Definition galaxy


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[ gal-uhk-see ]

/ ˈgæl ək si /


  1. a large system of stars held together by mutual gravitation and isolated from similar systems by vast regions of space.
  2. (usually initial capital letter)Milky Way.

any large and brilliant or impressive assemblage of persons or things: a galaxy of opera stars.



We could talk until we're blue in the face about this quiz on words for the color "blue," but we think you should take the quiz and find out if you're a whiz at these colorful terms.

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Which of the following words describes “sky blue”?

Origin of galaxy

1350–1400; Middle English galaxie, galaxias<Medieval Latin galaxia,galaxias, ultimately <Greek galaxías kýklos the Milky Way; see galacto-

Words nearby galaxy

Galatia, Galatian, Galatians, galatine, galax, galaxy, Galba, galbanum, Galbraith, Galbraith, John Kenneth, Galcha Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

How to use galaxy in a sentence

  • So somehow a galaxy that spans tens of thousands of light-years is intimately related to what is, in effect, a microscopic dot at its center.

    The Universe Has Made Almost All the Stars It Will Ever Make - Issue 89: The Dark Side|Caleb Scharf|August 19, 2020|Nautilus

  • Our galaxy, the Milky Way, has hundreds of billions of stars.

    Explainer: Stars and their families|Ken Croswell|August 18, 2020|Science News For Students

  • On an even grander scale, it’s tempting to imagine life physically exchanging informational algorithms across an entire galaxy.

    How Life Could Continue to Evolve - Issue 88: Love & Sex|Caleb Scharf|August 12, 2020|Nautilus

  • In particular, our measurement of the current rate of expansion of the universe is about 10 percent lower than the value found using direct methods of measuring distances to nearby galaxies.

    Dark Energy: Map Gives Clue About What It Is—but Deepens Dispute About the Cosmic Expansion Rate|Julian Bautista|July 31, 2020|Singularity Hub 

  • The filament looks magnetized throughout, not just near the galaxy clusters that are moving toward each other from either end.

    The Hidden Magnetic Universe Begins to Come Into View|Natalie Wolchover|July 2, 2020|Quanta Magazine

  • Compare that to Guardians of the Galaxy which opened in Korea on July 31.

    Propaganda, Protest, and Poisonous Vipers: The Cinema War in Korea|Rich Goldstein|December 30, 2014|DAILY BEAST

  • Pratt, of course, just exploded with Guardians of the Galaxy and the upcoming lead in Jurassic World.

    Exclusive: Sony Emails Reveal Channing Tatum and Chris Pratt’s Plans For ‘Ghostbusters’ Film|William Boot|December 15, 2014|DAILY BEAST

  • He says he has yet to experience any negative feedback from the galaxy of Whovians.

    Doctor Who: It’s Time For a Black, Asian, or Woman Doctor|Nico Hines|December 11, 2014|DAILY BEAST

  • These black holes are a type known as quasars: extremely massive objects that emit more light than the rest of the galaxy.

    The Black Hole Tango|Matthew R. Francis|November 24, 2014|DAILY BEAST

  • Xiaomi smartphones and Samsung Galaxy tablets captured photos of the warplanes in action before the shots were uploaded to Weibo.

    Beijing's New Stealth Jet: Made in China|Brendon Hong|November 16, 2014|DAILY BEAST

  • What are a few paltry, lumps of crystallised carbon compared to a galaxy of a million million suns?

    God and my Neighbour|Robert Blatchford

  • At the southern end there is a large gallery, overshadowing the noisiest galaxy of Sunday infants we ever encountered.

    Our Churches and Chapels|Atticus

  • To this galaxy of artistic talent I have yet to add Habeneck, who conducted the orchestra.

    Frederick Chopin as a Man and Musician|Frederick Niecks

  • It was Etruria that produced not only Dante, but also a galaxy of great men such as no other part of Europe has presented.

    Archaic England|Harold Bayley

  • Knowing this distance from experience, they were able to calculate the diameter of this galaxy.

    Islands of Space|John W Campbell

British Dictionary definitions for galaxy (1 of 2)


any of a vast number of star systems held together by gravitational attraction in an asymmetric shape (an irregular galaxy) or, more usually, in a symmetrical shape (a regular galaxy), which is either a spiral or an ellipseFormer names: island universe, extragalactic nebula Related adjective: galactic

a splendid gathering, esp one of famous or distinguished people

Word Origin for galaxy

C14 (in the sense: the Milky Way), from Medieval Latin galaxia, from Latin galaxias, from Greek, from gala milk; related to Latin lac milk

British Dictionary definitions for galaxy (2 of 2)


the Galaxythe spiral galaxy, approximately 100 000 light years in diameter, that contains the solar system about three fifths of the distance from its centreAlso known as: the Milky Way System See also Magellanic Cloud

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for galaxy

Any of numerous large-scale collections of stars, gas, and dust that make up the visible universe. Galaxies are held together by the gravitational attraction of the material contained within them, and most are organized around a galactic nucleus into elliptical or spiral shapes, with a small percentage of galaxies classed as irregular in shape. A galaxy may range in diameter from some hundreds of light-years for the smallest dwarfs to hundreds of thousands of light-years for the largest ellipticals, and may contain from a few million to several trillion stars. Many galaxies are grouped into clusters, with the clusters themselves often grouped into larger superclusters. See more at active galaxy. See also elliptical galaxyirregular galaxylenticular galaxyspiral galaxy.

the Galaxy. The Milky Way.

The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for galaxy

A large, self-contained mass of stars.

notes for galaxy

A common form for galaxies is a bright center with spiral arms radiating outward.

notes for galaxy

The universe contains billions of galaxies.

notes for galaxy

The sun belongs to the galaxy called the Milky Way.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


Meaning of galaxy in English:


Pronunciation /ˈɡaləksi/

See synonyms for galaxy

Translate galaxy into Spanish

nounplural noun galaxies

  • 1A system of millions or billions of stars, together with gas and dust, held together by gravitational attraction.

    ‘There are reckoned to be about 400 billion stars in our galaxy, the Milky Way.’

    • ‘New planetary star systems and galaxies are being discovered almost daily.’
    • ‘Or it may expand so fast that gravity could never pull galaxies together again.’
    • ‘Some massive clusters of galaxies are similarly held together against the cosmic flow.’
    • ‘Supermassive black holes are found in the centers of galaxies that contain billions of stars.’
    • ‘Section IV takes us off the land and into comets, galaxies and constellations of stars.’
    • ‘Near the geographic center of the galaxy was a binary star system called Theralia.’
    • ‘The clusters smashed together thousands of galaxies and trillions of stars.’
    • ‘He was thinking only of the chance of individual stars within our own galaxy, the Milky Way, acting this way.’
    • ‘The rishis tell us of the galactic center of the universe, which feeds energy to all the galaxies and solar systems of the universe.’
    • ‘Stars and planets, galaxies and nebulas unveil themselves close to the eyes of the visitors.’
    • ‘Just as stars are the building blocks of galaxies, galaxies are the building blocks of the universe.’
    • ‘This clumping in turn produced the galaxies and clusters of galaxies we see today.’
    • ‘With even greater, if not absolute certainty, we know that man can never be in a position to detect life in other solar systems of galaxies.’
    • ‘Eventually, these protogalactic fragments merged and galaxies and quasars formed.’
    • ‘That is, we can discover things about the stars and galaxies involved from the way in which the Moon cuts off their light.’
    • ‘There are a hundred billion stars in our galaxy alone and there are billions of other galaxies.’
    • ‘There really is a big mountain out there on some planet in some solar system in some galaxy.’
    • ‘In the Nature we observe, the Universe is filled with dust and gas in addition to stars, planets and galaxies.’
    • ‘But of course it has structures in it, stars and galaxies and clusters of galaxies.’

    star system, solar system, constellation, cluster, nebula

    View synonyms
    1. 1.1the GalaxyThe galaxy of which the solar system is a part; the Milky Way.

      ‘And he needs to be president of the Galaxy to do it.’

      • ‘We realized that our Galaxy was just one of many billions of galaxies in the universe.’
      • ‘On another note, this story takes place now, just in a different part of the galaxy.’
      • ‘Conquering the galaxy involves dealing with a couple hundred billion stars at least.’
      • ‘You can conquer the galaxy with your best friends in a single evening.’
    2. 1.2A large group of impressive people or things.

      ‘the four musicians have played with a galaxy of stars’

      • ‘To create his galaxy of impressions, Baxter watched television avidly, even backstage between stage shows.’
      • ‘The official website is now a positive galaxy of useful stuff.’
      • ‘RSS-fortified radio on mobile phones opens a whole galaxy of possibilities.’
      • ‘Wales were savaged by the absence of a galaxy of their players.’
      • ‘Archeologists unearthed a galaxy of merry swastikas when they excavated Troy.’
      • ‘Kalpana Chawla, her name now enshrined in the galaxy of super achievers, was a rare bird indeed.’
      • ‘Greetings from a convention of those who admire the galaxy of nameless icons!’
      • ‘There were other great athletes in those days as well, in fact a galaxy of them.’
      • ‘We don't have the galaxy of players we had when we had Woosnam and Lyle, Faldo and Ballesteros.’
      • ‘Auden was the next writer to sign on, bringing in his wake a galaxy of homeless talents.’
      • ‘I would like to say that he was an emperor amongst the present galaxy of saints.’
      • ‘In the galaxy of first-class hams, this one most definitely deserves its spot.’
      • ‘Major cash problems for Australian clubs means a galaxy of internationals will want to play in Britain next season.’
      • ‘Music fans flocked to hear a galaxy of international stars at a festival of global music and dance.’
      • ‘A whole galaxy of cool, disco artists are banding together for a fun show to remember.’

      brilliant gathering, dazzling assemblage, illustrious group

      View synonyms

The Galaxy in which the earth is located is a disc-shaped spiral galaxy with approximately 100,000 million stars. The sun is located about two thirds of the way out from the centre


Late Middle English (originally referring to the Milky Way): via Old French from medieval Latin galaxia, from Greek galaxias (kuklos) ‘milky (vault)’, from gala, galakt- ‘milk’.

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Look up a word, learn it forever.

A galaxy is a collection of stars and planets that are held together by gravity. In a galaxy, the celestial bodies rotate around a central object.

The Earth's galaxy is known as the Milky Way. Our solar system, made up of the Sun and the planets that encircle it, is a tiny part of that galaxy. A galaxy can also refer to a gathering of a lot of sparkly people –– like celebrities or sports stars. What gravity holds this galaxy together? Usually cameras.

Definitions of galaxy

  1. noun

    (astronomy) a collection of star systems; any of the billions of systems each having many stars and nebulae and dust
    synonyms:extragalactic nebula
  2. noun

    a splendid assemblage (especially of famous people)
  3. noun

    tufted evergreen perennial herb having spikes of tiny white flowers and glossy green round to heart-shaped leaves that become coppery to maroon or purplish in fall
    synonyms:Galax urceolata, beetleweed, coltsfoot, galax, wandflower
    see moresee less
    type of:
    herb, herbaceous plant

    a plant lacking a permanent woody stem; many are flowering garden plants or potherbs; some having medicinal properties; some are pests


Meaning of galaxy in English

Nevertheless, galaxies with highly peculiar morphological properties were found by us during our searches, and will be investigated in detail in the years to come.

From the Cambridge English Corpus

When we turn from galaxies to the distribution of galaxies, the distribution does not seem to exhibit design - it looks rather random and haphazard.

From the Cambridge English Corpus

On the outside of galaxies there is much less light, meaning that there is much less mass concentrated in these regions.

From the Cambridge English Corpus

The interactions of galaxies will also lead to large changes of angular momentum and energy.

From the Cambridge English Corpus

Research on barred galaxies has been performed using observations, theory and numerical simulations.

From the Cambridge English Corpus

Following the introductory chapter, the next section reviews the current state of cosmology and prevalent theories of how matter, stars, galaxies and the universe formed.

From the Cambridge English Corpus

I have distinguished three sorts of galaxies, but it remains an open question which of the three types are instantiated.

From the Cambridge English Corpus

Perhaps the strongest form of evidence for the existence of dark matter is the rotation curves of galaxies.

From the Cambridge English Corpus

There must have been something there to contribute to the gravity needed to keep the stars and galaxies gravitationally bound to their system.

From the Cambridge English Corpus

Such a large axion density would be incompatible with the formation of galaxies and so is anthropically disallowed.

From the Cambridge English Corpus

The possibility of this happening is about as likely as humans walking the surface of planets of distant galaxies.

From the Cambridge English Corpus

See all examples of galaxy

These examples are from corpora and from sources on the web. Any opinions in the examples do not represent the opinion of the Cambridge Dictionary editors or of Cambridge University Press or its licensors.


Galaxy definition

What Is a Galaxy?

The Short Answer:

A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems, all held together by gravity.

We live on a planet called Earth that is part of our solar system. But where is our solar system? It’s a small part of the Milky Way Galaxy.

A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems. A galaxy is held together by gravity. Our galaxy, the Milky Way, also has a supermassive black hole in the middle.

a diagram that shows that a planet is part of a solar system, and the solar system is part of a galaxy

When you look up at stars in the night sky, you’re seeing other stars in the Milky Way. If it’s really dark, far away from lights from cities and houses, you can even see the dusty bands of the Milky Way stretch across the sky.

a photo of the Milky Way seen from a dark desert. The dusty colors light up the sky.

The Milky Way Galaxy stretches across the sky at the Trona Pinnacles National Landmark in California. Photo by Ian Norman.

There are many galaxies besides ours, though. There are so many, we can’t even count them all yet! The Hubble Space Telescope looked at a small patch of space for 12 days and found 10,000 galaxies, of all sizes, shapes, and colors. Some scientists think there could be as many as one hundred billion galaxies in the universe.

a photo taken by the Hubble Space Telescope of thousands of small galaxies of all different shapes and colors.

This is the picture taken by the Hubble Space Telescope showing thousands of galaxies. Even the tiny dots are whole galaxies. The universe is a very big place!

Some galaxies are spiral-shaped like ours. They have curved arms that make it look like a pinwheel. Other galaxies are smooth and oval shaped. They’re called elliptical galaxies. And there are also galaxies that aren’t spirals or ovals. They have irregular shapes and look like blobs. The light that we see from each of these galaxies comes from the stars inside it.

A diagram of the different shapes of galaxies: spiral, elliptical, and irregular.

Sometimes galaxies get too close and smash into each other. Our Milky Way galaxy will someday bump into Andromeda, our closest galactic neighbor. But don’t worry. It won’t happen for about five billion years. But even if it happened tomorrow, you might not notice. Galaxies are so big and spread out at the ends that even though galaxies bump into each other, the planets and solar systems often don’t get close to colliding.



Gravitationally bound astronomical structure

This article is about the astronomical structure. For our galaxy, see Milky Way. For other uses, see Galaxy (disambiguation).

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

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

In 2021, data from NASA's New Horizons space probe was used to revise the previous estimate to roughly 200 billion galaxies (2×1011),[7] which followed a 2016 estimate that there were two trillion (2×1012) or more[8][9] galaxies in the observable universe, overall, and as many as an estimated 1×1024 stars[10][11] (more stars than all the grains of sand on all beaches of the planet Earth).[12] Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 ly) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million ly.)

The space between galaxies is filled with a tenuous gas (the intergalactic medium) with an average density of less than one atom per cubic meter. Most galaxies are gravitationally organized into groups, clusters and superclusters. The Milky Way is part of the Local Group, which it dominates along with Andromeda Galaxy. The group is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids.[13] Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea.[14]


The word galaxy was borrowed via French and Medieval Latin from the Greek term for the Milky Way, galaxías (kúklos)γαλαξίας (κύκλος)[15][16] 'milky (circle)', named after 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 the baby will drink her divine milk and 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 band of light known as the Milky Way.[17][18]

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 Chaucerc. 1380:

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

— Geoffrey Chaucer, The House of Fame[16]

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


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), the UGC (Uppsala General Catalogue of Galaxies), and the PGC (Catalogue of Principal Galaxies, also known as LEDA). All the well-known galaxies appear in one or more of these catalogs but each time under a different number. For example, Messier 109 (or "M109") is a spiral galaxy having the number 109 in the catalog of Messier. It also has the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, and PGC 37617 (or LEDA 37617). Millions of fainter galaxies are known by their identifiers in sky surveys such as the Sloan Digital Sky Survey, in which M109 is cataloged as SDSS J115735.97+532228.9.

Observation history[edit]

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

Milky Way[edit]

Main article: Milky Way

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.[20]Aristotle (384–322 BCE), however, believed the Milky Way was 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."[21]Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 CE) was critical of this view, arguing that if the Milky Way was 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 did not. In his view, the Milky Way was celestial.[22]

According to Mohani Mohamed, Arabian astronomer Alhazen (965–1037) made the first attempt at observing and measuring the Milky Way's parallax,[23] and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere."[24]Persian astronomer al-Bīrūnī (973–1048) proposed the Milky Way galaxy was "a collection of countless fragments of the nature of nebulous stars."[25]Andalusian astronomer Ibn Bâjjah ("Avempace", d. 1138) proposed that it was composed of many stars that almost touched one another, and appeared to be a continuous image due to the effect of refraction from sublunary material,[21][26] citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects were near.[21] In the 14th century, Syrian-born Ibn Qayyim proposed the Milky Way galaxy was "a myriad of tiny stars packed together in the sphere of the fixed stars."[27]

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

Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study it and discovered it was composed of a huge number of faint stars.[28][29] In 1750, English astronomer Thomas Wright, in his An Original Theory or New Hypothesis of the Universe, correctly speculated that it 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, and that the resulting disk of stars could be seen as a band on the sky from our perspective inside it.[30][31] In his 1755 treatise, Immanuel Kant elaborated on Wright's idea about the Milky Way's structure.[32]

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.[33][34] 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.[31] 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 emerged.[35]

Distinction from other nebulae[edit]

A few galaxies outside the Milky Way are visible on a dark night to the unaided eye, including the Andromeda Galaxy, Large Magellanic Cloud, the Small Magellanic Cloud, and the Triangulum Galaxy. In the 10th century, Persian astronomer Al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud".[36] In 964, he probably mentioned the Large Magellanic Cloud in his Book of Fixed Stars (referring to "Al Bakr of the southern Arabs",[37] since at a declination of about 70° south it was not visible where he lived); it was not well known to Europeans until Magellan's voyage in the 16th century.[38][37] The Andromeda Galaxy was later independently noted by Simon Marius in 1612.[36] In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there might be galaxies outside our own that were formed into galactic clusters that were minuscule parts of the universe that extended far beyond what we could see. These views "are remarkably close to the present-day views of the cosmos."[39] In 1745, Pierre Louis Maupertuis conjectured that some nebula-like objects were collections of stars with unique properties, including a glow exceeding the light its stars produced on their own, and repeated Johannes Hevelius's view that the bright spots were massive and flattened due to their rotation.[40] In 1750, Thomas Wright correctly speculated that the Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.[31][41]

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.[31] 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.[42]

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.[43][44]

In 1917, Heber Curtis observed nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy, Messier objectM31, 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.[45]

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.[46]

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.[47] 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.[48] In 1936 Hubble produced a classification of galactic morphology that is used to this day.[49]

Modern research[edit]

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

In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas;[50] 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.[51] 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.[52][53]

Scientists used the galaxies visible in the GOODSsurvey to recalculate the total number of galaxies.[54]

Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, its data helped establish that the missing dark matter in our galaxy could not consist solely of inherently faint and small stars.[55] The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the observable universe.[56] Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in the Zone of Avoidance (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.[57]

A 2016 study published in The Astrophysical Journal, led by Christopher Conselice of the University of Nottingham, used 20 years of Hubble images to estimate that the observable universe contained at least two trillion (2×1012) galaxies.[8][9] However, later observations with the New Horizons space probe from outside the zodiacal light reduced this to roughly 200 billion (2×1011).[58][59]

Types and morphology[edit]

Main article: Galaxy morphological classification

Types of galaxies according to the Hubble classification scheme: an Eindicates a type of elliptical galaxy; an Sis a spiral; and SBis a barred-spiral galaxy.[note 1]

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


Main article: Elliptical galaxy

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead, 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.[60]

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.[61]

Shell galaxy[edit]

NGC 3923Elliptical Shell Galaxy (Hubble photograph)

A shell galaxy is a type of elliptical galaxy where the stars in its 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. These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy NGC 3923 has over 20 shells.[62]


Main articles: Spiral galaxy and Barred spiral galaxy

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 which extends beyond the visible component, as demonstrated by the universal rotation curve concept.[63]

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) 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.[64] 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.[65] 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.[66]

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".[67] 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.[68]

Barred spiral galaxy[edit]

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.[69] 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 tidal interaction with another galaxy.[70] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.[71]

Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy[72] about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011)[73] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[74]

Super-luminous spiral[edit]

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.[75][76]

Other morphologies[edit]

  • 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.[77] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.[78]
  • 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[79] (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.[80] Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.
  • An ultra diffuse galaxy (UDG) is an extremely-low-density galaxy. It may be the same size as the Milky Way, but have a visible star count only one percent of the Milky Way's. Its lack of luminosity is due to a lack of star-forming gas, resulting in old stellar populations.


Main article: Dwarf galaxy

Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies. They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.[81]

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

A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses, regardless of whether it has thousands or millions of stars. This suggests 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.[83]

Other types of galaxies[edit]


Main article: Interacting galaxy

The Antennae Galaxiesare undergoing a collision that will result in their eventual merger.

Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.[84][85] Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies usually do not collide, but the gas and dust within the two forms interacts, sometimes triggering star formation. A collision can severely distort the galaxies' shapes, forming bars, rings or tail-like structures.[84][85]

At the extreme of interactions are galactic mergers, where the galaxies' relative momentums are insufficient to allow them to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to the galaxies' original morphology. If one of the galaxies is much more massive than the other, the result is known as cannibalism, where the more massive larger galaxy remains relatively undisturbed, and the smaller one 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.[84][85]


Main article: Starburst galaxy

M82, a starburst galaxy that has ten times the star formation of a "normal" galaxy[86]

Stars are created within galaxies from a reserve of cold gas that forms 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, they would consume their reserve of gas in a time span less than the galaxy's lifespan. Hence starburst activity usually lasts only about ten million years, a relatively brief period in a galaxy's history. Starburst galaxies were more common during the universe's early history,[87] but still contribute an estimated 15% to total star production.[88]

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.[89] These stars produce supernova explosions, creating 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 activity end.[87]

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.[90]

Active galaxy[edit]

Main article: Active galactic nucleus

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

Some observable galaxies are classified as "active" if they contain an active galactic nucleus (AGN). A significant portion of the galaxy's total energy output is emitted by the active nucleus instead of its stars, dust and interstellar medium. There are multiple classification and naming schemes for AGNs, but those in the lower ranges of luminosity are called Seyfert galaxies, while those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects or quasars. AGNs emit radiation throughout the electromagnetic spectrum from radio wavelengths to X-rays, though some of it may be absorbed by dust or gas associated with the AGN itself or with the host galaxy.

The standard model for an active galactic nucleus is based on an accretion disc that forms around a supermassive black hole (SMBH) at the galaxy's core region. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.[91] The AGN's luminosity depends on the SMBH's mass and the rate at which matter falls onto it. 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.[92]


Main article: Blazar

Blazars are believed to be active galaxies with a relativistic jet 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 observer's position.[92]


Main article: Low-ionization nuclear emission-line region

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.[93] Approximately one-third of nearby galaxies are classified as containing LINER nuclei.[91][93][94]

Seyfert galaxy[edit]

Main article: Seyfert galaxy

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


Main article: Quasar

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

Luminous infrared galaxy[edit]

Main article: Luminous infrared galaxy

Luminous infrared galaxies (LIRGs) are galaxies with luminosities—the measurement of electromagnetic power output—above 1011 L☉ (solar luminosities). In most cases, most of their energy comes from large numbers of young stars which heat surrounding dust, which reradiates the energy in the infrared. Luminosity high enough to be a LIRG requires a star formation rate of at least 18 M☉ yr−1. Ultra-luminous infrared galaxies (ULIRGs) are at least ten times more luminous still and form stars at rates >180 M☉ yr−1. Many LIRGs also emit radiation from an AGN. Infrared galaxies emit more energy in the infrared than all other wavelengths combined, with peak emission typically at wavelengths of 60 to 100 microns. LIRGs are uncommon in the local universe but were much more common when the universe was younger.


Magnetic fields[edit]

Galaxies have magnetic fields of their own.[95] They are strong enough to be dynamically important, as they:

  • Drive mass inflow into the centers of galaxies
  • Modify the formation of spiral arms
  • Can affect the rotation of gas in the galaxies' outer regions
  • Provide the transport of angular momentum required for the collapse of gas clouds, and hence the formation of new stars

The typical average equipartition strength for spiral galaxies is about 10 μG (microGauss) or 1 nT (nanoTesla). By 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 M33, 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, such as the centers of NGC 1097 and other barred galaxies.[95]

Formation and evolution[edit]

Main article: Galaxy formation and evolution

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


Artist's impression of a protocluster forming in the early universe[96]

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.[97][98] These primordial structures eventually became the galaxies we see today.

Artist's impression of a young galaxy accreting material

Early galaxy formation[edit]

Evidence for the appearance of galaxies very early in the Universe's history 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 earliest-to-form galaxy seen at that time.[99] 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[100] after the Big Bang (which was about 13.8 billion years ago),[101] is about 13.42 billion light travel distance years away. The existence of galaxies so soon after the Big Bang suggests that protogalaxies must have grown in the so-called "dark ages".[97] 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.[102][103][104][105][106]

The detailed process by which the earliest 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.[108] 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.[109] 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 more massive than 100 times the Sun's mass. If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium.[110] This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.[111]

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.[112][113]


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.[114] During this early epoch, galaxies undergo a major burst of star formation.[115]

During the following two billion years, the accumulated matter settles into a galactic disc.[116] A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its life.[117] 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.[118]

XDF view field compared to the angular size of the Moon. Several thousand galaxies, each consisting of billions of stars, are in this small view.

XDF (2012) view: Each light speck is a galaxy, some of which are as old as 13.2 billion years[119] – the observable universe is estimated to contain 200 billion to two trillion galaxies.

XDF image shows (from left) fully mature galaxies, nearly mature galaxies (from five to nine billion years ago), and protogalaxies, blazing with young stars (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.[120] 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[121] or the Antennae Galaxies.[122]

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.[123]

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.[124]

Future trends[edit]

Main article: Future of an expanding universe

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.[125] Elliptical galaxies are largely devoid of this gas, and so form few new stars.[126] 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.[127][128]

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.[127][129]

Larger-scale structures[edit]

Main articles: Observable universe § Large-scale structure, Galaxy filament, and Galaxy groups and clusters

Deep-sky surveys show that galaxies are often found in groups and clusters. Solitary galaxies that have not significantly interacted with other galaxies of comparable mass in the past billion years are relatively scarce. Only about 5% of the galaxies surveyed have been found to be truly isolated; however, they 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.[130]

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, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This ongoing merging process (as well as an influx of infalling gas) heats the intergalactic gas in a cluster to very high temperatures of 30–100 megakelvins.[131] About 70–80% of a cluster's mass is in the form of dark matter, with 10–30% consisting of this heated gas and the remaining few percent in the form of galaxies.[132]

Most galaxies 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; these formations contain the majority of galaxies (as well as most of the baryonic mass) in the universe.[133][134] 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.[135]

Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.[136] 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.[137]

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.[138] Above this scale, the universe appears to be the same in all directions (isotropic and homogeneous).,[139] though this notion has been challenged in recent years by numerous findings of large-scale structures that appear to be exceeding this scale. The Hercules-Corona Borealis Great Wall, currently the largest structure in the universe found so far, is 10 billion light-years (three gigaparsecs) in length.[140][141][142]

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.[143] 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.[144] And the Virgo Supercluster itself is a part of the Pisces-Cetus Supercluster Complex, a giant galaxy filament.

Southern plane of the Milky Way from submillimeter wavelengths[146]

Multi-wavelength observation[edit]

See also: Observational astronomy

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

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

The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared, which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail.[147] Infrared is also used to observe distant, red-shifted galaxies that were formed much earlier. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy.

The 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.)[148] 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.[149]

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.[150] 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.[151]


  • LEFT: ARP-MADORE2115-273 is a rare example of an interacting galaxy pair in the southern hemisphere. RIGHT: ARP-MADORE0002-503 is a large spiral galaxy with unusual, extended spiral arms, at a distance of 490 million light-years.[153]

See also[edit]


  1. ^Galaxies to the left side of the Hubble classification scheme are sometimes referred to as "early-type", while those to the right are "late-type".
  2. ^The term "field galaxy" is sometimes used to mean an isolated galaxy, although the same term is also used to describe galaxies that do not belong to a cluster but may be a member of a group of galaxies.


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Now discussing:

Hit the road abroad for a week or two with the whole family, we still celebrate the New Year all together - in our family circle. Over time, the girlfriends all became confused, mainly because of envy, because most (if not all) of them were much less fortunate than me.

Although they predicted that I would be left alone and die an old maid. Someone's husband drinks, another - beats, and a third - after work on the sofa with beer and sausage. My own, over the years lived together, showed himself to be an extremely intelligent person, never even raised his voice at me, and, although over.

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