Future trends

At present, most star formation occurs in smaller galaxies where cool gas is not so depleted. ^[71] Spiral galaxies, like the Milky Way, only produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms. See also Solar nebula A molecular cloud, sometimes called a stellar nursery if Star formation is occurring within is a type of Interstellar ^[76] Elliptical galaxies are already largely devoid of this gas, and so form no new stars. ^[77] 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. ^[78]

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (10¹³–10¹⁴ years), as the smallest, longest-lived stars in our astrosphere, tiny red dwarfs, begin to fade. According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool Star, of the Main sequence, either late K 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. In Astronomy, the term compact star (sometimes compact object) is used to refer collectively to White dwarfs Neutron stars other exotic Brown dwarfs are sub- stellar objects with a mass below that necessary to maintain Hydrogen -burning Nuclear fusion reactions in their cores as do stars A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. A black dwarf is a hypothetical Star, created when a White dwarf becomes sufficiently cool to no longer emit significant Heat or Light A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e 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. Relaxation time is a general concept in Physics for the Characteristic time in which a system changes to an Equilibrium condition from a non-equilibrium ^[79][78]

Larger scale structures

Main articles: Large-scale structure of the cosmos and Groups and clusters of galaxies

Deep sky surveys show that galaxies are often found in relatively close association with other galaxies. In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation Solitary galaxies that have not significantly interacted with another galaxy of comparable mass during the past billion years are relatively scarce. Only about 5% of the galaxies surveyed have been found to be truly isolated; however, these isolated formations may have interacted and even merged with other galaxies in the past, and may still be orbited by smaller, satellite galaxies. Isolated galaxies^[b] can produce stars at a higher rate than normal, as their gas is not being stripped by other, nearby galaxies. ^[80]

On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see Hubble's law). Hubble's law is the statement in Physical cosmology that the Redshift in light coming from distant galaxies is proportional to their distance 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. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic ^[81] 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. ^[82]

Seyfert's Sextet is an example of a compact galaxy group. Seyfert's Sextet is a group of galaxies about 190 million Light-years away in the Constellation Serpens Caput. Credit:Hubble Space Telescope/NASA/ESA. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member

Most galaxies in the universe are gravitationally bound to a number of other galaxies. These form a fractal-like hierarchy of clustered structures, with the smallest such associations being termed groups. A fractal is generally "a rough or fragmented geometric shape that can be split into parts each of which is (at least approximately a reduced-size copy of the whole" 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. Baryons are the family of Subatomic particles with a Baryon number of 1 ^[83][84] 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). In Mechanics, the virial theorem provides a general equation relating the average total Kinetic energy, \left\langle T \right\rangle of a stable system If there is insufficient kinetic energy, however, the group may evolve into a smaller number of galaxies through mergers. The kinetic energy of an object is the extra Energy which it possesses due to its motion ^[85]

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. Brightest cluster galaxy (BCG is defined as the brightest galaxy in a cluster of galaxies The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced ^[86]

Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. Superclusters are large groupings of smaller galaxy groups and clusters, and are among the largest structures of the Cosmos. At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids. In Physical cosmology, the term large-scale structure refers to the characterization of observable distributions of Matter and Light ^[87] Above this scale, the universe appears to be isotropic and homogeneous. Isotropy is uniformity in all directions Precise definitions depend on the subject area ^[88]

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 Local Group is the group of galaxies that includes our galaxy the Milky Way. 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. ^[89] 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. The Virgo Supercluster or Local Supercluster is the galactic Supercluster that contains the Local Group, the latter containing in its turn The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 Mly (18 ^[90]

Multi-wavelength observation

See also: Observational 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. Observational astronomy is a division of the astronomical Science that is concerned with getting data in contrast with Theoretical astrophysics which is Centaurus A (also known as NGC 5128) is a Lenticular galaxy about 14 million Light-years away in the Constellation Centaurus Credit:NASA. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program

After galaxies external to the Milky Way were found to exist, initial observations were made mostly using visible light. The peak radiation of most stars lies here, 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. An H II region (also known as Emission nebula) is a cloud of glowing Gas and plasma, sometimes several hundred Light-years across

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. Far infrared astronomy is the branch of Astronomy and Astrophysics which deals with objects visible in Far-infrared radiation (extending from 30 ^[91] Infrared is also used to observe distant, red-shifted galaxies that were formed much earlier in the history of the universe. In Physics and Astronomy, redshift occurs when Electromagnetic radiation – usually Visible light – emitted or reflected by 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. Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single Infrared astronomy is the branch of Astronomy and Astrophysics which deals with objects visible in Infrared (IR radiation

The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies. Radio astronomy is a subfield of Astronomy that studies celestial objects at radio frequencies. The atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. (The ionosphere blocks signals below this range. The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation )^[92] Large radio interferometers have been used to map the active jets emitted from active nuclei. Interferometry is the technique of using the pattern of Interference created by the superposition of two or more Waves to diagnose the properties of Radio telescopes can also be used to observe neutral hydrogen (via 21 centimetre radiation), including, potentially, the non-ionized matter in the early universe that later collapsed to form galaxies. A radio telescope is a form of directional Radio antenna used in Radio astronomy and in tracking and collecting data from Satellites The hydrogen line, 21 centimeter line or HI line refers to the Spectral line created by changes in the energy state of neutral Hydrogen and ^[93]

Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet astronomy is generally used to refer to observations at Ultraviolet wavelengths between approximately 10 and 320 nanometres X-ray astronomy is an observational branch of Astronomy, which deals with the study of X-ray emission from celestial objects An ultraviolet flare was observed when a star in a distant galaxy was torn apart from the tidal forces of a black hole. ^[94] The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of super-massive black holes at the cores of galaxies was confirmed through X-ray astronomy. ^[95]

See also

• List of galaxies
• List of nearest galaxies
• Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure

Notes

References

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.  

External links

• Galaxies, SEDS Messier pages
• An Atlas of The Universe
• Galaxies — Information and amateur observations
• The Oldest Galaxy Yet Found
• Galaxies — discussed on BBC Radio 4's "In Our Time" programme
• Galaxy classification project, harnessing the power of the internet and the human brain