document_title: Galaxy

question_text: who is the first person to classified galaxies into groups according to their structure

question_tokens: who is the first person to classified galaxies into groups according to their structure

text: Galaxy

text: Etymology

text: 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]

text: 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:

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

text: Certain astronomical objects known as spiral nebulae such as M31 would later be recognized as conglomerations of stars when the true distance to these objects began to be discovered, and they would be deemed island universes. However, the word Universe was later understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies.[16]

text: Nomenclature

text: 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.

text: Observation history

text: 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.

text: Milky Way

text: 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]

text: 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]

text: 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]

text: 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]

text: Distinction from other nebulae

text: A few galaxies outside the Milky Way are visible in the night sky to the unaided eye. 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 identified the Large Magellanic Cloud in his Book of Fixed Stars; it was not seen by Europeans until Magellan's voyage in the 16th century.[35][36] The Andromeda Galaxy was independently noted by Simon Marius in 1612.[34]

text: 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][37] In 1755, Immanuel Kant used the term "island Universe" to describe these distant nebulae.

text: 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.[38]

text: 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.[39][40]

text: 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.[41]

text: 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.[42]

text: 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.[43] 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.[44] In 1936 Hubble produced a classification of galactic morphology that is used to this day.[45]

text: Modern research

text: In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas;[46] 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.[47] 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.[48][49] A concept known as the universal rotation curve of spirals, moreover, shows that the problem is ubiquitous in these objects.

text: 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.[51] The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (7011125000000000000♠1.25×1011) galaxies in the observable universe.[52] Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allow detection of other galaxies that are not detected by Hubble. Particularly, galaxy surveys in the Zone of Avoidance (the region of the sky blocked by the Milky Way) have revealed a number of new galaxies.[53]

text: 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 (7012200000000000000♠2×1012) galaxies in the observable universe.[8][9][54][55]

text: Types and morphology

text: 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]

text: Ellipticals

text: 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.[56]

text: 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.[57]

text: Starburst galaxies are the result of a galactic collision that can result in the formation of an elliptical galaxy.[56]

text: Shell galaxy

text: 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.[58]

text: Spirals

text: 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.[59]

text: 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.[60] 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.[61] 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.[62]

text: 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".[63] 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.[64]

text: Barred spiral galaxy

text: 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.[65] 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.[66] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.[67]

text: Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy[68] about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011)[69] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[70]

text: Super-luminous spiral

text: 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.[71][72]

text: Other morphologies

text:
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.[73] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.[74]

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[75] (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.[76] 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.

text: Dwarfs

text: 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.[77]

text: 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.[78] 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.

text: 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.[79]

text: Other types of galaxies

text: Interacting

text: 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.[80][81] 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.[80][81]

text: 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.[80][81]

text: Starburst

text: 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,[83] and, at present, still contribute an estimated 15% to the total star production rate.[84]

text: 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.[85] 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.[83]

text: 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.[86]

text: Active galaxy

text: A portion of the observable galaxies are classified as active galaxies if the galaxy contains an active galactic nucleus. 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.

text: 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.[87] 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.[88]

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

text: Blazars

text: 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.[88]

text: LINERS

text: 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.[89] Approximately one-third of nearby galaxies are classified as containing LINER nuclei.[87][89][90]

text: Seyfert galaxy

text: 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.

text: Quasar

text: Quasars (/ˈkweɪzɑr/) or quasi-stellar radio sources are the most energetic and distant members of a class of objects called active galactic nuclei (AGN). 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.

text: Luminous infrared galaxy

text: 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.

text: Properties

text: Magnetic fields

text: Galaxies have magnetic fields of their own.[91] 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.

text: 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]

text: Formation and evolution

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

text: Formation

text: 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.

text: Early galaxies

text: 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]

text: Early galaxy formation

text: 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 theories (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]

text: 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]

text: 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]

text: Evolution

text: 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]

text: 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]

text: 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]

text: 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]

text: 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]

text: Future trends

text: 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]

text: 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]

text: Larger-scale structures

text: 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]

text: 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]

text: 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]

text: 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]

text: 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]

text: 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.

text: Multi-wavelength observation

text: 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.

text: 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.

text: 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]

text: 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]


document_title: Push-up

question_text: who holds the record for most push ups

question_tokens: who holds the record for most push ups

root

text: Push-up

text: Etymology[edit]

text: The American English term push-up was first used between 1905 and 1910,[2] while the British press-up was first recorded between 1945 and 1950.[3][4]

tag: p

text: Body mass supported during push-ups[edit]

text: According to the study published in Journal of Strength and Conditioning Research, the test subjects supported with their hands, on average, 69.16% of their body mass in the up position, and 75.04% in the down position during the traditional push-ups. In modified push-ups, where knees are used as the pivot point, subjects supported 53.56% and 61.80% of their body mass in up and down positions, respectively.[5]

text: Muscles worked[edit]

text: While the push-up primarily targets the muscles of the chest, arms, and shoulders, support required from other muscles results in a wider range of muscles integrated into the exercise.[6]

text: Abdominals[edit]

text: The rectus abdominis and transversus abdominis contract continually while performing push-ups to hold the body off the floor and keep the legs and torso aligned. The rectus abdominis spans the front of the abdomen and is the most prominent of the abdominal muscles. The transversus abdominis lies deep within the abdomen, wrapping around the entire abdominal area. Both muscles compress the abdomen, and the rectus abdominis also flexes the spine forward, although it does not execute this function when performing push-ups.

text: Deltoid[edit]

text: The anterior portion of the deltoid muscle is one of the major shoulder-joint horizontal adductors, moving the upper arms toward the chest during the upward phase of a push-up. It also helps control the speed of movement during the downward phase. The deltoid attaches to parts of the clavicle and scapula, just above the shoulder joint on one end, and to the outside of the humerus bone on the other. Along with horizontal adduction, the anterior deltoid assists with flexion and internal rotation of the humerus within the shoulder socket.

text: Chest muscles[edit]

text: The push up requires the work of many muscle groups, with one of the primary muscle groups being the chest muscles, the pectoralis major and the minor.[7] These are the two large chest muscles and the main pushing muscle group of the upper body. When pushing and lowering the body during a push up, the pectoralis major is doing most of the work. As a result, these muscles become very strong and can become defined lean muscle after doing push-ups regularly.

text: Stabilizers: back body[edit]

text: The push-up depends on stabilizer muscles as you push and lower the body. The erector spinae is the main stabilizer muscle in the back. Made up of 3 muscles including the spinalis, longissmus and iliocostalis. spinalis runs adjacent to the spine, the longissimus runs adjacent to the spinalis and the iliocostalis runs adjacent to the longissimus and over the ribs. 2 muscles called the gluteus medius and gluteus minimus stabilize the upper leg. The medius and minimus sit under the largest butt muscle, the gluteus maximus.

text: Triceps brachii[edit]

text: While the anterior deltoids and pectoralis major muscles work to horizontally adduct the upper arms during the upward phase of a push-up, the triceps brachii muscles, or triceps for short, are also hard at work extending the elbow joints so the arms can be fully extended. The triceps also control the speed of elbow-joint flexion during the downward phase of the exercise. The closer together the hands are placed during a push-up, the harder the triceps work. The muscle is divided into three heads — the lateral head, long head and medial head. The lateral and medial heads attach to the back of the humerus bone, and the long head attaches just behind the shoulder socket on one end; all three heads combine and attach to the back of the elbow on the other.[8] There is a special sub-set of the diamond push-up (so named for the diamond shaped space between the hands when the thumb and forefinger of the left hand is placed on the floor up against the thumb and forefinger of the right hand.) The special version of this push-up is when the diamond is placed directly below the nose instead of the solar plexus. The nose must almost touch the floor in the center of the diamond. This special diamond push-up is done by the United States Marine Corps. The lips must come within 1 inch of the floor while keeping the neck in line with the straight spine to qualify as a valid push-up. This can be verified by placing a 1 inch foam disposable earplug on the floor in the center of the diamond and picking it up with the lips.

text: Forearms[edit]

text: Stabilizers include wrist and forearm muscles, the knee extensors, and the hip/spine flexors which all work isometrically to maintain a proper plank position in the standard prone push-up.

text: Biceps[edit]

text: During the push-up exercise, the short head of the biceps brachii muscle acts as a dynamic stabilizer. This means the muscle activates at both ends—the elbow and the shoulder—to help stabilize the joints.

text: Joints and tendons[edit]

text: Inner muscles that support the operation of the fingers, wrists, forearms and elbows are also worked isometrically. Some push-up modifications that require to have the arms at different heights effectively engage the rotator cuff.[9]

text: Variations[edit]

text: In the "full push-up", the back and legs are straight and off the floor. There are several variations besides the common push-up. These include bringing the thumbs and index fingers of both hands together (a "diamond push-up") as well as having the elbows pointed towards the knees. These variations are intended to put greater emphasis on the triceps or shoulders, rather than the chest muscles. When both hands are unbalanced or on uneven surfaces, this exercise works the body core. Raising the feet or hands onto elevated surfaces during the exercise emphasizes the upper (minor) or lower (major) pectorals, respectively. Raising the hands with the aid of push-up bars or a dumbbell allows for greater ROM (range of motion), providing further stress for the muscles.

text: Planche push-ups[edit]

text: An extremely difficult variation is to perform a push-up using only hands, without resting the feet on the floor, i.e. starting from and returning to the planche position. These are known as "planche push-ups". To do this variation, the body's center of gravity must be kept over the hands while performing the push-up by leaning forward while the legs are elevated in the air, which requires great strength and a high level of balance. The entire body weight is lifted in this variation.

text: Knuckle push-ups[edit]

text: Another variation is to perform push-ups on the knuckles of the fist, rather than with palms of the hands on the floor. This method is also commonly used in martial arts, such as Karate and Tae Kwon Do, and may be used in boxing training while wearing boxing gloves.

text: The intent, in addition to building strength and conditioning, is to toughen the knuckles, wrist, and forearm in the punching position. This variation also reduces the amount of strain in the wrist, compared to the typical "palms on floor" approach, and so it is sometimes used by those with wrist injuries. Such practitioners will usually perform their knuckle push-ups on a padded floor or a rolled-up towel, unlike martial artists, who may do bare-knuckle push-ups on hard floors.

text: The most knuckle push-ups in one minute is 91 by Ron Cooper (Massachusetts, USA) on 15 December 2016.[10]

text: Maltese push-up[edit]

text: The Maltese push-up is a gymnastic variation of the push-up, in which the hands are positioned further down towards the hips (as opposed to roughly alongside the pectorals), but with a wide distance between them.

text: Hindu push-up[edit]

text: The Hindu push-up starts from the "downward dog" yoga position and transitions to a "cobra pose" position. It is also known as a Hanuman, judo, or dive bomber push-up. It is common in Indian and Pakistani physical culture and Indian martial arts, particularly Pehlwani.

text: Guillotine push-up[edit]

text: The guillotine push-up is a form of push-up exercise done from an elevated position (either hands on elevated platforms or traditionally medicine balls) where in the practitioner lowers the chest, head, and neck (thus the name) past the plane of the hands. The goal is to stretch the shoulders and put extra emphasis on the muscles there.

text: Backhanded push-up[edit]

text: The backhanded push-up is a form of push-ups performed using the back of the hands, rather than the palms. Currently the record holder of the backhanded push-ups is Bill Kathan who broke the world record in 2010, by performing 2,396 on Valentine's Day.[11]

text: One arm versions[edit]

text: Many of the push-up variations can be done using one arm instead of two. This will further increase the resistance put upon the trainee. The world record for the most one handed push ups in an hour was 1,868 and was set by Paddy Doyle from the United Kingdom. It was recorded in Munster Arms Hotel, Sparkbrook, United Kingdom on the 27 November 1993.[12]

text: Single-leg push-up[edit]

text: Lift one leg up off the ground and do a set. Switch legs on the next set.

text: Narrow-grip push-up[edit]

text: Do a normal push-up with your hands just a few inches apart from each other underneath the chest.

text: Clap push-up[edit]

text: At the peak of the push-up, push yourself up off the ground and quickly clap in midair. The fast jolting force of clap push-ups will help develop explosive power while also bulking up the pectoral muscles for a more defined chest.

text: Spiderman push-up[edit]

text: Do a normal push-up but raise one knee toward the elbow of the same side as you rise. Switch knees with each rep.

text: Other versions[edit]

text: There are some less difficult versions, which reduce the effort by supporting some of the body weight in some way. One can move on to the standard push-up after progress is made.

text: "Wall" push-ups are performed by standing close to a wall and pushing away from the wall with the arms; one can increase the difficulty by moving one's feet farther from the wall.

text: "Table" or "chair" push-ups are performed by pushing away from a table, chair, or other object. The lower the object, the more difficult the push-up. One should be sure that the object is securely stationary before attempting to push up from it.

text: "Modified" or "knee" push-ups are performed by supporting the lower body on the knees instead of the toes, which reduces the difficulty. This is useful for warm ups/downs, pyramids/drop sets, endurance training and rehab. It can also be used to train in a more explosive plyometric manner (like clapping push-ups) when one can't perform them with the feet. It can also be used with the 1-arm variations as a transition.

text: "Three phase" push-ups involve simply breaking a standard push up into three components and doing each one slowly and deliberately. Participants usually start face down on the floor with hands outstretched either perpendicular or parallel to the body. The first phase involves the arms being brought palms down on a 90 degree angle at the elbows. The second phase involves the body being pushed into the up position. The third phase is returning to the starting position. This technique is commonly used after a large block of regular push ups, as it poses less stress and requires less effort.

text: "Diamond" or "Triceps" push-ups are done by placing both palms on the ground and touching together both thumbs and pointer fingers. This technique requires stronger triceps muscles than regular push-ups due to the fact that, at the bottom of the stroke, the forearm is nearly parallel to the ground and the elbow is almost completely flexed, resulting in much higher mechanical load on the triceps.

text: "Hollow-Body" push-ups are performed in the position gymnasts call the "hollow body". In the plank version of the hollow body, the shoulders are protracted into a pronounced curve in the upper back while the abdominal muscles are tightened and the legs are locked and squeezed together. This variation requires full-body tension to execute and results in greater integration of the hips, shoulders, and core.

text: Plyometrics[edit]

text: Two platforms are placed beside the trainee, one on either side. The exercise begins with the hands on either platform supporting the body, then the subject drops to the ground and explosively rebounds with a push-up, extending the torso and arms completely off the ground and returning the hands to the platforms.

text: Another is simply an explosive push-up where a person attempts to push quickly and with enough force to raise his or her hands several centimeters off the ground, with the body completely suspended on the feet for a moment, a variation of the drop push. This is necessary for performing 'clap push ups' - i.e. clapping the hands while in the air.

text: Aztec push-ups

text: The Aztec push-up is one of the most difficult plyometric push-ups. A person performs an Aztec push-up by beginning in the normal push-up starting position and exploding upward with both the hands and feet, driving the entire body into the air. While in the air, the body is bent at the waist and the hands quickly touch the toes. The body is then quickly straightened and the hands and feet break the fall, returning the body to the normal push-up position for another repetition.[13]

text: 360 push ups

text: The 360 push-up is a variation of the superman push-up where one rotates 360 degrees while in the air.[14]

text:
Falling and explosive rebound push ups
Here one falls to the ground from standing position and then using an explosive push-up gets back to standing position.[15]

text: With push-ups, many possibilities for customization and increased intensity are possible. Some examples are: One hand can be set on a higher platform than the other or be farther away from the other to give more weight to the opposite arm/side of the body and also exercise many diverse muscles. One can perform push-ups by using only the tips of the fingers and thumb. For increased difficulty, push-ups can be performed on one arm or using weights.

text: Push-ups between chairs form an integral part of the "Dynamic Tension" Course devised by Charles Atlas, and similar systems.

text: Non-training[edit]

text: They are also commonly used as a fitness test or as a mild physical punishment on the spot (while benefiting the punished), to show off physically or as demonstration of submission.

text: In a competitive or disciplinary context especially, it is not rare to use more difficult or unpleasant variations, e.g., in mud, gravel, snow or dirt, hot ground, divested, and/or to make it physically harder, as by putting one's foot or a weight on the performer's back (possibly with sanctions if equilibrium is lost, such as spilling a glass) or to do the exercise resting on the knuckles or not use all fingers (not counting the thumb).

text: Record breakers and attempts[edit]

text:
The first record for push-ups was documented by Guinness World Records: 6,006 non-stop push-ups by Charles Linster in 1965, October 5.[16]
The record for the most push-ups non-stop was 10,507, set by Minoru Yoshida of Japan in October 1980.[17] Yoshida's record was the last for non-stop push-ups to be published by Guinness World Records. A new category, "Most Push-ups in 24 Hours," has since been introduced.
The current world record for most push-ups in 24 hours is by Charles Servizio (USA) who achieved 46,001 push-ups in just 21 hours, 6 minutes on 1993, April 24 to 25.[18]
The most push ups in one hour is 3,877 and was achieved by Bijender singh (India) on 20 sept 1988.[19] The Guinness world record for most push ups in one hour is 2,298 by David Escojido (USA) on 2 April 2016.[20]
Doug Pruden (Canada) performed 1,025 one-arm push-ups on the back of the hand on 8 November 2008.[21]
The flying push up record was set by Ahmed Valentino Kerigo on March 22, 2013, who reached a height of 1.36 meters.[22][23]
Paul Lynch (UK) performed 124 consecutive one finger push-ups on 21 April 1992.[24]
The most knuckle push ups in one hour is 2,175 and was achieved by Syed Taj Muhammad (Pakistan) on March 2016.[25]
The most knuckle push ups in one hour by a woman is 1,206 by Eva Clarke (Australia) on 31 January 2014.[26]
The most consecutive headstand push ups on one arm is 7 by Yury Tikhonovich (Russia) on 1 April 2010.[27][28][29] In recordholders the record is 12 by the same person[30][31][32]

tag: ul

text: Most push ups in one minute :

text:
using back of hands : 132 and was achieved by Abdul Latif Mahmoud Saadiq (Qatar) on November 2009.[33]
with the finger tips (10 fingers) : 105 by Graham Maly (UK) on May 2009.[34]
with claps : 90 by Stephen Buttler (UK) on November 2011.[35]
knuckle : 84 by Roy Berger (Canada) on June 2016.[36]
one arm using the back of the hand : 59 by Doug Pruden (Canada) on March 2007.[37]
two finger push ups (two arms) : 52 by Aryan Grover (India) on August 2015.[38]
Aztec push ups : 50 and was achieved by Jason Shen (USA) on January 2014.[39]
two finger push ups (one arm): 46 by Mohammed Mohammed Ali Zeinhom (Egypt) on March 2010 (it was in 49 seconds).[40]
with a 100 lb pack : 34 by Paddy Doyle (UK) on June 2011.[41]
one arm carrying a 40 lb pack : 33 by Hiroyuki Gondou (Japan) on May 2014.[42]

text: In the animal kingdom[edit]

text: There are zoology observations that certain animals emulate a push up action. Most notably various taxa of the fence lizard exhibit this display,[43] primarily involving the male engaging in postures to attract females. The western fence lizard is a particular species that also engages in this behavior.[44] (It may be noted that in Mexican Spanish push-ups are called "lagartijas", which means "lizards".)


document_title: Encapsulation (networking)

question_text: explain the concept of encapsulation in computer networks

text: Encapsulation (networking)

text: This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2013) (Learn how and when to remove this template message)

tag: table

text: In computer networking, encapsulation is a method of designing modular communication protocols in which logically separate functions in the network are abstracted from their underlying structures by inclusion or information hiding within higher level objects.

text: The physical layer is responsible for physical transmission of the data. Link encapsulation allows local area networking and Internet Protocol (IP) provides global addressing of individual computers; Transmission Control Protocol (TCP) adds application or process selection, i.e., the port specifies the service such as a Web or TFTP server.[2]

text: During encapsulation, each layer builds a protocol data unit (PDU) by adding a header (and sometimes trailer) containing control information to the PDU from the layer above. For example, in the Internet protocol suite, the contents of a web page are encapsulated with an HTTP header, then by a TCP header, an IP header, and, finally, by a frame header and trailer. The frame is forwarded to the destination node as a stream of bits, where it is decapsulated (or de-encapsulated) into the respective PDUs and interpreted at each layer by the receiving node.[3]

text: The result of encapsulation is that each lower layer provides a service to the layer or layers above it, while at the same time each layer communicates with its corresponding layer on the receiving node. These are known as adjacent-layer interaction and same-layer interaction, respectively.[3]

text: In discussions of encapsulation, the more abstract layer is often called the upper layer protocol while the more specific layer is called the lower layer protocol. Sometimes, however, the terms upper layer protocols and lower layer protocols are used to describe the layers above and below IP, respectively.[2]

text: Encapsulation is a characteristic feature of most networking models, including both the OSI model and TCP/IP suite of protocols.