A typical in the, is about 55,000 in diameter and approximately 60 million light-years from Earth.A galaxy is a bound system of,. The word galaxy is derived from the galaxias ( γαλαξίας), literally 'milky', a reference to the. Galaxies range in size from with just a few hundred million (10 8) stars to with one hundred (10 14) stars, each orbiting its galaxy's.Galaxies are categorized according to their visual as,. Many galaxies are thought to have at their centers. The Milky Way's central black hole, known as, has a mass four million times greater than the.

As of March 2016, is the oldest and most distant observed galaxy with a of 32 billion from Earth, and observed as it existed just 400 million years after the.Research released in 2016 revised the number of galaxies in the from a previous estimate of 200 billion ( 2 ×10 11) to a suggested two trillion ( 2 ×10 12) or more and, overall, as many as an estimated 1 ×10 24 stars (more stars than all the on planet ). Most of the galaxies are 1,000 to 100,000 in diameter (approximately 3000 to 300,000 ) 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, its nearest large neighbor, by 780,000 parsecs (2.5 million ly.)The between galaxies is filled with a tenuous gas (the ) having an average density of less than one per cubic meter. The majority of galaxies are gravitationally organized into,. The Milky Way is part of the, which is dominated by it and the Andromeda Galaxy and is part of the.

At the, these associations are generally arranged into surrounded by immense. Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named. — Geoffrey Chaucer,Galaxies were initially discovered telescopically and were known as. Most 18th to 19th Century astronomers considered them as either unresolved or anagalactic, 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, 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.

SDSS stands for, J for, and 1152+3313 for the and respectively.Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the, the, the, and the. Astronomers work with numbers from certain catalogues, such as the, the NGC , the IC , the CGCG , the MCG and UGC ( of Galaxies). All the well-known galaxies appear in one or more of these catalogues but each time under a different number.For example, is a spiral galaxy having the number 109 in the catalogue of Messier, and also having the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, and PGC 37617.Observation historyThe 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.Milky Way. Main article:The philosopher (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.(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, in the.' The philosopher ( c.

Xda-developers Samsung Galaxy S9 Samsung Galaxy S9 Real Life Review Screen on time by XDARealLifeReview. XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Apr 11, 2020  Nov. 21, 2019 — Scientists proved for the first time that the galaxy NGC 6240 contains three supermassive black holes. The unique observations show the black holes close to each other in the.

495–570 CE) was critical of this view, arguing that if the Milky Way is (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have, which it does not. In his view, the Milky Way is celestial.According to Mohani Mohamed, the astronomer (965–1037) made the first attempt at observing and measuring the Milky Way's parallax, and he thus 'determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere.'

The astronomer (973–1048) proposed the Milky Way galaxy to be 'a collection of countless fragments of the nature of nebulous stars.' The astronomer ('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 from sublunary material, citing his observation of the of Jupiter and Mars as evidence of this occurring when two objects are near. In the 14th century, the Syrian-born proposed the Milky Way galaxy to be 'a myriad of tiny stars packed together in the sphere of the fixed stars.' The shape of the Milky Way as estimated from star counts by 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 used a to study the Milky Way and discovered that it is composed of a huge number of faint stars.In 1750 the English astronomer, 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 forces, akin to the 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. In a treatise in 1755, elaborated on Wright's idea about the structure of the Milky Way.The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by 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. Using a refined approach, 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 based on the cataloguing of led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the by present in the, but after quantified this effect in 1930 by studying, the present picture of our host galaxy, the Milky Way, emerged. Mosaic of the Milky Way arching at a high inclination across the night sky, shot from a dark-sky location in Chile.

The, satellite galaxies of the Milky Way, appear near the left edge. Distinction from other nebulaeA few galaxies outside the Milky Way are visible on a dark night to the, including the, the, and the.

In the 10th century, the Persian astronomer made the earliest recorded identification of the Andromeda Galaxy, describing it as a 'small cloud'. In 964, Al-Sufi probably mentioned the Large Magellanic Cloud in his (referring to 'Al Bakr of the southern Arabs', since at a of about 70° south it was not visible where he lived); it was not well known to Europeans until 's voyage in the 16th century. The Andromeda Galaxy was later independently noted by in 1612.In 1734, philosopher in his Principia speculated that there may be galaxies outside our own that are formed into galactic clusters that are minuscule 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.' In 1745, conjectured that some -like objects are collections of stars with unique properties, including a its stars produce on their own, and repeated 's view that the bright spots are massive and flattened due to their rotation.In 1750, 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. Photograph of the 'Great Andromeda Nebula' by, 1899, later identified as theToward the end of the 18th century, compiled a containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae.

In 1845, 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.In 1912, made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high, 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.In 1917, observed nova within the 'Great Nebula' (as the Andromeda Galaxy, was then known). Searching the photographic record, he found 11 more. Curtis noticed that these novae were, on average, 10 fainter than those that occurred within our galaxy. As a result, he was able to come up with a distance estimate of 150,000. He became a proponent of the so-called 'island universes' hypothesis, which holds that spiral nebulae are actually independent galaxies.In 1920 a debate took place between and (the ), 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.In 1922, the astronomer gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. Using the new 100 inch telescope, was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.

In 1936 Hubble produced a classification of that is used to this day. Modern research. Of a typical spiral galaxy: predicted based on the visible matter (A) and observed (B). The distance is from the.In 1944, predicted that radiation with would be detectable from interstellar atomic gas; 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 in the center of our galaxy.

With improved, hydrogen gas could also be traced in other galaxies.In the 1970s, uncovered a discrepancy between observed galactic 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. Scientists used the galaxies visible in the survey to recalculate the total number of galaxies.Beginning in the 1990s, the 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. The, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion ( 1.25 ×10 11) galaxies in the observable universe. Improved technology in detecting the invisible to humans (radio telescopes, infrared cameras, and ) allow detection of other galaxies which are not detected by Hubble.

Particularly, galaxy surveys in the (the region of the sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.In 2016, a study published in and led by of the using 3D modeling of images collected over 20 years by the Hubble Space Telescope concluded that there are more than two trillion ( 2 ×10 12) galaxies in the observable universe. Types and morphology.

Types of galaxies according to the Hubble classification scheme: an E indicates a type of elliptical galaxy; an S is a spiral; and SB is a barred-spiral galaxy.Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the. Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as rate in and activity in the cores of. Main article: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 profile, giving them an elliptical appearance regardless of the viewing angle.

Their appearance shows little structure and they typically have relatively little. Consequently, these galaxies also have a low portion of and a reduced rate of new star formation. Instead they are dominated by generally older, more 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.The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the, 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.

Shell galaxy. Elliptical Shell Galaxy (Hubble 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 has over twenty shells. The, NGC 5457Spiral galaxies resemble spiraling.

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 which extends beyond the visible component, as demonstrated by the universal rotation curve concept.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 are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter ( a, b, or c) which 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.

A galaxy with poorly defined arms is sometimes referred to as a; in contrast to the that has prominent and well-defined spiral arms. 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. An example of aIn spiral galaxies, the spiral arms do have the shape of approximate, 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. The spiral arms are thought to be areas of high-density matter, or '. 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. An example of a Barred spiral galaxyA majority of spiral galaxies, including our own 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. In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter ( a, b or c) which 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 with another galaxy. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.Our own galaxy, the, is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec thick. The gardens between ps4. It contains about two hundred billion (2×10 11) stars and has a total mass of about six hundred billion (6×10 11) times the mass of the Sun.

Super-luminous spiralRecently, 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.

Other morphologies. are galactic formations that develop unusual properties due to tidal interactions with other galaxies. A 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. Such an event may have affected the, as it displays a multi-ring-like structure when viewed in radiation. A 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 ( receive Hubble classification SB0.).

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. Nearby examples of (dwarf) irregular galaxies include the. An (UDG) is an extremely-low-density galaxy. The galaxy may be the same size as the Milky Way but has a visible star count only one percent of the Milky Way.

The lack of luminosity is because there is a lack of star-forming gas in it, which results in old stellar populations.Dwarfs. Main article:Despite the prominence of large elliptical and spiral galaxies, most galaxies 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.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. Dwarf galaxies may also be classified as,.

Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called instead.A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million, regardless of whether the galaxy has thousands or millions of stars. This has led to the suggestion that galaxies are largely formed by, and that the minimum size may indicate a form of incapable of gravitational coalescence on a smaller scale. Other types of galaxies Interacting. The 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. Near misses between galaxies result in warping distortions due to, and may cause some exchange of gas and dust.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.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. 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 and the. A starburst galaxy that has ten times the star formation of a 'normal' galaxyStars are created within galaxies from a reserve of cold gas that forms into giant. 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 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, and, at present, still contribute an estimated 15% to the total star production rate.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. These massive stars produce explosions, resulting in expanding 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.Starbursts are often associated with merging or interacting galaxies.

The prototype example of such a starburst-forming interaction is, which experienced a close encounter with the larger. Irregular galaxies often exhibit spaced knots of starburst activity. Active galaxy. A jet of particles is being emitted from the core of the elliptical radio galaxy.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 of the galaxy.The standard model for an is based upon an that forms around a (SMBH) at the core region of the galaxy. The radiation from an active galactic nucleus results from the of matter as it falls toward the black hole from the disc. In about 10% of these galaxies, a diametrically opposed pair of ejects particles from the galaxy core at velocities close to the. The mechanism for producing these jets is not well understood. or, are classified depending on the luminosity, are active galaxies that emit high-energy radiation in the form of.Blazars.

Main article: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.Quasar. Main article:Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of brightness, above 10 11 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.

LIRGs are 100 billion times brighter than our Sun.Properties Magnetic fieldsGalaxies have of their own. 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 is about 10 μG or 1 nT. For comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss or 30 μT. Radio-faint galaxies like and, our '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, for example in M 82 and the, and in nuclear starburst regions, for example in the centers of NGC 1097 and of other. Formation and evolution. Artist's impression of a protocluster forming in the early universeCurrent cosmological models of the early universe are based on the theory. About 300,000 years after this event, atoms of and began to form, in an event called.

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 '. It was from density fluctuations (or irregularities) in this primordial matter that began to appear. As a result, masses of matter started to condense within halos. These primordial structures would eventually become the galaxies we see today.

Artist's impression of a young galaxy accreting material Early galaxiesEvidence for the early appearance of galaxies was found in 2006, when it was discovered that the galaxy has an unusually high of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and primordial galaxy yet seen.While some scientists have claimed other objects (such as ) 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 is the most distant object known and has a redshift value of 11.9. The object, estimated to have existed around '380 million years' after the (which was about 13.8 billion years ago), is about 13.42 billion away. The existence of such early suggests they must have grown in the so-called 'dark ages'. As of May 5, 2015, the galaxy is the most distant and earliest galaxy measured, forming 670 million years after the. 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 during 13 billion years.

Early galaxy formation. Different components of near-infrared background light detected by the in deep-sky surveysThe 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.

In bottom-up theories (such as the Searle-Zinn SZ model), small structures such as form first, and then a number of such bodies accrete to form a larger galaxy.Once protogalaxies began to form and contract, the first (called ) 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, releasing heavy elements into the.

This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.In June 2015, astronomers reported evidence for in the 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 heavier than that are needed for the later formation of planets and life as we know it. EvolutionWithin a billion years of a galaxy's formation, key structures begin to appear., the central supermassive black hole, and a of metal-poor 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. During this early epoch, galaxies undergo a major burst of star formation.During the following two billion years, the accumulated matter settles into a. A galaxy will continue to absorb infalling material from and throughout its life. This matter is mostly hydrogen and helium.

The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the of. Image shows (from left) fully mature galaxies, nearly mature galaxies (from five to nine billion years ago), and, blazing with (beyond nine 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. 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 or the.The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130, 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.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 about ten billion years ago.

Future trends. Main article:Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense of interstellar hydrogen in their spiral arms.

Elliptical galaxies are largely devoid of this gas, and so form few new stars. 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.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 (10 13–10 14 years), as the smallest, longest-lived stars in our universe, tiny, begin to fade.

At the end of the stellar age, galaxies will be composed of:, that are cooling or cold ('),. Eventually, as a result of, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions. Larger-scale structures. Is an example of a compact galaxy group.Deep sky surveys show that galaxies are often found in groups. Solitary galaxies that have not significantly interacted with another galaxy of comparable mass during the past billion years are relatively scarce. Only about five percent 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 can produce stars at a higher rate than normal, as their gas is not being stripped by other nearby galaxies.On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see ). Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early, 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. 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.Most galaxies are gravitationally bound to a number of other galaxies.

These form a -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 mass) in the universe. To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (see ). If there is insufficient, however, the group may evolve into a smaller number of galaxies through mergers.Unsolved problem in physics. The in the universe are larger than expected. Are these actual structures or random density fluctuations?Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.

Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the, which, over time, destroys its satellite galaxies and adds their mass to its own.contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the, galaxies are arranged into sheets and filaments surrounding vast empty voids.

Above this scale, the universe appears to be the same in all directions ( and )., though this notion has been challenged in recent years by numerous findings of large-scale structures that appear to be exceeding this scale. The, currently the in the universe found so far, is 10 billion (three gigaparsecs) in length.The Milky Way galaxy is a member of an association named the, 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. The Local Group itself is a part of a cloud-like structure within the, a large, extended structure of groups and clusters of galaxies centered on the. And the Virgo Supercluster itself is a part of the, a giant. This ultraviolet image of shows blue regions containing young, massive stars.The peak radiation of most stars lies in the, so the observation of the stars that form galaxies has been a major component of. It is also a favorable portion of the spectrum for observing ionized, and for examining the distribution of dusty arms.The present in the interstellar medium is opaque to visual light.

It is more transparent to, which can be used to observe the interior regions of giant molecular clouds and in great detail. Infrared is also used to observe distant, galaxies that were formed much earlier. Water vapor and absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for.The first non-visual study of galaxies, particularly active galaxies, was made using. The Earth's atmosphere is nearly transparent to radio between 5 and 30 GHz. (The blocks signals below this range.) Large radio have been used to map the active jets emitted from active nuclei. Dead state download pt br.

Can also be used to observe neutral hydrogen (via ), including, potentially, the non-ionized matter in the early niverse that later collapsed to form galaxies.and 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. The distribution of hot gas in galactic clusters can be mapped by X-rays.

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