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A galaxy is a
gravitation Gravity (), or gravitation, is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germination seedlings, three days after germination. Germination is t ...

gravitation
ally bound system of
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
s,
stellar remnant In astronomy, the term compact star (or compact object) refers collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects hav ...
s,
interstellar gas Interstellar or Interstella may refer to: Space * Interstellar space ** Interstellar medium * Interstellar travel * Interstellar communication * Interstellar probe Art, entertainment, and media Films and soundtracks * ''Interstella 5555: The 5tor ...
,
dust Dust is made of s of solid . On Earth, it generally consists of particles in the that come from various sources such as lifted by wind (an ), , and . Dust in homes is composed of about 20–50% dead . The rest, and in offices, and other ...
, and
dark matter Dark matter is a hypothetical form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, ...

dark matter
. The word is derived from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
' (), literally "milky", a reference to the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

Milky Way
. Galaxies range in size from dwarfs with just a few hundred million () stars to
giants Giant or Giants may refer to: Mythology and religion *Giant **Giants (Greek mythology) **Giants (Norse mythology) **Giants (Welsh folklore) **Giants (esotericism) **Nephilim, a Hebrew term loosely translated as giants in some Bibles Arts, ente ...

giants
with one hundred
trillion A trillion is a number with two distinct definitions: * 1,000,000,000,000, i.e. one million million, or (ten to the twelfth power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted ...
() stars, each orbiting its galaxy's
center of mass In physics, the center of mass of a distribution of mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of " ...
. Galaxies are categorized according to their visual
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology) In archaeology, morphology is the study of the shape of Artifact (archaeology), artefacts and ecofacts. Morphology is a major consid ...
as elliptical,
spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:irregular Something that is irregular does not follow the expected pattern; not even or balanced in shape or arrangement; abnormal. The term is used in many different fields, with various meanings. Accounting * Accounting irregularity Astronomy * Irreg ...
. Many are thought to have
supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. ...

supermassive black hole
s at their centers. The Milky Way's central black hole, known as
Sagittarius A* Sagittarius A* (pronounced "Sagittarius A-Star", abbreviated Sgr A*) is a bright and very compact astronomical radio source at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius (const ...

Sagittarius A*
, has a mass four million times greater than the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
. As of March 2016,
GN-z11 GN-z11 is a high-redshift galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

GN-z11
is the oldest and most distant galaxy observed. It has a
comoving distance In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects. ''Proper distance'' roughly corresponds to where a distant object would be at a speci ...
of 32 billion
light-years A light-year, alternatively spelt lightyear, is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metric un ...
from
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, and is seen as it existed just 400 million years after the
Big Bang The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

Big Bang
. In 2021, data from NASA's
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...

New Horizons
space probe was used to revise the previous estimate to roughly 200 billion galaxies (), which followed a 2016 estimate that there were two trillion () or more galaxies in the
observable universe The observable universe is a ball-shaped region of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang th ...
, overall, and as many as an estimated stars (more stars than all the on all beaches of the planet
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
). Most of the galaxies are 1,000 to 100,000
parsec The parsec (symbol: pc) is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metric units, used in every ...

parsec
s in diameter (approximately 3,000 to 300,000
light year The light-year is a unit of length used to express astronomical distances and is equivalent to about 9.46 Orders of magnitude (numbers)#1012, trillion kilometres () or 5.88 trillion miles ().One trillion here is long and short scales, t ...
s) 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 The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg, 350px, NGC 1300, viewed nearly face-on; Hubbl ...

Andromeda Galaxy
, its nearest large neighbor, by 780,000 parsecs (2.5 million ly.) The
space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Gre ...
between galaxies is filled with a tenuous gas (the intergalactic medium) with an average density of less than one
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

atom
per cubic meter. Most galaxies are gravitationally organized into
groups A group is a number of people or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic identi ...
, clusters and
supercluster A supercluster is a large group of smaller galaxy clusters or galaxy groups; it is among the largest known structures of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, ...
s. The
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

Milky Way
is part of the
Local Group Local group may refer to: * The Local Group Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way The Local Group is the galaxy group that includes the Milky Way The Milky Way is th ...

Local Group
, which it dominates along with
Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg, 350px, NGC 1300, viewed nearly face-on; Hubbl ...

Andromeda Galaxy
. The group is part of the
Virgo Supercluster The Virgo Supercluster (Virgo SC) or the Local Supercluster (LSC or LS) is a mass concentration of galaxy, galaxies containing the Virgo Cluster and Local Group, which in turn contains the Milky Way and Andromeda Galaxy, Andromeda galaxies. At ...
. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense
voids Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, a s ...
. Both the Local Group and the
Virgo Supercluster The Virgo Supercluster (Virgo SC) or the Local Supercluster (LSC or LS) is a mass concentration of galaxy, galaxies containing the Virgo Cluster and Local Group, which in turn contains the Milky Way and Andromeda Galaxy, Andromeda galaxies. At ...
are contained in a much larger cosmic structure named
Laniakea or is a Hawaiian language, Hawaiian word that means 'immense heaven', 'open skies', or 'wide horizons'. Laniakea or Laniākea may also refer to: * Laniakea Supercluster, a supercluster of galaxies that includes the Milky Way * Laniākea, a buildi ...
.


Etymology

The word ''galaxy'' was borrowed via and
Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share ...
from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
term for the Milky Way, ' () 'milky (circle)', named after its appearance as a milky band of light in the sky. In
Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A belief is an Attitude (psyc ...
,
Zeus Zeus or , , ; grc, Δῐός, ''Diós'', label=genitive In grammar In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Ling ...

Zeus
places his son born by a mortal woman, the infant
Heracles Heracles ( ; grc-gre, Ἡρακλῆς, , glory/fame of Hera Hera (; grc-gre, Ἥρα, Hḗrā; grc, Ἥρη, Hḗrē, label=none in Ionic Ionic or Ionian may refer to: Arts and entertainment * Ionic meter, a poetic metre in anci ...

Heracles
, on
Hera Hera (; grc-gre, Ἥρα, Hḗrā; grc, Ἥρη, Hḗrē, label=none in Ionic Ionic or Ionian may refer to: Arts and entertainment * Ionic meter, a poetic metre in ancient Greek and Latin poetry * Ionian mode, a musical mode or a diatonic ...

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. In the astronomical literature, the capitalized word "Galaxy" is often used to refer to our galaxy, the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

Milky Way
, to distinguish it from the other galaxies in our
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

universe
. The English term ''Milky Way'' can be traced back to a story by
Chaucer Geoffrey Chaucer (; – 25 October 1400) was an English poet and author. Widely considered the greatest English poet of the Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th ...

Chaucer
: Galaxies were initially discovered telescopically and were known as ''
spiral nebula Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''
e''. Most 18th to 19th century astronomers considered them as either unresolved
star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of hundreds to millions of old stars which are gravitationally bound, while open clusters are more loosely clustered ...
s or anagalactic
nebula A nebula (Latin for 'cloud' or 'fog'; pl. nebulae, nebulæ or nebulas) is a distinct body of interstellar clouds (which can consist of cosmic dust, hydrogen, helium, molecular clouds; possibly as Plasma (physics), ionized gases). Originally, th ...

nebula
e, 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 The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg, 350px, NGC 1300, viewed nearly face-on; Hubbl ...

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.


Nomenclature

Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the
Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg, 350px, NGC 1300, viewed nearly face-on; Hubbl ...

Andromeda Galaxy
, the
Magellanic Clouds The Magellanic Clouds (or ''Nubeculae Magellani'') are two irregular dwarf galaxies visible in the Southern Celestial Hemisphere; they are members of the Local Group and are orbit In physics, an orbit is the gravitationally curved tr ...
, the
Whirlpool Galaxy The Whirlpool Galaxy, also known as Messier 51a, M51a, and NGC 5194, is an Interacting galaxy, interacting Grand design spiral galaxy, grand-design spiral galaxy with a Seyfert galaxy, Seyfert 2 active galactic nucleus. It lies in the constellat ...

Whirlpool Galaxy
, and the
Sombrero Galaxy The Sombrero Galaxy (also known as Messier Object 104, M104 or NGC 4594) is a spiral galaxy Spiral galaxies form a originally described by in his 1936 work ''The Realm of the Nebulae''
. Astronomers work with numbers from certain catalogues, such as the
Messier catalogue The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his ''Catalogue des Nébuleuses et des Amas d'Étoiles'' (''Catalogue of Nebulae and Star Clusters''). Because Messier was only int ...
, the NGC (
New General Catalogue The ''New General Catalogue of Nebulae and Clusters of Stars'' (abbreviated NGC) is an astronomical catalogue Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural scien ...
), the IC (
Index Catalogue The ''New General Catalogue of Nebulae and Clusters of Stars'' (abbreviated NGC) is an astronomical catalog, astronomical catalogue of deep-sky objects compiled by John Louis Emil Dreyer in 1888. The NGC contains 7,840 objects, including galaxy, g ...
), the CGCG ( Catalogue of Galaxies and of Clusters of Galaxies), the MCG (
Morphological Catalogue of Galaxies Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
), the UGC (
Uppsala General CatalogueThe Uppsala General Catalogue of Galaxies (UGC) is a catalogue of 12,921 galaxy, galaxies visible from the northern hemisphere. It was first published in 1973. The catalogue includes essentially all galaxies north of declination -02°30' and to a li ...
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 Messier 109 (also known as NGC 3992) is a barred spiral galaxy exhibiting a weak inner ring structure around the central bar approximately away in the celestial hemisphere, northern constellation Ursa Major. M109 can be seen south-east of ...
(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 The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after ...
, in which M109 is cataloged as SDSS J115735.97+532228.9.


Observation history

The realization that we live in a galaxy that is one among many, parallels major discoveries about the Milky Way and other
nebula A nebula (Latin for 'cloud' or 'fog'; pl. nebulae, nebulæ or nebulas) is a distinct body of interstellar clouds (which can consist of cosmic dust, hydrogen, helium, molecular clouds; possibly as Plasma (physics), ionized gases). Originally, th ...

nebula
e.


Milky Way

Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
philosopher
Democritus Democritus (; el, Δημόκριτος, ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient w ...

Democritus
(450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...

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 An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
, in the region of the World that is continuous with the heavenly motions."
Neoplatonist Neoplatonism is a strand of Platonic Plato's influence on Western culture was so profound that several different concepts are linked by being called Platonic or Platonist, for accepting some assumptions of Platonism, but which do not imply accept ...
philosopher
Olympiodorus the Younger Olympiodorus the Younger ( el, Ὀλυμπιόδωρος ὁ Νεώτερος; c. 495 – 570) was a Neoplatonist Neoplatonism is a strand of Platonic philosophy Philosophy (from , ) is the study of general and fundamental questions, ...
(–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 Parallax is a displacement or difference in the apparent positionThe apparent place of an object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Entity, something that is tangible and within the ...

parallax
, which it did not. In his view, the Milky Way was celestial. According to Mohani Mohamed,
Arabian The Arabian Peninsula (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs The Arabs (singular Arab ; singular ar, عَرَبِيٌّ, ISO 233: , Arabic pronunciati ...
astronomer
Alhazen Ḥasan Ibn al-Haytham (Latinization of names, Latinized as Alhazen ; full name ; ) was a Muslim Arab Mathematics in medieval Islam, mathematician, Astronomy in the medieval Islamic world, astronomer, and Physics in the medieval Islamic world, ...
(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."
Persian Persian may refer to: * People and things from Iran, historically called ''Persia'' in the English language ** Persians, Persian people, the majority ethnic group in Iran, not to be conflated with the Iranian peoples ** Persian language, an Iranian ...

Persian
astronomer al-Bīrūnī (973–1048) proposed the Milky Way galaxy was "a collection of countless fragments of the nature of nebulous stars." astronomer Ibn Bâjjah ("Avempace", 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 In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...

refraction
from sublunary material, citing his observation of the
conjunction Conjunction may refer to: * Conjunction (astronomy), in which two astronomical bodies appear close together in the sky * Conjunction (astrology), astrological aspect in horoscopic astrology * Conjunction (grammar), a part of speech * Logical conjun ...
of Jupiter and Mars as evidence of this occurring when two objects were near. In the 14th century, Syrian-born
Ibn Qayyim Shams al-Dīn Abū ʿAbd Allāh Muḥammad ibn Abī Bakr ibn Ayyūb al-Zurʿī l-Dimashqī l-Ḥanbalī (29 January 1292–15 September 1350 CE / 691 AH–751 AH), commonly known as Ibn Qayyim al-Jawziyya ("The son of the principal of he school ...
proposed the Milky Way galaxy was "a myriad of tiny stars packed together in the sphere of the fixed stars." Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific q ...

Galileo Galilei
used a
telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common ...
to study it and discovered it was composed of a huge number of faint stars. In 1750, English astronomer , 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
gravitation Gravity (), or gravitation, is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germination seedlings, three days after germination. Germination is t ...

gravitation
al forces, akin to the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

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. In his 1755 treatise,
Immanuel Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about r ...

Immanuel Kant
elaborated on Wright's idea about the Milky Way's structure. The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by
William Herschel Sir Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a spe ...

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. 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
Harlow Shapley Harlow Shapley (November 2, 1885 – October 20, 1972) was an American scientist, head of the Harvard College Observatory The Harvard College Observatory (HCO) is an institution managing a complex of buildings and multiple instruments used for ...
based on the cataloguing of
globular cluster A globular cluster is a sphere, spherical collection of stars. wiktionary:globular, Globular clusters are very tightly bound by gravity, with a high concentration of stars towards their centers. Their name is derived from Latin —a small sphere. ...
s 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 absorption of light by interstellar dust present in the
galactic plane The galactic plane is the plane on which the majority of a disk-shaped galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''la ...
; but after
Robert Julius Trumpler Robert Julius Trumpler (until 1915 Robert Trümpler, born October 2, 1886 in Zürich , neighboring_municipalities = Adliswil, Dübendorf, Fällanden, Kilchberg, Zurich, Kilchberg, Maur, Switzerland, Maur, Oberengstringen, Opfikon, Regensdorf, R ...
quantified this effect in 1930 by studying
open cluster An open cluster is a group of up to a few thousand star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest sta ...
s, the present picture of our host galaxy emerged.


Distinction from other nebulae

A few galaxies outside the Milky Way are visible on a dark night to the unaided eye, including the
Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg, 350px, NGC 1300, viewed nearly face-on; Hubbl ...

Andromeda Galaxy
,
Large Magellanic Cloud The Large Magellanic Cloud (LMC), or Nubecula Major is a satellite galaxy A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and Luminosity, luminous host galaxy ( ...

Large Magellanic Cloud
, the
Small Magellanic Cloud The Small Magellanic Cloud (SMC), or ''Nubecula Minor'', is a dwarf galaxy A dwarf galaxy is a small galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is ...

Small Magellanic Cloud
, and the
Triangulum Galaxy The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598. The Triangulum Galaxy is the third-largest member of the Local Group Distributi ...

Triangulum Galaxy
. In the 10th century, Persian astronomer
Al-Sufi 'Abd al-Rahman al-Sufi ( fa, عبدالرحمن صوفی (December 7, 903 in Rey, Iran Shahr-e Rey ( fa, شهر ری, "City of Ray") or simply Ray (Rey; ) is the capital of Ray County, Iran, Ray County in Tehran Province, Iran. Formerly a disti ...
made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud". In 964, he probably mentioned the Large Magellanic Cloud in his ''
Book of Fixed Stars The ''Book of Fixed Stars'' ( ar, كتاب صور الكواكب ', literally ''The Book of the Shapes of Stars'') is an astronomical text written by Abd al-Rahman al-Sufi 'Abd al-Rahman al-Sufi ( fa, عبدالرحمن صوفی (December 7, 9 ...
'' (referring to "Al Bakr of the southern Arabs", since at a
declination In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses math ...
of about 70° south it was not visible where he lived); it was not well known to Europeans until
Magellan Ferdinand Magellan ( or ; pt, Fernão de Magalhães, ; es, link=no, Fernando de Magallanes, ; 4 February 1480 – 27 April 1521) was a Portuguese people, Portuguese explorer and a subject of the Habsburg Spain, Hispanic Monarchy from 1518. H ...

Magellan
's voyage in the 16th century. The Andromeda Galaxy was later independently noted by
Simon Marius Simon Marius ( latinized form of Simon Mayr; January 10, 1573 – January 5, 1625) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Ea ...

Simon Marius
in 1612. In 1734, philosopher
Emanuel Swedenborg Emanuel Swedenborg (, ; born Emanuel Swedberg; 29 March 1772) was a -Christian , , and . He became best known for his book on the , (1758). Swedenborg had a prolific career as an and . In 1741, at 53, he entered into a phase in which he be ...

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." In 1745,
Pierre Louis Maupertuis Pierre Louis Moreau de Maupertuis (; ; 1698 – 27 July 1759) was a French mathematician, philosopher and man of letters An intellectual is a person who engages in critical thinking, research, and Human self-reflection, reflection about t ...
conjectured that some
nebula A nebula (Latin for 'cloud' or 'fog'; pl. nebulae, nebulæ or nebulas) is a distinct body of interstellar clouds (which can consist of cosmic dust, hydrogen, helium, molecular clouds; possibly as Plasma (physics), ionized gases). Originally, th ...

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 Johannes is a Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, ...
's view that the bright spots were massive and flattened due to their rotation. In 1750, 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. Toward the end of the 18th century,
Charles Messier Charles Messier (; 26 June 1730 – 12 April 1817) was a French astronomer. He published an astronomical catalogue consisting of 110 nebulae and faint star clusters, which came to be known as the ''Messier objects''. Messier's pur ...

Charles Messier
compiled a catalog 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,
Vesto Slipher Vesto Melvin Slipher (; November 11, 1875 – November 8, 1969) was an American astronomer who performed the first measurements of radial velocities for galaxies. He was the first to discover that distant galaxies are redshifts, redshifted, thus pr ...
made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high
Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer (physics), observer who is moving relative to the wave source. It is named after the Austrian physicist ...

Doppler shift
s, 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,
Heber Curtis Heber Doust Curtis (June 27, 1872 – January 9, 1942) was an American astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astron ...
observed nova
S Andromedae SN 1885A (also S Andromedae) was a supernova (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526 A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar ...
within the "Great
Andromeda Andromeda most commonly refers to: * Andromeda (mythology), a princess from Greek mythology * Andromeda (constellation), a region of the Earth's night sky * The Andromeda Galaxy, an astronomical object within the constellation Andromeda may also r ...
Nebula" (as the Andromeda Galaxy,
Messier object The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his ''Catalogue des Nébuleuses et des Amas d'Étoiles'' (''Catalogue of Nebulae and Star Clusters''). Because Messier was only int ...
, was then known). Searching the photographic record, he found 11 more
nova A nova (plural novae or novas) is a transient astronomical eventA transient astronomical event, often shortened by astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field out ...

nova
e. 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 
parsec The parsec (symbol: pc) is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metric units, used in every ...

parsec
s. 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
Harlow Shapley Harlow Shapley (November 2, 1885 – October 20, 1972) was an American scientist, head of the Harvard College Observatory The Harvard College Observatory (HCO) is an institution managing a complex of buildings and multiple instruments used for ...
and
Heber Curtis Heber Doust Curtis (June 27, 1872 – January 9, 1942) was an American astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astron ...
(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. In 1922, the
Estonia Estonia ( et, Eesti ), officially the Republic of Estonia ( et, Eesti Vabariik, links=no), is a country in northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the Baltic Sea across from Sweden ...

Estonia
n astronomer Ernst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. Using the new 100 inch Mt. Wilson telescope,
Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects previously ...
was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some
Cepheid variable A Cepheid variable () is a type of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the ...
s, 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 galactic morphology that is used to this day.


Modern research

In 1944,
Hendrik van de Hulst Hendrik Christoffel "Henk" van de Hulst (19 November 1918 – 31 July 2000) was a Dutch astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They ...
predicted that
microwave Microwave is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space a ...

microwave
radiation with wavelength of 21 cm would be detectable from interstellar atomic
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
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 bar structure in the center of our galaxy. With improved
radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...

radio telescope
s, hydrogen gas could also be traced in other galaxies. In the 1970s,
Vera Rubin Vera Florence Cooper Rubin (; July 23, 1928 – December 25, 2016) was an American astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They ...

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 Dark matter is a hypothetical form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, ...

dark matter
. Beginning in the 1990s, the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first ...

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. The
Hubble Deep Field The Hubble Deep Field (HDF) is an image of a small region in the constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, myt ...
, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion () galaxies in the observable universe. Improved technology in detecting the invisible to humans (radio telescopes, infrared cameras, and
x-ray telescopes An X-ray telescope (XRT) is a telescope A telescope is an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects, or various devices used to observe distant objects by their emission, absorption, o ...
) 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. A 2016 study published in ''
The Astrophysical Journal ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( ...
,'' led by Christopher Conselice of the
University of Nottingham , mottoeng = A city is built on wisdom , established = 1798 – teacher training college1881 – University College Nottingham1948 – university status , type = Public In public relations Public relations (PR) ...
, used 20 years of Hubble images to estimate that the observable universe contained at least two trillion () galaxies. However, later observations with the
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...

New Horizons
space probe from outside the
zodiacal light The zodiacal light (also called false dawn when seen before sunrise ''Before Sunrise'' is a 1995 American romantic drama film In film and television show, television, drama is a category of narrative fiction (or docudrama, semi-fiction) ...

zodiacal light
reduced this to roughly 200 billion ().


Types and morphology

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 sequenceThe Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning fork diagram because the shape in which it is traditionally represented resembles a tu ...

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 Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", Jeans instability, collapse and form stars. As a branch of astronomy, star fo ...

star formation
rate in
starburst galaxies A starburst galaxy is a galaxy undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star form ...
and activity in the cores of active galaxies.


Ellipticals

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
ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a Surface (mathemat ...

ellipsoid
al profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little
interstellar matter In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmi ...
. Consequently, these galaxies also have a low portion of
open cluster An open cluster is a group of up to a few thousand star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest sta ...
s 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
globular cluster A globular cluster is a sphere, spherical collection of stars. wiktionary:globular, Globular clusters are very tightly bound by gravity, with a high concentration of stars towards their centers. Their name is derived from Latin —a small sphere. ...
s. 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.


Shell galaxy

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 has over 20 shells.


Spirals

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 Dark matter is a hypothetical form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, ...

dark matter
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 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. A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the
grand design spiral galaxy Image:Ssc2003-06c.jpg, A Spitzer Space Telescope image of Messier 81, a grand design spiral. A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm and Flocculent spiral galaxy, ...
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. In spiral galaxies, the spiral arms do have the shape of approximate
logarithmic spiral A logarithmic spiral, equiangular spiral, or growth spiral is a self-similar __NOTOC__ has an infinitely repeating self-similarity when it is magnified. In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of s ...

logarithmic spiral
s, 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 In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

angular velocity
. The spiral arms are thought to be areas of high-density matter, or " density waves". As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.


Barred spiral galaxy

A majority of spiral galaxies, including our own
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

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. 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. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms. Our own galaxy, the
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

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


Super-luminous spiral

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


Other morphologies

*
Peculiar galaxies A peculiar galaxy is a galaxy of unusual size, shape, or composition. Between five and ten percent of known galaxies are categorized as peculiar. Astronomers have identified two types of peculiar galaxies: Interacting galaxy, ''interacting 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. Such an event may have affected the
Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula (see below), is a barred spiral galaxy Image:Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg, 350px, NGC 1300, viewed nearly face-on; Hubbl ...

Andromeda Galaxy
, as it displays a multi-ring-like structure when viewed in
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

infrared
radiation. * A
lenticular galaxy A lenticular galaxy (denoted S0) is a type of galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in t ...
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 (
barred lenticular galaxies A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical galaxy, elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scal ...
receive Hubble classification SB0.) *
Irregular galaxies An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a Spiral galaxy, spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic ...
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
Magellanic Clouds The Magellanic Clouds (or ''Nubeculae Magellani'') are two irregular dwarf galaxies visible in the Southern Celestial Hemisphere; they are members of the Local Group and are orbit In physics, an orbit is the gravitationally curved tr ...
. * 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.


Dwarfs

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. 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 dwarf elliptical galaxy, elliptical, dwarf spiral galaxy, spiral, or
irregular Something that is irregular does not follow the expected pattern; not even or balanced in shape or arrangement; abnormal. The term is used in many different fields, with various meanings. Accounting * Accounting irregularity Astronomy * Irreg ...
. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxy, 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 Dark matter is a hypothetical form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, ...

dark matter
, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.


Other types of galaxies


Interacting

Interactions between galaxies are relatively frequent, and they can play an important role in galaxy formation and evolution, galactic evolution. Near misses between galaxies result in warping distortions due to galactic tide, tidal interactions, 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 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. 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 Interacting galaxy#Galactic cannibalism, 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.


Starburst

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, but still contribute an estimated 15% to total star production. 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. These stars produce supernova explosions, creating expanding supernova remnant, 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. Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is Messier 82, M82, which experienced a close encounter with the larger Messier 81, M81. Irregular galaxies often exhibit spaced knots of starburst activity.


Active galaxy

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 galaxy, 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 A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. ...

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. 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 Astrophysical jet, 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.


Blazars

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.


LINERS

Possibly related to active galactic nuclei (as well as starburst (astronomy), 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-Asymptotic giant branch, AGB stars, AGN, and shocks. Approximately one-third of nearby galaxies are classified as containing LINER nuclei.


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.


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

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.


Properties


Magnetic fields

Galaxies have magnetic fields of their own. 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 theorem, equipartition strength for Spiral galaxy, spiral galaxies is about 10 μG (Gauss (unit), microGauss) or 1nT (Tesla (unit), nanoTesla). By comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss or 30 μT (Tesla (unit), microTesla). Radio-faint galaxies like Andromeda Galaxy, M 31 and Triangulum Galaxy, M33, our
Milky Way The Milky Way is the galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Property (''latin: Res Privata'') in the Abstract and con ...

Milky Way
's neighbors, have weaker fields (about 5μG), 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 A starburst galaxy is a galaxy undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star form ...
—for example, in M 82 and the Antennae Galaxies, Antennae; and in nuclear starburst regions, such as the centers of NGC 1097 and other Barred spiral galaxy, barred galaxies.


Formation and evolution

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


Formation

Current cosmological models of the early universe are based on the
Big Bang The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

Big Bang
theory. About 300,000 years after this event, atoms of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
and helium began to form, in an event called Recombination (cosmology), 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 "Timeline of the Big Bang#Dark Ages, dark ages". It was from density fluctuations (or anisotropy, anisotropic irregularities) in this primordial matter that structure formation, larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos. These primordial structures eventually became the galaxies we see today.


Early galaxy formation

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. While some scientists have claimed other objects (such as Galaxy Abell 1835 IR1916, 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 after the
Big Bang The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

Big Bang
(which was about 13.8 billion years ago), is about 13.42 billion Distance measures (cosmology), light travel distance years away. The existence of galaxies so soon after the Big Bang suggests that protogalaxy, protogalaxies must have grown in the so-called "dark ages". 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 Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity of consciously making sense of things, applying logic ...

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. 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. In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as
globular cluster A globular cluster is a sphere, spherical collection of stars. wiktionary:globular, Globular clusters are very tightly bound by gravity, with a high concentration of stars towards their centers. Their name is derived from Latin —a small sphere. ...
s form first, and then a number of such bodies accrete to form a larger galaxy. Once protogalaxies began to form and contract, the first halo stars (called Population 3 stars, 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. 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 Population 3 stars, Population III stars in the Cosmos Redshift 7 galaxy at . 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 Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
that are needed for the later formation of planets and life as we know it.


Evolution

Within a billion years of a galaxy's formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a bulge (astronomy), galactic bulge of metal-poor metallicity, 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. During this early epoch, galaxies undergo a major burst of star formation. During the following two billion years, the accumulated matter settles into a disc (galaxy), galactic disc. A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxy, dwarf galaxies 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 planetary formation, formation of planets. 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 NGC 4676 or the Antennae Galaxies. The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 metre per second, 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. 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

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. 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 (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 star, compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes. Eventually, as a result of Relaxation (physics)#Relaxation in astronomy, gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.


Larger-scale structures

Deep-sky surveys show that galaxies are often found in groups and Clusters of galaxies, 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 galaxiesThe 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. 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 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. 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. 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. 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. 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 brightest cluster galaxy, which, over time, tidal force, tidally destroys its satellite galaxies and adds their mass to its own. Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the large-scale structure of the Cosmos, supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids. Above this scale, the universe appears to be the same in all directions (isotropy, isotropic and wikt:Homogeneity, homogeneous)., 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 List of largest cosmic structures, largest structure in the universe found so far, is 10 billion light-years (three gigaparsecs) in length. The Milky Way galaxy is a member of an association named the
Local Group Local group may refer to: * The Local Group Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way The Local Group is the galaxy group that includes the Milky Way The Milky Way is th ...

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. The Local Group itself is a part of a cloud-like structure within the
Virgo Supercluster The Virgo Supercluster (Virgo SC) or the Local Supercluster (LSC or LS) is a mass concentration of galaxy, galaxies containing the Virgo Cluster and Local Group, which in turn contains the Milky Way and Andromeda Galaxy, Andromeda galaxies. At ...
, a large, extended structure of groups and clusters of galaxies centered on the Virgo Cluster. And the Virgo Supercluster itself is a part of the Pisces-Cetus Supercluster Complex, a giant galaxy filament.


Multi-wavelength observation

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 Dust is made of s of solid . On Earth, it generally consists of particles in the that come from various sources such as lifted by wind (an ), , and . Dust in homes is composed of about 20–50% dead . The rest, and in offices, and other ...
present in the interstellar medium is opaque to visual light. It is more transparent to far infrared astronomy, far-infrared, which can be used to observe the interior regions of giant molecular clouds and Bulge (astronomy), galactic cores in great detail. Infrared is also used to observe distant, redshift, 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 astronomy, radio frequencies. The Earth's atmosphere is nearly transparent to radio between 5 Hertz, MHz and 30 GHz. (The ionosphere blocks signals below this range.) Large radio interferometry, interferometers have been used to map the active jets emitted from active nuclei. Radio telescopes can also be used to observe neutral hydrogen (via hydrogen line, 21 cm radiation), including, potentially, the non-ionized matter in the early universe that later collapsed to form galaxies. UV astronomy, Ultraviolet and X-ray astronomy, 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. 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.


Gallery

File:Squabbling Galactic Siblings.jpg, Squabbling Galactic Siblings File:Hubble Returns to Science Operations.jpg, 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.


See also

* Dark galaxy * Galactic orientation * Galaxy formation and evolution * Illustris project * List of galaxies * List of nearest galaxies * Luminous infrared galaxy * Outline of galaxies * Supermassive black hole * Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure * UniverseMachine


Notes


References


Sources

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Bibliography

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External links


NASA/IPAC Extragalactic Database (NED)NED-Distances
*
An Atlas of The Universe

Galaxies—Information and amateur observations



Galaxy classification project, harnessing the power of the internet and the human brain

How many galaxies are in our universe?

3-D Video (01:46) – Over a Million Galaxies of Billions of Stars each – BerkeleyLab/animated.
{{Authority control Galaxies, Galaxies Concepts in astronomy Articles containing video clips