A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.[1][2] The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars,[3] each orbiting its galaxy's center of mass.

Galaxies are categorized according to their visual morphology as elliptical,[4] spiral, or irregular.[5] Many galaxies are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun.[6] As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang.

Recent estimates of the number of galaxies in the observable universe range from 200 billion (2×1011)[7] to 2 trillion (2×1012) or more,[8][9] containing more stars than all the grains of sand on planet Earth.[10] Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3000 to 300,000 light years) and separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 LY) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million LY).

The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which is dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids.[11] The largest structure of galaxies yet recognised is a cluster of superclusters that has been named Laniakea, which contains the Virgo supercluster.[12]

NGC 4414 (NASA-med)
NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years from Earth.


The origin of the word galaxy derives from the Greek term for the Milky Way, galaxias (γαλαξίας, "milky one"), or kyklos galaktikos ("milky circle")[13] due to its appearance as a "milky" band of light in the sky. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the faint band of light known as the Milky Way.[14][15]

In the astronomical literature, the capitalized word "Galaxy" is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be traced back to a story by Chaucer c. 1380:

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

— Geoffrey Chaucer, The House of Fame[13]

Galaxies were initially discovered telescopically and were known as spiral nebulae. Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called island universes, but this term quickly fell into disuse, as the word universe implied the entirety of existence. Instead, they became known simply as galaxies.[16]


Probing the distant past SDSS J1152+3313
Galaxy cluster SDSS J1152+3313. SDSS stands for Sloan Digital Sky Survey, J for Julian epoch, and 1152+3313 for the declination and right ascension respectivelly.

Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC (New General Catalogue), the IC (Index Catalogue), the CGCG (Catalogue of Galaxies and of Clusters of Galaxies), the MCG (Morphological Catalogue of Galaxies) and UGC (Uppsala General Catalogue of Galaxies). All of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a spiral galaxy having the number 109 in the catalogue of Messier, but also codes NGC3992, UGC6937, CGCG 269-023, MCG +09-20-044, and PGC 37617.

Observation history

The realization that we live in a galaxy which is one among many galaxies, parallels major discoveries that were made about the Milky Way and other nebulae.

Milky Way

The Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.[17] Aristotle (384–322 BCE), however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions."[18] The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 CE) was critical of this view, arguing that if the Milky Way is sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way is celestial.[19]

According to Mohani Mohamed, the Arabian astronomer Alhazen (965–1037) made the first attempt at observing and measuring the Milky Way's parallax,[20] and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere."[21] The Persian astronomer al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars."[22][23] The Andalusian astronomer Ibn Bâjjah ("Avempace", d. 1138) proposed that the Milky Way is made up of many stars that almost touch one another and appear to be a continuous image due to the effect of refraction from sublunary material,[18][24] citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects are near.[18] In the 14th century, the Syrian-born Ibn Qayyim proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars."[25]

The shape of the Milky Way as estimated from star counts by William Herschel in 1785; the Solar System was assumed to be near the center.

Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.[26][27] In 1750 the English astronomer Thomas Wright, in his An original theory or new hypothesis of the Universe, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the Solar System but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk.[28][29] In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.[30]

The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center.[31][32] 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.[29] Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our host galaxy, the Milky Way, emerged.[33]

Milky Way Arch
A fish-eye mosaic of the Milky Way arching at a high inclination across the night sky, shot from a dark-sky location in Chile. The Magellanic Clouds, satellite galaxies of the Milky Way, appear near the left edge.

Distinction from other nebulae

A few galaxies outside the Milky Way are visible on a dark night to the unaided eye, including the Andromeda Galaxy, Large Magellanic Cloud and the Small Magellanic Cloud. In the 10th century, the Persian astronomer Al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud".[34] In 964, Al-Sufi probably mentioned the Large Magellanic Cloud in his Book of Fixed Stars (referring to "Al Bakr of the southern Arabs",[35] since at a declination of about 70° south it was not visible where he lived); it was not well known to Europeans until Magellan's voyage in the 16th century.[36][35] The Andromeda Galaxy was later independently noted by Simon Marius in 1612.[34] In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there may be galaxies outside our own that are formed into galactic clusters that are miniscule parts of the universe which extends far beyond what we can see. These views "are remarkably close to the present-day views of the cosmos."[37] In 1750, Thomas Wright speculated (correctly) that the Milky Way is a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.[29][38] In 1755, Immanuel Kant used the term "island Universe" to describe these distant nebulae.

Pic iroberts1
Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy

Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae.[29] 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.[39]

In 1912, Vesto Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high Doppler shifts, indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us.[40][41]

In 1917, Heber Curtis observed nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy, Messier object M31, was then known). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.[42]

In 1920 a debate took place between Harlow Shapley and Heber Curtis (the Great Debate), concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[43]

In 1922, the Estonian astronomer Ernst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object.[44] Using the new 100 inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.[45] In 1936 Hubble produced a classification of galactic morphology that is used to this day.[46]

Modern research

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

In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas;[47] and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in our galaxy. These observations led to the hypothesis of a rotating bar structure in the center of our galaxy.[48] With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.[49][50]

GOODS South field
Scientists used the galaxies visible in the GOODS survey to recalculate the total number of galaxies.[51]

Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, Hubble data helped establish that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars.[52] The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the observable universe.[53] 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 at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.[54]

In 2016, a study published in The Astrophysical Journal and led by Christopher Conselice of the University of Nottingham using 3D modeling of images collected over 20 years by the Hubble Space Telescope concluded that there are over 2 trillion (2×1012) galaxies in the observable universe.[8][9][55][56]

Types and morphology

Hubble sequence photo
Types of galaxies according to the Hubble classification scheme: an E indicates a type of elliptical galaxy; an S is a spiral; and SB is a barred-spiral galaxy.[note 1]

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies.[5]


The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller globular clusters.[57]

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.[58]

Shell galaxy

NGC 3923 Elliptical Shell Galaxy
NGC 3923 Elliptical Shell Galaxy-Hubble Space Telescope photograph

A shell galaxy is a type of elliptical galaxy where the stars in the galaxy's halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. The shell-like structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy. As the two galaxy centers approach, the centers start to oscillate around a center point, the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy NGC 3923 has over twenty shells.[59]


M101 hires STScI-PRC2006-10a
The Pinwheel Galaxy, NGC 5457

Spiral galaxies resemble spiraling pinwheels. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter that extends beyond the visible component, as demonstrated by the universal rotation curve concept.[60]

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.[61] A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.[62] The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense.[63]

In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity. The spiral arms are thought to be areas of high-density matter, or "density waves".[64] 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.[65]

Hoag's object
Hoag's Object, an example of a ring galaxy

Barred spiral galaxy

A majority of spiral galaxies, including our own Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.[66] 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.[67] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.[68]

Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy[69] about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011)[70] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[71]

Super-luminous spiral

Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 100,000 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than the Milky Way.[72][73]

Other morphologies

  • Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies.
    • A ring galaxy has a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.[74] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.[75]
  • 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[76] (barred lenticular galaxies receive Hubble classification SB0.)
  • Irregular galaxies are galaxies that can not be readily classified into an elliptical or spiral morphology.
    • An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.
    • Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.[77] Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.
  • An ultra diffuse galaxy (UDG) is an extremely-low-density galaxy. The galaxy may be the same size as the Milky Way but has a visible star count of only 1% of the Milky Way. The lack of luminosity is because there is a lack of star-forming gas in the galaxy which results in old stellar populations.


Despite the prominence of large elliptical and spiral galaxies, most galaxies in the Universe are dwarf galaxies. These galaxies are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.[78]

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.[79] Dwarf galaxies may also be classified as elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.

A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses, regardless of whether the galaxy has thousands or millions of stars. This has led to the suggestion that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.[80]

Other types of galaxies


Antennae galaxies xl
The Antennae Galaxies are undergoing a collision that will result in their eventual merger.

Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.[81][82] Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies will usually not collide, but the gas and dust within the two forms will interact, sometimes triggering star formation. A collision can severely distort the shape of the galaxies, forming bars, rings or tail-like structures.[81][82]

At the extreme of interactions are galactic mergers. In this case the relative momentum of the two galaxies is insufficient to allow the galaxies to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to morphology, as compared to the original galaxies. If one of the merging galaxies is much more massive than the other merging galaxy then the result is known as cannibalism. The more massive larger galaxy will remain relatively undisturbed by the merger, while the smaller galaxy is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy.[81][82]


M82 HST ACS 2006-14-a-large web
M82, a starburst galaxy that has ten times the star formation of a "normal" galaxy[83]

Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, which is known as a starburst. If they continue to do so, then they would consume their reserve of gas in a time span less than the lifespan of the galaxy. Hence starburst activity usually lasts for only about ten million years, a relatively brief period in the history of a galaxy. Starburst galaxies were more common during the early history of the Universe,[84] and, at present, still contribute an estimated 15% to the total star production rate.[85]

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.[86] These massive stars produce supernova explosions, resulting in expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger a chain reaction of star building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the starburst activity end.[84]

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.[87]

Active galaxy

M87 jet
A jet of particles is being emitted from the core of the elliptical radio galaxy M87.

A portion of the observable galaxies are classified as active galaxies if the galaxy contains an active galactic nucleus (AGN). A significant portion of the total energy output from the galaxy is emitted by the active galactic nucleus, instead of the stars, dust and interstellar medium of the galaxy.

The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole (SMBH) at the core region of the galaxy. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.[88] In about 10% of these galaxies, a diametrically opposed pair of energetic jets ejects particles from the galaxy core at velocities close to the speed of light. The mechanism for producing these jets is not well understood.[89]

  • Seyfert galaxies or quasars, are classified depending on the luminosity, are active galaxies that emit high-energy radiation in the form of x-rays.


Blazars are believed to be an active galaxy with a relativistic jet that is pointed in the direction of Earth. A radio galaxy emits radio frequencies from relativistic jets. A unified model of these types of active galaxies explains their differences based on the viewing angle of the observer.[89]


Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear emission-line regions (LINERs). The emission from LINER-type galaxies is dominated by weakly ionized elements. The excitation sources for the weakly ionized lines include post-AGB stars, AGN, and shocks.[90] Approximately one-third of nearby galaxies are classified as containing LINER nuclei.[88][90][91]

Seyfert galaxy

Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses but unlike quasars, their host galaxies are clearly detectable. Seyfert galaxies account for about 10% of all galaxies. Seen in visible light, most Seyfert galaxies look like normal spiral galaxies, but when studied under other wavelengths, the luminosity of their cores is equivalent to the luminosity of whole galaxies the size of the Milky Way.


Quasars (/ˈkweɪzɑr/) or quasi-stellar radio sources are the most energetic and distant members of active galactic nuclei. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared to be similar to stars, rather than extended sources similar to galaxies. Their luminosity can be 100 times greater than that of the Milky Way.

Luminous infrared galaxy

Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of brightness, above 1011 L☉. LIRGs are more abundant than starburst galaxies, Seyfert galaxies and quasi-stellar objects at comparable total luminosity. Infrared galaxies emit more energy in the infrared than at all other wavelengths combined. A LIRG's luminosity is 100 billion times that of our Sun.


Magnetic fields

Galaxies have magnetic fields of their own.[92] They are strong enough to be dynamically important: they drive mass inflow into the centers of galaxies, they modify the formation of spiral arms and they can affect the rotation of gas in the outer regions of galaxies. Magnetic fields provide the transport of angular momentum required for the collapse of gas clouds and hence the formation of new stars.

The typical average equipartition strength for spiral galaxies is about 10 μG (microGauss) or 1 nT (nanoTesla). For comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss or 30 μT (microTesla). Radio-faint galaxies like M 31 and M 33, our Milky Way's neighbors, have weaker fields (about 5 μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83 and NGC 6946, have 15 μG on average. In prominent spiral arms the field strength can be up to 25 μG, in regions where cold gas and dust are also concentrated. The strongest total equipartition fields (50–100 μG) were found in starburst galaxies, for example in M 82 and the Antennae, and in nuclear starburst regions, for example in the centers of NGC 1097 and of other barred galaxies.[92]

Formation and evolution

Galactic formation and evolution is an active area of research in astrophysics.


Artist's impression of a protocluster forming in the early Universe
Artist's impression of a protocluster forming in the early Universe[93]

Current cosmological models of the early Universe are based on the Big Bang theory. About 300,000 years after this event, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result, this period has been called the "dark ages". It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos.[94][95] These primordial structures would eventually become the galaxies we see today.

Young Galaxy Accreting Material
Artist's impression of a young galaxy accreting material

Early galaxies

Evidence for the early appearance of galaxies was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and primordial galaxy yet seen.[96] While some scientists have claimed other objects (such as Abell 1835 IR1916) have higher redshifts (and therefore are seen in an earlier stage of the Universe's evolution), IOK-1's age and composition have been more reliably established. In December 2012, astronomers reported that UDFj-39546284 is the most distant object known and has a redshift value of 11.9. The object, estimated to have existed around "380 million years"[97] after the Big Bang (which was about 13.8 billion years ago),[98] is about 13.42 billion light travel distance years away. The existence of such early protogalaxies suggests that they must have grown in the so-called "dark ages".[94] As of May 5, 2015, the galaxy EGS-zs8-1 is the most distant and earliest galaxy measured, forming 670 million years after the Big Bang. The light from EGS-zs8-1 has taken 13 billion years to reach Earth, and is now 30 billion light-years away, because of the expansion of the universe during 13 billion years.[99][100][101][101][102][103]

Early galaxy formation

Signatures of the Earliest Galaxies
Different components of near-infrared background light detected by the Hubble Space Telescope in deep-sky surveys[104]

The detailed process by which early galaxies formed is an open question in astrophysics. Theories can be divided into two categories: top-down and bottom-up. In top-down correlations (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.[105] 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.[106]

Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them. These were composed almost entirely of hydrogen and helium, and may have been massive. If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium.[107] This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.[108]

In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it.[109][110]


Within a billion years of a galaxy's formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added.[111] During this early epoch, galaxies undergo a major burst of star formation.[112]

During the following two billion years, the accumulated matter settles into a galactic disc.[113] A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its life.[114] 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.[115]

XDF view field compared to the angular size of the Moon. Several thousand galaxies, each consisting of billions of stars, are in this small view.
Constellation Fornax, EXtreme Deep Field
XDF (2012) view: Each light speck is a galaxy, some of which are as old as 13.2 billion years[116] – the observable universe is estimated to contain 200 billion to 2 trillion galaxies.
XDF image shows (from left) fully mature galaxies, nearly mature galaxies (from 5 to 9 billion years ago), and protogalaxies, blazing with young stars (beyond 9 billion years).

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.[117] 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[118] or the Antennae Galaxies.[119]

The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s, and—depending upon the lateral movements—the two might collide in about five to six billion years. Although the Milky Way has never collided with a galaxy as large as Andromeda before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing.[120]

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.[121]

Future trends

Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms.[122] Elliptical galaxies are largely devoid of this gas, and so form few new stars.[123] 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.[124][125]

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest, longest-lived stars in our universe, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.[124][126]

Larger-scale structures

Deep sky surveys show that galaxies are often found in groups and clusters. Solitary galaxies that have not significantly interacted with another galaxy of comparable mass during the past billion years are relatively scarce. Only about 5% of the galaxies surveyed have been found to be truly isolated; however, these isolated formations may have interacted and even merged with other galaxies in the past, and may still be orbited by smaller, satellite galaxies. Isolated galaxies[note 2] can produce stars at a higher rate than normal, as their gas is not being stripped by other nearby galaxies.[127]

On the largest scale, the Universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see Hubble's law). Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early in the Universe, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This on-going merger process (as well as an influx of infalling gas) heats the inter-galactic gas within a cluster to very high temperatures, reaching 30–100 megakelvins.[128] 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.[129]

Seyfert Sextet full
Seyfert's Sextet is an example of a compact galaxy group.

Most galaxies in the Universe are gravitationally bound to a number of other galaxies. These form a fractal-like hierarchical distribution of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster, and these formations contain a majority of the galaxies (as well as most of the baryonic mass) in the Universe.[130][131] To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (see Virial theorem). If there is insufficient kinetic energy, however, the group may evolve into a smaller number of galaxies through mergers.[132]

Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.[133] 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.[134]

Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids.[135] Above this scale, the Universe appears to be the same in all directions (isotropic and homogeneous).[136]

The Milky Way galaxy is a member of an association named the Local Group, a relatively small group of galaxies that has a diameter of approximately one megaparsec. The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf companions of these two galaxies.[137] The Local Group itself is a part of a cloud-like structure within the Virgo Supercluster, a large, extended structure of groups and clusters of galaxies centered on the Virgo Cluster.[138] And the Virgo Supercluster itself is a part of the Pisces-Cetus Supercluster Complex, a giant galaxy filament.

Multi-wavelength observation

Andromeda galaxy
This ultraviolet image of Andromeda shows blue regions containing young, massive stars.

The peak radiation of most stars lies in the visible spectrum, so the observation of the stars that form galaxies has been a major component of optical astronomy. It is also a favorable portion of the spectrum for observing ionized H II regions, and for examining the distribution of dusty arms.

The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared, which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail.[139] Infrared is also used to observe distant, red-shifted galaxies that were formed much earlier in the history of the Universe. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy.

The southern plane of the Milky Way from the ATLASGAL survey
The southern plane of the Milky Way from submillimeter wavelengths[140]

The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies. The Earth's atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. (The ionosphere blocks signals below this range.)[141] Large radio interferometers have been used to map the active jets emitted from active nuclei. Radio telescopes can also be used to observe neutral hydrogen (via 21 cm radiation), including, potentially, the non-ionized matter in the early Universe that later collapsed to form galaxies.[142]

Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole.[143] The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies was confirmed through X-ray astronomy.[144]

See also


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  2. ^ The term "field galaxy" is sometimes used to mean an isolated galaxy, although the same term is also used to describe galaxies that do not belong to a cluster but may be a member of a group of galaxies.


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External links

Andromeda Galaxy

The Andromeda Galaxy (), also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth, and the nearest major galaxy to the Milky Way. Its name stems from the area of the Earth's sky in which it appears, the constellation of Andromeda.

The 2006 observations by the Spitzer Space Telescope revealed that the Andromeda Galaxy contains approximately one trillion stars, more than twice the number of the Milky Way's estimated 200 to 400 billion stars. The Andromeda Galaxy's mass is estimated to be around 1.76 times that of the Milky Way Galaxy (~0.8-1.5×1012 solar masses vs the Milky Way's 8.5×1011 solar masses), though a 2018 study found that the Andromeda Galaxy's mass is roughly the same as the Milky Way's. The Andromeda Galaxy, spanning approximately 220,000 light-years, is the largest galaxy in the Local Group, which is also home to the Triangulum Galaxy and other minor galaxies.

The Milky Way and Andromeda galaxies are expected to collide in ~4.5 billion years, merging to form a giant elliptical galaxy or a large disc galaxy.

With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects making it visible to the naked eye from Earth on moonless nights, even when viewed from areas with moderate light pollution.

Chris Pratt

Christopher Michael Pratt (born June 21, 1979) is an American actor. He rose to prominence for his television roles, particularly as Andy Dwyer in the NBC sitcom Parks and Recreation (2009–2015), for which he received critical acclaim and was nominated for the Critics' Choice Television Award for Best Supporting Actor in a Comedy Series in 2013. He also starred earlier in his career as Bright Abbott in The WB drama series Everwood (2002–2006) and has notable roles in Wanted (2008), Jennifer's Body (2009), Moneyball (2011), The Five-Year Engagement (2012), Zero Dark Thirty (2013), Delivery Man (2013), and Her (2013).

Pratt achieved leading man status in 2014 after starring in two critically and commercially successful films, Warner Animation Group's The Lego Movie as Emmet Brickowski and Marvel Studios' Guardians of the Galaxy as Star-Lord. In 2015, he starred in Jurassic World, the fourth installment in the Jurassic Park franchise, which was his most financially successful film up until the release of Infinity War; he reprised the former role in the sequel Jurassic World: Fallen Kingdom in 2018. In 2015, Time named Pratt one of the 100 most influential people in the world on the annual Time 100 list.Pratt continued his leading man run in 2016 with The Magnificent Seven and Passengers. He reprises his role as Star-Lord in the Marvel Cinematic Universe in Guardians of the Galaxy Vol. 2 (2017), Avengers: Infinity War (2018) and its upcoming sequel, Avengers: Endgame (2019).

David Beckham

David Robert Joseph Beckham, (; born 2 May 1975) is an English retired professional footballer and current President of Inter Miami CF. He played for Manchester United, Preston North End, Real Madrid, Milan, LA Galaxy, Paris Saint-Germain and the England national team, for which he held the appearance record for an outfield player until 2016. He is the first English player to win league titles in four countries: England, Spain, the United States and France. He retired in May 2013 after a 20-year career, during which he won 19 major trophies.Known for his range of passing, crossing ability and bending free-kicks as a right winger, Beckham has been hailed as one of the greatest midfielders of all time. He was runner-up in the Ballon d'Or, twice runner-up for FIFA World Player of the Year and in 2004 was named by Pelé in the FIFA 100 list of the world's greatest living players. He was inducted into the English Football Hall of Fame in 2008. A global ambassador for the sport, Beckham is regarded as a British cultural icon.Beckham's professional club career began with Manchester United, where he made his first-team debut in 1992 aged 17. With United, he won the Premier League title six times, the FA Cup twice, and the UEFA Champions League in 1999. He then played four seasons with Real Madrid, winning the La Liga championship in his final season with the club. In July 2007 Beckham signed a five-year contract with Major League Soccer club LA Galaxy. While a Galaxy player, he spent two loan spells in Italy with Milan in 2009 and 2010. He was the first British footballer to play 100 UEFA Champions League games.In international football, Beckham made his England debut on 1 September 1996 at the age of 21. He was captain for six years, earning 58 caps during his tenure. He made 115 career appearances in total, appearing at three FIFA World Cup tournaments, in 1998, 2002 and 2006, and two UEFA European Championship tournaments, in 2000 and 2004.

Beckham has consistently ranked among the highest earners in football, and in 2013 was listed as the highest-paid player in the world, having earned over $50 million in the previous 12 months. He has been married to Victoria Beckham since 1999 and they have four children. He has been a UNICEF UK ambassador since 2005, and in 2015 he launched 7: The David Beckham UNICEF Fund. In 2014, MLS announced that Beckham and a group of investors would own an Inter Miami CF, which will begin in 2020.

Galaxy cluster

A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity with typical masses ranging from 1014–1015 solar masses. They are the largest known gravitationally bound structures in the universe and were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with star clusters, such as open clusters, which are structures of stars within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.

Notable galaxy clusters in the relatively nearby Universe include the Virgo Cluster, Fornax Cluster, Hercules Cluster, and the Coma Cluster. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma Cluster, is massive enough to affect the local expansion of the Universe. Notable galaxy clusters in the distant, high-redshift Universe include SPT-CL J0546-5345 and SPT-CL J2106-5844, the most massive galaxy clusters found in the early Universe. In the last few decades, they are also found to be relevant sites of particle acceleration, a feature that has been discovered by observing non-thermal diffuse radio emissions, such as radio halos and radio relics. Using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters.

Guardians of the Galaxy (film)

Guardians of the Galaxy (retroactively referred to as Guardians of the Galaxy Vol. 1) is a 2014 American superhero film based on the Marvel Comics superhero team of the same name, produced by Marvel Studios and distributed by Walt Disney Studios Motion Pictures. It is the tenth film in the Marvel Cinematic Universe (MCU). The film was directed by James Gunn, who wrote the screenplay with Nicole Perlman, and features an ensemble cast including Chris Pratt, Zoe Saldana, Dave Bautista, Vin Diesel, and Bradley Cooper as the titular Guardians, along with Lee Pace, Michael Rooker, Karen Gillan, Djimon Hounsou, John C. Reilly, Glenn Close, and Benicio del Toro. In Guardians of the Galaxy, Peter Quill forms an uneasy alliance with a group of extraterrestrial criminals who are fleeing after stealing a powerful artifact.

Perlman began working on the screenplay in 2009. Producer Kevin Feige first publicly mentioned Guardians of the Galaxy as a potential film in 2010 and Marvel Studios announced it was in active development at the San Diego Comic-Con International in July 2012. Gunn was hired to write and direct the film that September. In February 2013, Pratt was hired to play Peter Quill / Star-Lord, and the supporting cast members were subsequently confirmed. Principal photography began in July 2013 at Shepperton Studios in England, with filming continuing in London before wrapping up in October 2013. Post-production was finished on July 7, 2014.

The film premiered in Hollywood on July 21, 2014, and was released in theaters on August 1, 2014 in the United States in the 3D and IMAX 3D formats. The film became a critical and commercial success, grossing $773.3 million worldwide and becoming the highest-grossing superhero film of 2014, as well as the third-highest-grossing film of 2014. The film was praised for its humor, acting, direction, soundtrack, visual effects, and action sequences. At the 87th Academy Awards, the film received nominations for Best Visual Effects and Best Makeup and Hairstyling. A sequel, Guardians of the Galaxy Vol. 2, was released on May 5, 2017. A third film, Guardians of the Galaxy Vol. 3, which was initially being developed for a 2020 release, has been put on hold after the firing of Gunn.

Guardians of the Galaxy Vol. 2

Guardians of the Galaxy Vol. 2 is a 2017 American superhero film based on the Marvel Comics superhero team Guardians of the Galaxy, produced by Marvel Studios and distributed by Walt Disney Studios Motion Pictures. It is the sequel to 2014's Guardians of the Galaxy and the fifteenth film in the Marvel Cinematic Universe (MCU). Written and directed by James Gunn, the film stars an ensemble cast featuring Chris Pratt, Zoe Saldana, Dave Bautista, Vin Diesel, Bradley Cooper, Michael Rooker, Karen Gillan, Pom Klementieff, Elizabeth Debicki, Chris Sullivan, Sean Gunn, Sylvester Stallone, and Kurt Russell. In Guardians of the Galaxy Vol. 2, the Guardians travel throughout the cosmos as they help Peter Quill learn more about his mysterious parentage.

The film was officially announced at the 2014 San Diego Comic-Con International before the theatrical release of the first film, along with James Gunn's return from the first film, with the title of the sequel revealed a year later in June 2015. Principal photography began in February 2016 at Pinewood Atlanta Studios in Fayette County, Georgia, with many crew changes from the first film due to other commitments. Filming concluded in June 2016. James Gunn chose to set the sequel shortly after the first film to explore the characters' new roles as the Guardians, and to follow the storyline of Quill's father established throughout that previous film. Russell was confirmed as Quill's father in July 2016, portraying Ego, a departure from Quill's comic father.

Guardians of the Galaxy Vol. 2 premiered in Tokyo on April 10, 2017 and was released in the United States on May 5, in 3D and IMAX 3D. It grossed more than $863 million worldwide, making it the eighth highest-grossing film of 2017, while also outgrossing its predecessor. The film received praise for its visuals, soundtrack, humor, and cast, though some critics deemed it not as "fresh" as the original. It also received a nomination for Best Visual Effects at the 90th Academy Awards. A sequel, Guardians of the Galaxy Vol. 3, was initially being developed for a 2020 release, but has been put on hold after the firing of James Gunn.

LA Galaxy

The LA Galaxy, also known as the Los Angeles Galaxy, is an American professional soccer franchise based in the Los Angeles suburb of Carson, California, that competes in Major League Soccer (MLS), as a member of the Western Conference. The club began play in 1996 as one of the league's eight charter members.

The Galaxy was founded in 1994 and is owned by Anschutz Entertainment Group. In their early years, the club played its home games at the Rose Bowl in Pasadena, California. Since 2003, they have played at Dignity Health Sports Park in Carson, California. The team holds a rivalry with the San Jose Earthquakes in the California Clásico and used to play the SuperClasico against city rivals Chivas USA before that team folded in 2014. A new rival emerged in 2018 in the form of Los Angeles FC in the El Trafico derby.

The franchise is Major League Soccer's most decorated club, with a record five MLS Cups, four Supporters' Shields, and two U.S. Open Cups. The club has appeared in a record nine MLS Cups, and won the Western Conference regular season title eight times. In 2017, the club added the dubious MLS Wooden Spoon to its trophy case for finishing bottom of the MLS league table. In international play, the team won the CONCACAF Champions' Cup in 2000, one of just two American clubs, along with D.C. United, to win the regional tournament.

In 2007, the club made international headlines with the signing of English player David Beckham from Real Madrid, the most high-profile transaction with Major League Soccer to that point. The club has fielded other high-profile international players including Ashley Cole, Robbie Keane, Luis Hernández, Giovani dos Santos, Jonathan dos Santos, Jorge Campos, Steven Gerrard, Zlatan Ibrahimović and American Landon Donovan who is the all-time leading scorer for both the club and the league. Forbes estimates the franchise is the most valuable in the league, worth more than $300 million.

List of galaxies

The following is a list of notable galaxies.

There are about 51 galaxies in the Local Group (see list of nearest galaxies for a complete list), on the order of 100,000 in our Local Supercluster and an estimated number of about one to two trillion in all of the observable universe.

The discovery of the nature of galaxies as distinct from other nebulae (interstellar clouds) was made in the 1920s. The first attempts at systematic catalogues of galaxies were made in the 1960s, with the Catalogue of Galaxies and Clusters of Galaxies listing 29,418 galaxies and galaxy clusters, and with the Morphological Catalogue of Galaxies, a putatively complete list of galaxies with photographic magnitude above 15, listing 30,642. In the 1980s, the Lyons Groups of Galaxies listed 485 galaxy groups with 3,933 member galaxies. Galaxy Zoo is a project aiming at a more comprehensive list: launched in July 2007, it has classified over one million galaxy images from The Sloan Digital Sky Survey, The Hubble Space Telescope and the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey.There is no universal naming convention for galaxies, as they are mostly catalogued before it is established whether the object is or isn't a galaxy. Mostly they are identified by their celestial coordinates together with the name of the observing project (HUDF, SDSS, 3C, CFHQS, NGC/IC, etc.)

Lockheed C-5 Galaxy

The Lockheed C-5 Galaxy is a large military transport aircraft originally designed and built by Lockheed, and now maintained and upgraded by its successor, Lockheed Martin. It provides the United States Air Force (USAF) with a heavy intercontinental-range strategic airlift capability, one that can carry outsized and oversized loads, including all air-certifiable cargo. The Galaxy has many similarities to its smaller Lockheed C-141 Starlifter predecessor, and the later Boeing C-17 Globemaster III. The C-5 is among the largest military aircraft in the world.

The C-5 Galaxy's development was complicated, including significant cost overruns, and Lockheed suffered significant financial difficulties. Shortly after entering service, cracks in the wings of many aircraft were discovered and the C-5 fleet was restricted in capability until corrective work was completed. The C-5M Super Galaxy is an upgraded version with new engines and modernized avionics designed to extend its service life beyond 2040.

The USAF has operated the C-5 since 1969. In that time, the airlifter supported US military operations in all major conflicts including Vietnam, Iraq, Yugoslavia, and Afghanistan, as well as allied support, such as Israel during the Yom Kippur War and operations in the Gulf War. The Galaxy has also been used to distribute humanitarian aid and disaster relief, and supported the US Space Shuttle program.

Milky Way

The Milky Way is the galaxy that contains our Solar System. The name describes the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle"). From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

The Milky Way is a barred spiral galaxy with a diameter between 150,000 and 200,000 light-years (ly). It is estimated to contain 100–400 billion stars and more than 100 billion planets. The Solar System is located at a radius of 26,490 (± 100) light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, likely a supermassive black hole of 4.100 (± 0.034) million solar masses.

Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way is invisible to our telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter". The rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.

Robbie Keane

Robert David Keane (born 8 July 1980) is an Irish former professional footballer who played as a forward and is the current assistant coach of the Republic of Ireland national team. He served as captain of Ireland from March 2006 until his international retirement in August 2016.

Keane began his professional career at Wolverhampton Wanderers, scoring twice on his first-team debut aged 17. The following season he was the club's leading goalscorer and scored his first international goal for the Republic of Ireland. He changed club frequently between 1999 and 2002, breaking transfer fee records, but his brief spells at Inter Milan and Leeds United were unexceptional. He joined Tottenham Hotspur in 2002 and played there for seven-and-a-half years over two spells and amassed 306 appearances across all competitions, scoring 122 goals. The 2007–08 season was the most fruitful of his career as he set a career record of 23 goals in a season, including a landmark 100th competitive goal and won his first senior honour, the League Cup.

Keane moved to Liverpool in July 2008 but he spent only six months at the club before returning to Tottenham, where he was made first team captain. In January 2010, he moved on loan to Scottish Premier League side Celtic for the rest of the season, and spent the second half of the following season loaned to West Ham United. He left Tottenham for LA Galaxy in 2011, and in January 2012 went to Aston Villa on a two-month loan during the Major League Soccer off-season. He departed LA Galaxy in 2016 having scored 104 goals across six seasons, before a short stint with Indian club ATK. Keane announced his retirement from professional football in November 2018. In total, he scored 126 Premier League goals for six different clubs, which ranks him as the 14th-most successful goalscorer in the history of the Premier League.

Keane scored 68 goals for the Republic of Ireland national team over an 18-year international career, making him the all-time record Irish scorer. His 146 caps is also an Irish record. Keane is the joint fifth-highest European international goalscorer of all time, and the only player in the history of world football to have scored at least one international goal in 19 consecutive years. He is also the all-time top scorer in European Championship qualification matches. Keane was Ireland's top scorer at the 2002 FIFA World Cup with three goals as they reached the last 16, and also played at UEFA Euro 2012 and UEFA Euro 2016. Throughout his club and international career he was known for his goal celebration where he performed a cartwheel followed by a forward roll.

Samsung Galaxy

Samsung Galaxy (stylized as SAMSUNG Galaxy since 2015, previously stylized as Samsung GALAXY) is a series of mobile computing devices designed, manufactured and marketed by Samsung Electronics. The product line includes the Galaxy S series of high-end smartphones, the Galaxy Tab series of tablets, the Galaxy Note series of tablets and phablets with the added functionality of a stylus, and the first version of the Galaxy Gear smartwatch, with later versions dropping the Galaxy branding.

Samsung Galaxy devices use the Android operating system produced by Google, usually with a custom user interface called Samsung Experience (formerly known as TouchWiz). However, The Galaxy TabPro S is the first Galaxy-branded Windows 10 device that was announced in CES 2016

All Galaxy devices starting from the S2 up to present come with a file transfer app (Called Smart Switch) pre-loaded, with no app icon.

Samsung Galaxy S4

The Samsung Galaxy S4 is an Android smartphone produced by Samsung Electronics and was first shown publicly on March 14, 2013 at Samsung Mobile Unpacked in New York City. It is the successor to the Galaxy S III which maintains a similar design, but with upgraded hardware and an increased focus on software features that take advantage of its hardware capabilities—such as the ability to detect when a finger is hovered over the screen, and expanded eye tracking functionality. A hardware variant of the S4 became the first smartphone to support the emerging LTE Advanced mobile network standard.The S4 was made available in late April 2013 on 327 carriers in 155 countries. It became Samsung's fastest selling smartphone and eventually Samsung’s best selling smartphone with 20 million sold worldwide in the first two months, and 40 million in the first six months.While some users called all new Galaxy S4 features "innovation", others called them feature creep or just gimmicks. Those features are, for example, Smart Pause, Smart Rotation, Smart Scroll, Air View, Air gesture, Story Album and Temperature and humidity sensors.

The Galaxy S4's successor, the Samsung Galaxy S5, was announced by Samsung in February 2014 ahead of a release in April of that year.

Samsung Galaxy S5

The Samsung Galaxy S5 is an Android smartphone produced by Samsung Electronics. Unveiled on 24 February 2014 at Mobile World Congress in Barcelona, Spain, it was released on 11 April 2014 in 150 countries as the immediate successor to the Galaxy S4. As with the S4, the S5 is an evolution of the prior year's model, placing a particular emphasis on an improved build with a textured rear cover and IP67 certification for dust and water resistance, a more refined user experience, new security features such as a fingerprint reader and private mode, expanded health features (such as a built-in heart rate monitor), and an updated camera.

The Galaxy S5 received mostly positive reviews; The phone was praised for its display, hardware, camera, long battery life, and incorporating water resistance while retaining a removable battery and MicroSD slot. However, the S5 was criticized for bloated software, the inconsistent fingerprint scanner featured on the phone, and its plastic design in light of rival smartphones' more premium metal bodies.In August 2015, following the release of its latest flagship, the Galaxy S6, Samsung released an updated version called the "Galaxy S5 Neo" which has an Exynos 7 Octa (7580) processor clocked at 1.6 GHz. It has 2 GB of RAM, 16 GB of internal storage and comes with Android 6.0.1 "Marshmallow".

Samsung Galaxy S7

Samsung Galaxy S7, Samsung Galaxy S7 Edge and Samsung Galaxy S7 Active are Android smartphones manufactured and marketed by Samsung Electronics. The S7 series serves as the successor to the Galaxy S6, S6 Edge, S6 Edge+ and S6 Active released in 2015. The S7 and S7 Edge were officially unveiled on 21 February 2016 during a Samsung press conference at Mobile World Congress, with a European and North American release on 11 March 2016. The S7 Active was unveiled on 4 June 2016, and released on AT&T in the United States on 10 June 2016.The Galaxy S7 is an evolution of the prior year's model, with upgraded hardware, design refinements, and the restoration of features removed from the Galaxy S6, such as IP certification for water and dust resistance, as well as expandable storage. Succeeding the S6 and S6 Edge+, respectively, the S7 is produced in a standard model with a display size of 5.1-inch (130 mm) as well as an Edge variant whose display is curved along the wide sides of the screen and also has a larger 5.5-inch (140 mm) display. The S7 Active features a thicker more rugged frame and an increased battery capacity. The Galaxy S7 and S7 Edge are the last two phones in the Samsung Galaxy S series to have a physical home button with a front-sided fingerprint sensor embedded in the button (the S7 Active being the last in the Active series to feature three physical buttons with the fingerprint reader embedded home button) and the last Samsung Galaxy S series phones to use MicroUSB charging.

Samsung Galaxy S8

The Samsung Galaxy S8, Samsung Galaxy S8+ (shortened to S8 and S8+, respectively) and Samsung Galaxy S8 Active are Android smartphones (with the S8+ being the phablet smartphone) produced by Samsung Electronics as the eighth generation of the Samsung Galaxy S series. The S8 and S8+ were unveiled on 29 March 2017 and directly succeeded the Samsung Galaxy S7 and S7 edge, with a North American release on 21 April 2017 and international rollout throughout April and May. The S8 Active was announced on 8 August 2017 and is exclusive to certain U.S. cellular carriers.

The S8 and S8+ contain upgraded hardware and major design changes over the S7 line, including larger screens with a taller aspect ratio and curved sides on both the smaller and larger models, iris and face recognition, a new suite of virtual assistant features known as Bixby (along with a new dedicated physical button for launching the assistant), a shift from MicroUSB to USB-C charging, and Samsung DeX, a docking station accessory that allows the phones to be used with a desktop interface with keyboard and mouse input support. The S8 Active features tougher materials designed for protection against shock, shatter, water and dust, with a metal frame and a tough texture for improved grip that makes the S8 Active have a rugged design. The Active's screen measures the same size as the standard S8 model but loses the curved edges in favor of a metal frame.

The S8 and S8+ received mostly positive reviews. Their design and form factor received praise, while critics also liked the updated software and camera optimizations. They received criticism for duplicate software apps, lackluster Bixby features at launch, for the placement of the fingerprint sensor on the rear next to the camera. A video published after the phones' release proved that the devices' facial and iris scanners can be fooled by suitable photographs of the user.

The S8 and S8+ were in high demand at release. During the pre-order period, a record one million units were booked in South Korea, and overall sales numbers were 30% higher than the Galaxy S7. However, subsequent reports in May announced sales of over five million units, a notably lower first-month sales number than previous Galaxy S series models.

On March 11, 2018, Samsung launched the successor to the S8, the Samsung Galaxy S9.

Samsung Galaxy S9

The Samsung Galaxy S9 and Samsung Galaxy S9+ (shortened to S9 and S9+, respectively) are Android smartphones produced by Samsung Electronics as part of the Samsung Galaxy S series. The devices were revealed at the Mobile World Congress in Barcelona on 25 February 2018, as the successors to the Samsung Galaxy S8 and S8+.The Galaxy S9 and S9+ have nearly identical features to the S8 batch, with the same display size and aspect ratio, just like their predecessor. One highly regarded change to distinguish between the models is the location of the fingerprint sensor. While the S8's is found beside the camera, the S9's is directly underneath it. Most notably, however, the S9 line is equipped with several camera improvements over the S8.

The phone has received generally favorable reviews, with critics mostly noting the enhanced camera and better positioned fingerprint scanner. Critics, however, still criticize the lack of improvement to other features compared to its predecessor.

Spiral galaxy

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.

Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to their barless cousins has likely changed over the history of the Universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.Our own Milky Way is a barred spiral, although the bar itself is difficult to observe from the Earth's current position within the galactic disc. The most convincing evidence for the stars forming a bar in the galactic center comes from several recent surveys, including the Spitzer Space Telescope.Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.

The Hitchhiker's Guide to the Galaxy

The Hitchhiker's Guide to the Galaxy (sometimes referred to as HG2G, HHGTTG or H2G2) is a comedy science fiction series created by Douglas Adams. Originally a radio comedy broadcast on BBC Radio 4 in 1978, it was later adapted to other formats, including stage shows, novels, comic books, a 1981 TV series, a 1984 video game, and 2005 feature film.

A prominent series in British popular culture, The Hitchhiker's Guide to the Galaxy has become an international multi-media phenomenon; the novels are the most widely distributed, having been translated into more than 30 languages by 2005. In 2017, BBC Radio 4 announced a 40th-anniversary celebration with Dirk Maggs, one of the original producers, in charge. This sixth series of the sci-fi spoof has been based on Eoin Colfer's book And Another Thing, with additional unpublished material by Douglas Adams. The first of six new episodes was broadcast on 8 March 2018.The broad narrative of Hitchhiker follows the misadventures of the last surviving man, Arthur Dent, following the demolition of the planet Earth by a Vogon constructor fleet to make way for a hyperspace bypass. Dent is rescued from Earth's destruction by Ford Prefect, a human-like alien writer for the eccentric, electronic travel guide The Hitchhiker's Guide to the Galaxy, by hitchhiking onto a passing Vogon spacecraft. Following his rescue, Dent explores the galaxy with Prefect and encounters Trillian, another human who had been taken from Earth prior to its destruction by the President of the Galaxy, the two-headed Zaphod Beeblebrox, and the depressed Marvin, the Paranoid Android. Certain narrative details were changed between the various adaptations.

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