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The Milky Way or Milky Way Galaxy is the
galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
that includes 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 ...
, with the name describing the galaxy's appearance from
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
: a hazy band of light seen in the
night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include moonlig ...
formed from stars in other arms of the galaxy, which are so far away that they cannot be individually distinguished by the
naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...
. The Milky Way is a
barred spiral galaxy A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about two thirds of all spiral galaxies in the local universe, and generally affect both the motions of stars and interstellar gas ...
with a D25 isophotal diameter estimated at , but only about 1,000 light-years thick at the spiral arms (more at the bulge). Recent simulations suggest that a
dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...
area, also containing some visible stars, may extend up to a diameter of almost 2 million light-years (613 kpc). The Milky Way has several
satellite galaxies 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 (also known as the primary galaxy). Satellite galaxies and their constituents ...
and is part of the
Local Group The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a " dumbbell" shape; the Milky Way ...
of galaxies, forming part of the
Virgo Supercluster The Local Supercluster (LSC or LS), or Virgo Supercluster is a formerly defined supercluster containing the Virgo Cluster and Local Group, which itself contains the Milky Way and Andromeda galaxies, as well as others. At least 100 galaxy group ...
which is itself a component of the
Laniakea Supercluster The Laniakea Supercluster (; Hawaiian language, Hawaiian for "open skies" or "immense heaven") or the Local Supercluster (LSC or LS) is the galaxy supercluster that is home to the Milky Way and approximately 100,000 other nearby galaxies. It w ...
. It is estimated to contain 100–400 billion stars and at least that number of
planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
s. The Solar System is located at a radius of about 27,000 light-years (8.3 kpc) from the
Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...
, on the inner edge of the
Orion Arm The Orion Arm, also known as the Orion–Cygnus Arm, is a minor spiral arm within the Milky Way Galaxy spanning in width and extending roughly in length. This galactic structure encompasses the Solar System, including Earth. It is sometimes r ...
, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a
bulge __NOTOC__ Bulge may refer to: Astronomy and geography *Bulge (astronomy), a tightly packed group of stars at the center of a spiral galaxy *Equatorial bulge, a bulge around the equator of a planet due to rotation *Tharsis bulge, vast volcanic pla ...
and one or more bars that radiate from the bulge. The Galactic Center is an intense radio source known as
Sagittarius A* Sagittarius A*, abbreviated as Sgr A* ( ), is the supermassive black hole at the Galactic Center of the Milky Way. Viewed from Earth, it is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south o ...
, a
supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (). Black holes are a class of astronomical ...
of 4.100 (± 0.034) million
solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...
es. The oldest stars in the Milky Way are nearly as old as the
Universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...
itself and thus probably formed shortly after the Dark Ages of the
Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...
.
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers
Harlow Shapley Harlow Shapley (November 2, 1885 – October 20, 1972) was an American astronomer, who served as head of the Harvard College Observatory from 1921–1952, and political activist during the latter New Deal and Fair Deal. Shapley used Cepheid var ...
and
Heber Doust Curtis Heber Doust Curtis (June 27, 1872 – January 9, 1942) was an American astronomer. He participated in 11 expeditions for the study of solar eclipses, and, as an advocate and theorist that additional galaxies existed outside of the Milky Way, wa ...
, observations by
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 ...
in 1923 showed that the Milky Way is just one of many galaxies.


Mythology

In the
Babylonia Babylonia (; , ) was an Ancient history, ancient Akkadian language, Akkadian-speaking state and cultural area based in the city of Babylon in central-southern Mesopotamia (present-day Iraq and parts of Kuwait, Syria and Iran). It emerged as a ...
n epic poem ''
Enūma Eliš ' ( Akkadian Cuneiform: , also spelled "Enuma Elish"), meaning "When on High", is a Babylonian creation myth ( named after its opening words) from the late 2nd millennium BCE and the only complete surviving account of ancient near eastern cosmol ...
'', the Milky Way is created from the severed tail of the primeval salt water
dragon A dragon is a Magic (supernatural), magical legendary creature that appears in the folklore of multiple cultures worldwide. Beliefs about dragons vary considerably through regions, but European dragon, dragons in Western cultures since the Hi ...
ess
Tiamat In Mesopotamian religion, Tiamat ( or , ) is the primordial sea, mating with Abzû (Apsu), the groundwater, to produce the gods in the Babylonian epic '' Enûma Elish'', which translates as "when on high". She is referred to as a woman, an ...
, set in the sky by
Marduk Marduk (; cuneiform: Dingir, ᵈAMAR.UTU; Sumerian language, Sumerian: "calf of the sun; solar calf"; ) is a god from ancient Mesopotamia and patron deity of Babylon who eventually rose to prominence in the 1st millennium BC. In B ...
, the Babylonian
national god A national god or tribal god is a guardian deity whose special concern is supposed to be the safety and well-being of an 'ethnic group' (''nation''). This is contrasted with other guardian figures such as family gods responsible for the well-be ...
, after slaying her. This story was once thought to have been based on an older
Sumer Sumer () is the earliest known civilization, located in the historical region of southern Mesopotamia (now south-central Iraq), emerging during the Chalcolithic and Early Bronze Age, early Bronze Ages between the sixth and fifth millennium BC. ...
ian version in which Tiamat is instead slain by
Enlil Enlil, later known as Elil and Ellil, is an List of Mesopotamian deities, ancient Mesopotamian god associated with wind, air, earth, and storms. He is first attested as the chief deity of the Sumerian pantheon, but he was later worshipped by t ...
of
Nippur Nippur (Sumerian language, Sumerian: ''Nibru'', often logogram, logographically recorded as , EN.LÍLKI, "Enlil City;"I. E. S. Edwards, C. J. Gadd, N. G. L. Hammond, ''The Cambridge Ancient History: Prolegomena & Prehistory'': Vol. 1, Part 1, Ca ...
, but is now thought to be purely an invention of Babylonian propagandists with the intention to show Marduk as superior to the Sumerian deities.


Etymology

In
Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...
,
Zeus Zeus (, ) is the chief deity of the List of Greek deities, Greek pantheon. He is a sky father, sky and thunder god in ancient Greek religion and Greek mythology, mythology, who rules as king of the gods on Mount Olympus. Zeus is the child ...
places his son born by a mortal woman, the infant
Heracles Heracles ( ; ), born Alcaeus (, ''Alkaios'') or Alcides (, ''Alkeidēs''), was a Divinity, divine hero in Greek mythology, the son of ZeusApollodorus1.9.16/ref> and Alcmene, and the foster son of Amphitryon.By his adoptive descent through ...
, on
Hera In ancient Greek religion, Hera (; ; in Ionic Greek, Ionic and Homeric Greek) is the goddess of marriage, women, and family, and the protector of women during childbirth. In Greek mythology, she is queen of the twelve Olympians and Mount Oly ...
's breast while she is asleep so the baby will drink her divine milk and 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 another Greek story, the abandoned Heracles is given by
Athena Athena or Athene, often given the epithet Pallas, is an ancient Greek religion, ancient Greek goddess associated with wisdom, warfare, and handicraft who was later syncretism, syncretized with the Roman goddess Minerva. Athena was regarde ...
to Hera for feeding, but Heracles' forcefulness causes Hera to rip him from her breast in pain. In Western culture, the name "Milky Way" is derived from its appearance as a dim unresolved "milky" glowing band arching across the night sky. The term is a translation of the
Classical Latin Classical Latin is the form of Literary Latin recognized as a Literary language, literary standard language, standard by writers of the late Roman Republic and early Roman Empire. It formed parallel to Vulgar Latin around 75 BC out of Old Latin ...
''via lactea'', in turn derived from the
Hellenistic Greek Koine Greek (, ), also variously known as Hellenistic Greek, common Attic, the Alexandrian dialect, Biblical Greek, Septuagint Greek or New Testament Greek, was the common supra-regional form of Greek spoken and written during the Hellenistic ...
, short for ('), meaning "milky circle". The
Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
(') – from root -, ("milk") + (forming adjectives) – is also the root of "galaxy", the name for our, and later all such, collections of stars. The Milky Way, or "milk circle", was just one of 11 "circles" the Greeks identified in the sky, others being the
zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north and south celestial latitude of the ecliptic – the apparent path of the Sun across the celestial sphere over the course of the year. Within this zodiac ...
, the meridian, the
horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
, the
equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...
, the tropics of Cancer and Capricorn, the
Arctic Circle The Arctic Circle is one of the two polar circles, and the northernmost of the five major circle of latitude, circles of latitude as shown on maps of Earth at about 66° 34' N. Its southern counterpart is the Antarctic Circle. The Arctic Circl ...
and the
Antarctic Circle The Antarctic Circle is the most southerly of the five major circles of latitude that mark maps of Earth. The region south of this circle is known as the Antarctic, and the zone immediately to the north is called the Southern Temperate Zone. So ...
, and two colure circles passing through both poles. The English term can be traced back to a story by
Geoffrey Chaucer Geoffrey Chaucer ( ; – 25 October 1400) was an English poet, author, and civil servant best known for ''The Canterbury Tales''. He has been called the "father of English literature", or, alternatively, the "father of English poetry". He w ...
:


Common names

* "Birds' Path" is used in several
Uralic The Uralic languages ( ), sometimes called the Uralian languages ( ), are spoken predominantly in Europe and North Asia. The Uralic languages with the most native speakers are Hungarian, Finnish, and Estonian. Other languages with speakers abo ...
and
Turkic languages The Turkic languages are a language family of more than 35 documented languages, spoken by the Turkic peoples of Eurasia from Eastern Europe and Southern Europe to Central Asia, East Asia, North Asia (Siberia), and West Asia. The Turkic langua ...
and in the
Baltic languages The Baltic languages are a branch of the Indo-European languages, Indo-European language family spoken natively or as a second language by a population of about 6.5–7.0 million people
. Northern peoples observed that
migratory birds Bird migration is a seasonal movement of birds between breeding and wintering grounds that occurs twice a year. It is typically from north to south or from south to north. Migration is inherently risky, due to predation and mortality. The ...
follow the course of the galaxy while migrating at the Northern Hemisphere. The name "Birds' Path" (in Finnish, Estonian, Latvian, Lithuanian, Bashkir and Kazakh) has some variations in other languages, e.g. "Way of the grey (wild) goose" in Chuvash, Mari and Tatar and "Way of the Crane" in Erzya and Moksha. *House river: The
Kaurna people The Kaurna people (, ; also Coorna, Kaura, Gaurna and other variations) are a group of Aboriginal people whose traditional lands include the Adelaide Plains of South Australia. They were known as the Adelaide tribe by the early settlers. Kau ...
of the
Adelaide Plains The Adelaide Plains (Kaurna name Tarndanya) is a plain in South Australia lying between the coast ( Gulf St Vincent) on the west and the Mount Lofty Ranges on the east. The southernmost tip of the plain is in the southern seaside suburbs of A ...
of South Australia called the Milky Way ''wodliparri'' in the
Kaurna language Kaurna ( or ) is a Pama-Nyungan language historically spoken by the Kaurna peoples of the Adelaide Plains of South Australia. The Kaurna peoples are made up of various tribal clan groups, each with their own ''parnkarra'' district of land and ...
, meaning "house river". *Emu in the Sky: The Gomeroi people between
New South Wales New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...
and
Queensland Queensland ( , commonly abbreviated as Qld) is a States and territories of Australia, state in northeastern Australia, and is the second-largest and third-most populous state in Australia. It is bordered by the Northern Territory, South Austr ...
called the Milky Way ''Dhinawan'', the giant Emu in the Sky that it stretches across the night sky. *Milky Way: Many European languages have borrowed, directly or indirectly, the Greek name for the Milky Way, including English and Latin. *Road to Santiago: the Milky Way was traditionally used as a guide by pilgrims traveling to the holy site at Compostela, hence the use of "The Road to Santiago" as a name for the Milky Way. Curiously, ''La Voje Ladee'' "The Milky Way" was also used to refer to the pilgrimage road. *River Ganga of the Sky: this Sanskrit name ( ''Ākāśagaṃgā'') is used in many Indian languages following a Hindu belief . *Silver River: this Chinese name "Silver River" () is used throughout East Asia, including Korea and Vietnam (''Ngân hà''). In Japan and Korea, "Silver River" (; ) means galaxies in general. *River of Heaven: The Japanese name for the Milky Way is the "River of Heaven" (, ''Amanokawa''), as well as an alternative name in Chinese (). In Vietnamese, "River of Heaven" (''Thiên hà'') means galaxies in general. *Straw Way: In West Asia, Central Asia and parts of the Balkans the name for the Milky Way is related to the word for
straw Straw is an agricultural byproduct consisting of the dry wikt:stalk, stalks of cereal plants after the grain and chaff have been removed. It makes up about half of the crop yield, yield by weight of cereal crops such as barley, oats, rice, ry ...
. Today, Persians, Pakistanis, and Turks use it in addition to Arabs. It has been suggested that the term was spread by medieval
Arab Arabs (,  , ; , , ) are an ethnic group mainly inhabiting the Arab world in West Asia and North Africa. A significant Arab diaspora is present in various parts of the world. Arabs have been in the Fertile Crescent for thousands of years ...
s who in turn borrowed it from Armenians. *Walsingham Way: In England the Milky Way was called the Walsingham Way in reference to the shrine of
Our Lady of Walsingham Our Lady of Walsingham is a title given to Mary, the mother of Jesus, venerated by Catholics and high-church Anglicans. According to tradition, the title is linked to a Marian vision experienced in 1061 by Lady Richeldis de Faverches, an Angl ...
which is in
Norfolk Norfolk ( ) is a Ceremonial counties of England, ceremonial county in England, located in East Anglia and officially part of the East of England region. It borders Lincolnshire and The Wash to the north-west, the North Sea to the north and eas ...
, England. It was understood to be either a guide to the pilgrims who flocked there, or a representation of the pilgrims themselves. *Winter Street: Scandinavian peoples, such as Swedes, have called the galaxy Winter Street (''Vintergatan'') as the galaxy is most clearly visible during the winter at the northern hemisphere, especially at high latitudes where the glow of the Sun late at night can obscure it during the summer.


Appearance

The Milky Way is visible as a hazy band of white light, some 30° wide, arching the
night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include moonlig ...
. Although all the individual naked-eye stars in the entire sky are part of the Milky Way Galaxy, the term "Milky Way" is limited to this band of light. The light originates from the accumulation of unresolved stars and other material located in the direction of the
galactic plane The galactic plane is the plane (geometry), plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galac ...
. Brighter regions around the band appear as soft visual patches known as star clouds. The most conspicuous of these is the Large Sagittarius Star Cloud, a portion of the central
bulge __NOTOC__ Bulge may refer to: Astronomy and geography *Bulge (astronomy), a tightly packed group of stars at the center of a spiral galaxy *Equatorial bulge, a bulge around the equator of a planet due to rotation *Tharsis bulge, vast volcanic pla ...
of the galaxy. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where
interstellar dust Cosmic dustalso called extraterrestrial dust, space dust, or star dustis dust that occurs in outer space or has fallen onto Earth. Most cosmic dust particles measure between a few molecules and , such as micrometeoroids (30 μm). Cosmic dust can ...
blocks light from distant stars. Peoples of the southern hemisphere, including the
Inca The Inca Empire, officially known as the Realm of the Four Parts (, ), was the largest empire in pre-Columbian America. The administrative, political, and military center of the empire was in the city of Cusco. The History of the Incas, Inca ...
and
Australian Aborigines Aboriginal Australians are the various indigenous peoples of the Australian mainland and many of its islands, excluding the ethnically distinct people of the Torres Strait Islands. Humans first migrated to Australia 50,000 to 65,000 years ...
, identified these regions as
dark cloud constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellations were likely defin ...
s. The area of sky that the Milky Way obscures is called the Zone of Avoidance. The Milky Way has a relatively low
surface brightness In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on ...
. Its visibility can be greatly reduced by background light, such as
light pollution Light pollution is the presence of any unwanted, inappropriate, or excessive artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting sources, during the ...
or moonlight. The sky needs to be darker than about 20.2
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
per square arcsecond in order for the Milky Way to be visible. It should be visible if the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1. This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but very prominent when viewed from
rural area In general, a rural area or a countryside is a geographic area that is located outside towns and cities. Typical rural areas have a low population density and small settlements. Agricultural areas and areas with forestry are typically desc ...
s when the Moon is below the horizon. Maps of artificial night sky brightness show that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution. As viewed from Earth, the visible region of the Milky Way's
galactic plane The galactic plane is the plane (geometry), plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galac ...
occupies an area of the sky that includes 30
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...
s. The
Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...
lies in the direction of Sagittarius, where the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass around to the galactic anticenter in Auriga. The band then continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two roughly equal
hemispheres Hemisphere may refer to: In geometry * Hemisphere (geometry), a half of a sphere As half of Earth or any spherical astronomical object * A hemispheres of Earth, hemisphere of Earth ** Northern Hemisphere ** Southern Hemisphere ** Eastern Hemisphe ...
. The galactic plane is inclined by about 60° to the
ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...
(the plane of
Earth's orbit Earth orbits the Sun at an astronomical unit, average distance of , or 8.317 light-second, light-minutes, in a retrograde and prograde motion, counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes & ...
). Relative to the
celestial equator The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. By extension, it is also a plane of reference in the equatorial coordinate system. Due to Earth's axial tilt, the celestial ...
, it passes as far north as the constellation of
Cassiopeia Cassiopeia or Cassiopea may refer to: Greek mythology * Cassiopeia (mother of Andromeda), queen of Aethiopia and mother of Andromeda * Cassiopeia (wife of Phoenix), wife of Phoenix, king of Phoenicia * Cassiopeia, wife of Epaphus, king of Egy ...
and as far south as the constellation of
Crux CRUX is a lightweight x86-64 Linux distribution targeted at experienced Linux users and delivered by a tar.gz-based package system with BSD-style initscripts. It is not based on any other Linux distribution. It also utilizes a ports system ...
, indicating the high inclination of Earth's
equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...
ial plane and the plane of the ecliptic, relative to the galactic plane. The north galactic pole is situated at
right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...
12h 49m,
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or ...
+27.4° (
B1950 In astronomy, an equinox is either of two places on the celestial sphere at which the ecliptic intersects the celestial equator. Although there are two such intersections, the equinox associated with the Sun's ascending node is used as the conv ...
) near β Comae Berenices, and the south galactic pole is near α Sculptoris. Because of this high inclination, depending on the time of night and year, the Milky Way arch may appear relatively low or relatively high in the sky. For observers from latitudes approximately 65° north to 65° south, the Milky Way passes directly overhead twice a day.


Astronomical history


Ancient, naked eye observations

In ''
Meteorologica ''Meteorology'' ( Greek: ; Latin: ''Meteorologica'' or ''Meteora'') is a treatise by Aristotle. The text discusses what Aristotle believed to have been all the affections common to air and water, and the kinds and parts of the Earth and the affec ...
'',
Aristotle Aristotle (; 384–322 BC) was an Ancient Greek philosophy, Ancient Greek philosopher and polymath. His writings cover a broad range of subjects spanning the natural sciences, philosophy, linguistics, economics, politics, psychology, a ...
(384–322 BC) states that the
Greek philosophers Ancient Greek philosophy arose in the 6th century BC. Philosophy was used to make sense of the world using reason. It dealt with a wide variety of subjects, including astronomy, epistemology, mathematics, political philosophy, ethics, metaphysics ...
Anaxagoras Anaxagoras (; , ''Anaxagóras'', 'lord of the assembly'; ) was a Pre-Socratic Greek philosopher. Born in Clazomenae at a time when Asia Minor was under the control of the Persian Empire, Anaxagoras came to Athens. In later life he was charged ...
(–428 BC) and
Democritus Democritus (, ; , ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greece, Ancient Greek Pre-Socratic philosophy, pre-Socratic philosopher from Abdera, Thrace, Abdera, primarily remembered today for his formulation of an ...
(460–370 BC) proposed that the Milky Way is the glow of stars not directly visible due to Earth's shadow, while other stars receive their light from the Sun, but have their glow obscured by solar rays. Aristotle himself believed that the Milky Way was part of the Earth's upper atmosphere, along with the stars, and that it was a byproduct of stars burning that did not dissipate because of its outermost location in the atmosphere, composing its
great circle In mathematics, a great circle or orthodrome is the circular intersection of a sphere and a plane passing through the sphere's center point. Discussion Any arc of a great circle is a geodesic of the sphere, so that great circles in spher ...
. He said that the milky appearance of the Milky Way
Galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
is due to the refraction of the Earth's atmosphere. The
Neoplatonist Neoplatonism is a version of Platonic philosophy that emerged in the 3rd century AD against the background of Hellenistic philosophy and religion. The term does not encapsulate a set of ideas as much as a series of thinkers. Among the common id ...
philosopher
Olympiodorus the Younger Olympiodorus the Younger (; born , died after 565) was a Neoplatonist philosopher, astrologer and teacher who lived in the early years of the Byzantine Empire, after Justinian's Decree of 529 AD which closed Plato's Academy in Athens and other p ...
(–570 AD) criticized this view, arguing that if the Milky Way were sublunary, 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 position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
, which it does not. In his view, the Milky Way is celestial. This idea would be influential later in the
Muslim world The terms Islamic world and Muslim world commonly refer to the Islamic community, which is also known as the Ummah. This consists of all those who adhere to the religious beliefs, politics, and laws of Islam or to societies in which Islam is ...
. The
Persian Persian may refer to: * People and things from Iran, historically called ''Persia'' in the English language ** Persians, the majority ethnic group in Iran, not to be conflated with the Iranic peoples ** Persian language, an Iranian language of the ...
astronomer
Al-Biruni Abu Rayhan Muhammad ibn Ahmad al-Biruni (; ; 973after 1050), known as al-Biruni, was a Khwarazmian Iranian scholar and polymath during the Islamic Golden Age. He has been called variously "Father of Comparative Religion", "Father of modern ...
(973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars". The Andalusian astronomer
Avempace Abū Bakr Muḥammad ibn Yaḥyà ibn aṣ-Ṣā’igh at-Tūjībī ibn Bājja (), known simply as Ibn Bajja () or his Latinized name Avempace (;  – 1138), was an Arab polymath, whose writings include works regarding astronomy, physic ...
( 1138) proposed that the Milky Way was made up of many stars but appeared to be a continuous image in the Earth's atmosphere, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence. The Persian astronomer
Nasir al-Din al-Tusi Muḥammad ibn Muḥammad ibn al-Ḥasan al-Ṭūsī (1201 – 1274), also known as Naṣīr al-Dīn al-Ṭūsī (; ) or simply as (al-)Tusi, was a Persians, Persian polymath, architect, Early Islamic philosophy, philosopher, Islamic medicine, phy ...
(1201–1274) in his ''Tadhkira'' wrote: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color."
Ibn Qayyim al-Jawziyya Shams ad-Dīn Abū ʿAbd Allāh Muḥammad ibn Abī Bakr ibn Ayyūb az-Zurʿī d-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 scho ...
(1292–1350) proposed that the Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars".


Telescopic observations

Proof of the Milky Way consisting of many stars came in 1610 when
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
used a
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
to study the Milky Way and discovered that it is composed of a huge number of faint stars. Galileo also concluded that the appearance of the Milky Way was due to
refraction In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...
of the Earth's atmosphere. In a treatise in 1755,
Immanuel Kant Immanuel Kant (born Emanuel Kant; 22 April 1724 – 12 February 1804) was a German Philosophy, philosopher and one of the central Age of Enlightenment, Enlightenment thinkers. Born in Königsberg, Kant's comprehensive and systematic works ...
, drawing on earlier work by Thomas Wright, speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by
gravitation In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...
al forces akin to the Solar System but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Wright and Kant also conjectured that some of the
nebula A nebula (; or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the Pillars of Creation in ...
e visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s. The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by
William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...
in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Milky Way with the Solar System close to the center. In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture. In 1904, studying the
proper motion Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...
s of stars,
Jacobus Kapteyn Jacobus Cornelius Kapteyn (19 January 1851 – 18 June 1922) was a Dutch astronomy, astronomer. He carried out extensive studies of the Milky Way. He found that the apparent movement of stars was not randomly distributed but had two preferentia ...
reported that these were not random, as it was believed in that time; stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of the Milky Way, which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort. In 1917,
Heber Doust Curtis Heber Doust Curtis (June 27, 1872 – January 9, 1942) was an American astronomer. He participated in 11 expeditions for the study of solar eclipses, and, as an advocate and theorist that additional galaxies existed outside of the Milky Way, wa ...
had observed the nova S Andromedae within the Great Andromeda Nebula (
Messier object The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his ' (''Catalogue of Nebulae and Star Clusters''). Because Messier was interested only in finding comets, he created a list of th ...
31). Searching the photographic record, he found 11 more
nova A nova ( novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", Latin for "new") that slowly fades over weeks or months. All observed novae involve white ...
e. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within the Milky Way. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were independent galaxies. In 1920 the Great Debate took place between
Harlow Shapley Harlow Shapley (November 2, 1885 – October 20, 1972) was an American astronomer, who served as head of the Harvard College Observatory from 1921–1952, and political activist during the latter New Deal and Fair Deal. Shapley used Cepheid var ...
and Heber Curtis, 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 The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described t ...
. The controversy was conclusively settled by
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 ...
in the early 1920s using the Mount Wilson observatory Hooker telescope. With the light-gathering power of this new telescope, he was able to produce astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some
Cepheid variable A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...
s that he could use as a
benchmark Benchmark may refer to: Business and economics * Benchmarking, evaluating performance within organizations * Benchmark price * Benchmark (crude oil), oil-specific practices Science and technology * Experimental benchmarking, the act of defining a ...
to estimate the distance to the nebulae. He found that the Andromeda Nebula is 275,000 parsecs from the Sun, far too distant to be part of the Milky Way.


Satellite observations

The ESA spacecraft ''
Gaia In Greek mythology, Gaia (; , a poetic form of ('), meaning 'land' or 'earth'),, , . also spelled Gaea (), is the personification of Earth. Gaia is the ancestral mother—sometimes parthenogenic—of all life. She is the mother of Uranus (S ...
'' provides distance estimates by determining the
parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
of a billion stars and is mapping the Milky Way with four planned releases of maps in 2016, 2018, 2021 and 2024. Data from ''Gaia'' has been described as "transformational". It has been estimated that ''Gaia'' has expanded the number of observations of stars from about 2 million stars as of the 1990s to 2 billion. It has expanded the measurable volume of space by a factor of 100 in radius and a factor of 1,000 in precision. A study in 2020 concluded that ''Gaia'' detected a wobbling motion of the galaxy, which might be caused by "
torques In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Whe ...
from a misalignment of the disc's rotation axis with respect to the principal axis of a non-spherical halo, or from accreted matter in the halo acquired during late infall, or from nearby, interacting satellite galaxies and their consequent tides". In April 2024, initial studies (and related maps) involving the
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
s of the Milky Way were reported.


Astrography


Sun's location and neighborhood

The
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
is near the inner rim of the
Orion Arm The Orion Arm, also known as the Orion–Cygnus Arm, is a minor spiral arm within the Milky Way Galaxy spanning in width and extending roughly in length. This galactic structure encompasses the Solar System, including Earth. It is sometimes r ...
, within the Local Fluff of the
Local Bubble The Local Bubble, or Local Cavity, is a relative superbubble, cavity in the interstellar medium (ISM) of the Orion Arm in the Milky Way. It contains the List of nearest stars and brown dwarfs, nearest stars and brown dwarfs and, among others, the ...
, between the Radcliffe wave and ''Split'' linear structures (formerly
Gould Belt The Gould Belt is a local ring of stars in the Milky Way, tilted away from the galactic plane by about 16–20 degrees, first reported by John Herschel and Benjamin Gould in the 19th century. It contains many O- and B-type stars, and many o ...
). Based upon studies of stellar orbits around Sgr A* by Gillessen ''et al.'' (2016), the Sun lies at an estimated distance of from the Galactic Center. Boehle ''et al.'' (2016) found a smaller value of , also using a star orbit analysis. The Sun is currently above, or north of, the central plane of the Galactic disk. The distance between the local arm and the next arm out, the
Perseus Arm The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called the Scutum–Centaurus Arm. The Perseus Arm begins from the distal end of the long Milky Way central bar. Previously thought to be 13,000 ligh ...
, is about . The Sun, and thus the Solar System, is located in the Milky Way's galactic habitable zone. There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of from the Sun, giving a density of one star per 69 cubic parsecs, or one star per 2,360 cubic light-years (from
List of nearest bright stars The following nearest bright stars are found within of the closest star, the Sun, and have an absolute magnitude of +8.5 or brighter, which is approximately comparable to a listing of stars more luminous than a red dwarf. Right ascension and ...
). On the other hand, there are 64 known stars (of any magnitude, not counting 4 
brown dwarf Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...
s) within of the Sun, giving a density of about one star per 8.2 cubic parsecs, or one per 284 cubic light-years (from
List of nearest stars This list covers all known stars, white dwarfs, brown dwarfs, and sub-brown dwarfs within of the Sun. So far, 131 such objects have been found. Only List of nearest bright stars, 22 are bright enough to be visible without a telescope, for whi ...
). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than
apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...
 4 but 15.5 million stars brighter than apparent magnitude 14. The apex of the Sun's way, or the
solar apex The solar apex, or the apex of the Sun's way, refers to the direction that the Sun travels with respect to the local standard of rest. This is not to be confused with the Sun's apparent motion through all constellations of the zodiac, which i ...
, is the direction that the Sun travels through the
Local standard of rest In astronomy, the local standard of rest or LSR is a reference frame which follows the mean motion of material in the Milky Way in the neighborhood of the Sun (stars in radius 100 pc from the Sun), on average sharing the same velocity around the ...
in the Milky Way. The general direction of the Sun's Galactic motion is towards the star
Deneb Deneb () is a blue supergiant star in the constellation of Cygnus. It is the brightest star in the constellation and the 19th brightest in the night sky, with an apparent magnitude slightly varying between +1.21 and +1.29. Deneb is one ...
near the constellation of Cygnus, at an angle of roughly 90 sky degrees to the direction of the Galactic Center. The Sun's orbit about the Milky Way is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun passes through the Galactic plane approximately 2.7 times per orbit. This is very similar to how a
simple harmonic oscillator In mechanics and physics, simple harmonic motion (sometimes abbreviated as ) is a special type of periodic function, periodic motion an object experiences by means of a restoring force whose magnitude is directly proportionality (mathematics), ...
works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass lifeform extinction periods on Earth. A reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation. It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a
galactic year The galactic year, also known as a cosmic year, is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy.
), so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the origin of humans. The
orbital speed In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter (the combined center of mass) or ...
of the Solar System about the center of the Milky Way is approximately or 0.073% of the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...
. The Sun moves through the heliosphere at . At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (
astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...
). The Solar System is headed in the direction of the zodiacal constellation
Scorpius Scorpius is a zodiac constellation located in the Southern celestial hemisphere, where it sits near the center of the Milky Way, between Libra to the west and Sagittarius to the east. Scorpius is an ancient constellation whose recognition pred ...
, which follows the ecliptic.


Galactic quadrants

A galactic quadrant, or quadrant of the Milky Way, refers to one of four circular sectors in the division of the Milky Way. In astronomical practice, the delineation of the galactic quadrants is based upon the
galactic coordinate system The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane (spherical coordin ...
, which places the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
as the origin of the mapping system. Quadrants are described using ordinalsfor example, "1st galactic quadrant", "second galactic quadrant", or "third quadrant of the Milky Way". Viewing from the
north galactic pole The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane (spherical coordin ...
with 0° (zero degrees) as the ray that runs starting from the Sun and through the Galactic Center, the quadrants are: :: with the galactic longitude (ℓ) increasing in the counter-clockwise direction ( positive rotation) as viewed from
north North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating Direction (geometry), direction or geography. Etymology T ...
of the
Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...
(a view-point several hundred thousand
light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...
s distant from Earth in the direction of the constellation
Coma Berenices Coma Berenices is an ancient asterism in the northern sky, which has been defined as one of the 88 modern constellations. It is in the direction of the fourth galactic quadrant, between Leo and Boötes, and it is visible in both hemispher ...
); if viewed from south of the Galactic Center (a view-point similarly distant in the constellation
Sculptor Sculpture is the branch of the visual arts that operates in three dimensions. Sculpture is the three-dimensional art work which is physically presented in the dimensions of height, width and depth. It is one of the plastic arts. Durable sc ...
),
L, or l, is the twelfth letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''el'' (pronounced ), plural ''els''. History Lam ...
would increase in the clockwise direction ( negative rotation).


Size and mass


Size

The Milky Way is one of the two largest galaxies in the
Local Group The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a " dumbbell" shape; the Milky Way ...
(the other being the
Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...
), although the size for its
galactic disc A galactic disc (or galactic disk) is a component of disc galaxies, such as spiral galaxies like the Milky Way and lenticular galaxies. Galactic discs consist of a stellar component (composed of most of the galaxy's stars) and a gaseous compone ...
and how much it defines the isophotal diameter is not well understood. It is estimated that the significant bulk of stars in the galaxy lies within the diameter, and that the number of stars beyond the outermost disc dramatically reduces to a very low number, with respect to an extrapolation of the exponential disk with the scale length of the inner disc. There are several methods being used in astronomy in defining the size of a galaxy, and each of them can yield different results with respect to one another. The most commonly employed method is the D25 standard – the isophote where the photometric brightness of a galaxy in the B-band (445 nm wavelength of light, in the blue part of the
visible spectrum The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light). The optica ...
) reaches 25 mag/arcsec2. An estimate from 1997 by Goodwin and others compared the distribution of
Cepheid variable A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...
stars in 17 other spiral galaxies to the ones in the Milky Way, and modelling the relationship to their surface brightnesses. This gave an isophotal diameter for the Milky Way at , by assuming that the galactic disc is well represented by an exponential disc and adopting a central surface brightness of the galaxy (μ0) of ''B''-mag/arcsec−2 and a disk scale length (''h'') of . This is significantly smaller than the Andromeda Galaxy's isophotal diameter, and slightly below the mean isophotal sizes of the galaxies being at . The paper concludes that the Milky Way and Andromeda Galaxy were not overly large spiral galaxies, nor were among the largest known (if the former not being the largest) as previously widely believed, but rather average ordinary spiral galaxies. To compare the relative physical scale of the Milky Way, if 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 ...
out to
Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
were the size of a
US quarter The quarter, formally known as the quarter dollar, is a coin in the United States valued at 25 cents, representing one-quarter of a dollar. Adorning its obverse is the profile of George Washington, while its reverse design has undergone frequent ...
(), the Milky Way would be approximately at least the greatest north–south line of the
contiguous United States The contiguous United States, also known as the U.S. mainland, officially referred to as the conterminous United States, consists of the 48 adjoining U.S. states and the District of Columbia of the United States in central North America. The te ...
. An even older study from 1978 gave a lower diameter for Milky Way about . A 2015 paper discovered that there is a ring-like filament of stars called Triangulum–Andromeda Ring (TriAnd Ring) rippling above and below the relatively flat
galactic plane The galactic plane is the plane (geometry), plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galac ...
, which alongside Monoceros Ring were both suggested to be primarily the result of disk oscillations and wrapping around the Milky Way, at a diameter of at least , which may be part of the Milky Way's outer disk itself, hence making the stellar disk larger by increasing to this size. A more recent 2018 paper later somewhat ruled out this hypothesis, and supported a conclusion that the Monoceros Ring, A13 and TriAnd Ring were stellar overdensities rather kicked out from the main stellar disk, with the velocity dispersion of the RR Lyrae stars found to be higher and consistent with halo membership. Another 2018 study revealed the very probable presence of disk stars at from the Galactic Center or perhaps even farther, significantly beyond approximately , in which it was once believed to be the abrupt drop-off of the stellar density of the disk, meaning that few or no stars were expected to be above this limit, save for stars that belong to the old population of the galactic halo. A 2020 study predicted the edge of the Milky Way's
dark matter halo In modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo ma ...
being around , which translates to a diameter of . The Milky Way's stellar disk is also estimated to be approximately up to thick.


Mass

The Milky Way is approximately 0.88 trillion times the mass of the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
in total (0.88 solar masses), using a cutoff of 200kpc to define the galaxy. Estimates of the mass of the Milky Way vary, depending upon the method and data used. The low end of the estimate range is 5.8 
solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...
es (), somewhat less than that of the
Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...
. Measurements using the
Very Long Baseline Array The Very Long Baseline Array (VLBA) is a system of ten radio telescopes which are operated remotely from their Array Operations Center located in Socorro, New Mexico, as a part of the National Radio Astronomy Observatory (NRAO). These ten radio ...
in 2009 found velocities as large as for stars at the outer edge of the Milky Way. Because the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7  within of its center. In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kilo
parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
s is 7 . In a 2014 study, the mass of the entire Milky Way is estimated to be 8.5 , but this is only half the mass of the Andromeda Galaxy. A recent 2019 mass estimate for the Milky Way is 1.29 . Much of the mass of the Milky Way seems to be
dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...
, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A
dark matter halo In modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo ma ...
is conjectured to spread out relatively uniformly to a distance beyond one hundred kiloparsecs (kpc) from the Galactic Center. Mathematical models of the Milky Way suggest that the mass of dark matter is 1–1.5 . 2013 and 2014 studies indicate a range in mass, as large as 4.5  and as small as 8 . By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6  and 6.43 . In addition to the stars, there is also interstellar gas, comprising 90%
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and 10%
helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
by mass, with two thirds of the hydrogen found in the atomic form and the remaining one-third as
molecular hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
. The mass of the Milky Way's interstellar gas is equal to between 10% and 15% of the total mass of its stars.
Interstellar dust Cosmic dustalso called extraterrestrial dust, space dust, or star dustis dust that occurs in outer space or has fallen onto Earth. Most cosmic dust particles measure between a few molecules and , such as micrometeoroids (30 μm). Cosmic dust can ...
accounts for an additional 1% of the total mass of the gas. In March 2019, astronomers reported that the virial mass of the Milky Way Galaxy is 1.54 trillion
solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...
es within a
radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
of about , over twice as much as was determined in earlier studies, suggesting that about 90% of the mass of the galaxy is
dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...
. In September 2023, astronomers reported that the virial mass of the Milky Way Galaxy is only 2.06 x 1011
solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...
es, only a 10th of the mass of previous studies. The mass was determined from data of the ''Gaia'' spacecraft.


Contents

The Milky Way contains between 100 and 400 billion stars and at least that many planets. An exact figure would depend on counting the number of very-low-mass stars, which are difficult to detect, especially at distances of more than from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars. The Milky Way may contain ten billion
white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...
s, a billion
neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
s, and a hundred million stellar
black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...
s. Filling the space between the stars is a disk of gas and dust called the
interstellar medium The interstellar medium (ISM) is the matter and radiation that exists 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 cosmic rays. It f ...
. This disk has at least a comparable extent in radius to the stars, whereas the thickness of the gas layer ranges from hundreds of light-years for the colder gas to thousands of light-years for warmer gas. The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars decreases with distance from the center of the Milky Way. Beyond a radius of roughly 40,000 light years (13 kpc) from the center, the number of stars per cubic
parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
drops much faster with radius. Surrounding the galactic disk is a spherical
galactic halo A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of a galaxy comprise its halo: * the stellar halo * the galactic corona (hot gas, i.e. a plas ...
of stars and
globular cluster A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...
s that extends farther outward, but is limited in size by the orbits of two Milky Way satellites, the Large and Small
Magellanic Clouds The Magellanic Clouds (''Magellanic system'' or ''Nubeculae Magellani'') are two irregular dwarf galaxies in the southern celestial hemisphere. Orbiting the Milky Way galaxy, these satellite galaxies are members of the Local Group. Because both ...
, whose closest approach to the Galactic Center is about . At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be around −20.9. Both
gravitational microlensing Gravitational microlensing is an astronomical phenomenon caused by the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronom ...
and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way, and microlensing measurements indicate that there are more
rogue planet A rogue planet, also termed a free-floating planet (FFP) or an isolated planetary-mass object (iPMO), is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf. Rogue planets may originate from ...
s not bound to host stars than there are stars. The Milky Way contains an average of at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 by the
Kepler Johannes Kepler (27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws of p ...
space observatory. A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...
s reside in the Milky Way. In November 2013, astronomers reported, based on ''Kepler'' space mission data, that there could be as many as 40 billion Earth-sized
planets A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the te ...
orbiting in the
habitable zone In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressu ...
s of Sun-like stars and
red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...
s within the Milky Way. 11 billion of these estimated planets may be orbiting Sun-like stars. The nearest exoplanet may be 4.2 light-years away, orbiting the
red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...
Proxima Centauri Proxima Centauri is the nearest star to Earth after the Sun, located 4.25 light-years away in the southern constellation of Centaurus. This object was discovered in 1915 by Robert T. A. Innes, Robert Innes. It is a small, low-mass st ...
, according to a 2016 study. Such Earth-sized planets may be more numerous than gas giants, though harder to detect at great distances given their small size. Besides exoplanets, "
exocomet An exocomet, or extrasolar comet, is a comet outside the Solar System, which includes rogue comets and comets that orbit stars other than the Sun. The first exocomets were detected in 1987 around Beta Pictoris, a very young A-type main-sequence ...
s",
comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
s beyond the Solar System, have also been detected and may be common in the Milky Way. More recently, in November 2020, over 300 million habitable exoplanets are estimated to exist in the Milky Way Galaxy. When compared to other more distant galaxies in the universe, the Milky Way galaxy has a below average amount of
neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...
luminosity making our galaxy a "neutrino desert".


Structure

The Milky Way consists of a bar-shaped core region surrounded by a warped disk of gas, dust and stars. The mass distribution within the Milky Way closely resembles the type Sbc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms. Astronomers first began to conjecture that the Milky Way is a
barred spiral galaxy A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about two thirds of all spiral galaxies in the local universe, and generally affect both the motions of stars and interstellar gas ...
, rather than an ordinary
spiral galaxy Spiral galaxies form a galaxy morphological classification, class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''
, in the 1960s. These conjectures were confirmed by the
Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003, that was deactivated when operations ended on 30 January 2020. Spitzer was the third space telescope dedicate ...
observations in 2005 that showed the Milky Way's central bar to be larger than previously thought.


Galactic Center

The Sun is from the Galactic Center. This value is estimated using
geometric Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician w ...
-based methods or by measuring selected astronomical objects that serve as
standard candles The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible ...
, with different techniques yielding various values within this approximate range. In the inner few kiloparsecs (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge. It has been proposed that the Milky Way lacks a
bulge __NOTOC__ Bulge may refer to: Astronomy and geography *Bulge (astronomy), a tightly packed group of stars at the center of a spiral galaxy *Equatorial bulge, a bulge around the equator of a planet due to rotation *Tharsis bulge, vast volcanic pla ...
due to a collision and merger between previous galaxies, and that instead it only has a ''pseudobulge'' formed by its central bar. However, confusion in the literature between the (peanut shell)-shaped structure created by instabilities in the bar, versus a possible bulge with an expected half-light radius of 0.5 kpc, abounds. The Galactic Center is marked by an intense radio source named (pronounced ''Sagittarius A-star''). The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object. This concentration of mass is best explained as a
supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (). Black holes are a class of astronomical ...
(SMBH) with an estimated mass of 4.1–4.5 million times the
mass of the Sun The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies a ...
. The rate of accretion of the SMBH is consistent with an inactive galactic nucleus, being estimated at   per year. Observations indicate that there are SMBHs located near the center of most normal galaxies. The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from and 10–50 degrees relative to the line of sight from Earth to the Galactic Center. Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other. However, RR Lyrae-type stars do not trace a prominent Galactic bar. The bar may be surrounded by a ring called the "5 kpc ring" that contains a large fraction of the molecular hydrogen present in the Milky Way, as well as most of the Milky Way's
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, sta ...
activity. Viewed from the
Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...
, it would be the brightest feature of the Milky Way. X-ray emission from the core is aligned with the massive stars surrounding the central bar and the
Galactic ridge The galactic ridge is a region of the inner galaxy that is coincident with the galactic plane of the Milky Way. It can be seen from Earth as a band of stars which is interrupted by 'dust lanes'. In these 'dust lanes' the dust in the gaseous galacti ...
. In June 2023, astronomers led by Naoko Kurahashi Neilson reported using a new cascade neutrino technique to detect, for the first time, the release of
neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...
s from the
galactic plane The galactic plane is the plane (geometry), plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galac ...
of the Milky Way
galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
, creating the first neutrino view of the Milky Way.


Gamma rays and x-rays

Since 1970, various gamma-ray detection missions have discovered 511-
keV In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an electric potential difference of one volt in vacuum. When us ...
gamma rays A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
coming from the general direction of the Galactic Center. These gamma rays are produced by positrons (antielectrons) annihilating with
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s. In 2008 it was found that the distribution of the sources of the gamma rays resembles the distribution of low-mass
X-ray binaries An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelengt ...
, seeming to indicate that these X-ray binaries are sending positrons (and electrons) into interstellar space where they slow down and annihilate. The observations were made by both
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
and ESA's satellites. In 1970 gamma ray detectors found that the emitting region was about 10,000 light-years across with a luminosity of about 10,000 Suns. In 2010, two gigantic spherical bubbles of high energy gamma-emission were detected to the north and the south of the Milky Way core, using data from the
Fermi Gamma-ray Space Telescope The Fermi Gamma-ray Space Telescope (FGST, also FGRST), formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is ...
. The diameter of each of the bubbles is about (or about 1/4 of the galaxy's estimated diameter); they stretch up to Grus and to
Virgo Virgo may refer to: Arts and entertainment * Virgo (film), a 1970 Egyptian film * Virgo (character), several Marvel Comics characters * Virgo Asmita, a character in the manga ''Saint Seiya: The Lost Canvas'' * ''Virgo'' (album), by Virgo Four, ...
on the night-sky of the southern hemisphere. Subsequently, observations with the Parkes Telescope at radio frequencies identified polarized emission that is associated with the Fermi bubbles. These observations are best interpreted as a magnetized outflow driven by star formation in the central of the Milky Way. Later, on January 5, 2015,
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
reported observing an
X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an
asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
falling into the black hole or by the entanglement of
magnetic field lines A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
within gas flowing into Sagittarius A*.


Spiral arms

Outside the gravitational influence of the Galactic bar, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms. Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by
H II region An H II region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light year ...
s and
molecular cloud A molecular cloud—sometimes called a stellar nursery if star formation is occurring within—is a type of interstellar cloud of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
s. The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's arms. Perfect logarithmic spiral patterns only crudely describe features near the Sun, because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity. The possible scenario of the Sun within a spur / Local arm emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way. Estimates of the pitch angle of the arms range from about 7° to 25°. There are thought to be four spiral arms that all start near the Milky Way Galaxy's center. These are named as follows, with the positions of the arms shown in the image: Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum–Centaurus Arm contains approximately 30% more
red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...
s than would be expected in the absence of a spiral arm. This observation suggests that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars. In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way. Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear. The '' Near 3 kpc Arm'' (also called the ''Expanding 3 kpc Arm'' or simply the ''3 kpc Arm'') was discovered in the 1950s by astronomer van Woerden and collaborators through 21 centimeter radio measurements of H (
atomic hydrogen A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral hydrogen atom contains a single positively charged proton in the nucleus, and a single negatively charged electron bound to the nucleus by the Coulomb for ...
). It was found to be expanding away from the central bulge at more than 50  km/s. It is located in the fourth galactic quadrant at a distance of about 5.2  kpc from the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
and 3.3 kpc from the
Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...
. The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame ( Center for Astrophysics Harvard & Smithsonian). It is located in the first galactic quadrant at a distance of 3  kpc (about 10,000  ly) from the Galactic Center. A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy. It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer pseudoring, and the two patterns would be connected by the Cygnus arm. Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped
thick disk The thick disk is one of the structural components of about 2/3 of all disk galaxies, including the Milky Way. It was discovered first in external edge-on galaxies. Soon after, it was proposed as a distinct galactic structure in the Milky Way, di ...
of the Milky Way. The structure of the Milky Way's disk is warped along an "S" curve.


Halo

The Galactic disk is surrounded by a spheroidal halo of old stars and
globular cluster A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...
s, of which 90% lie within of the Galactic Center. However, a few globular clusters have been found farther, such as PAL 4 and AM 1 at more than 200,000 light-years from the Galactic Center. About 40% of the Milky Way's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation. The globular clusters can follow rosette orbits about the Milky Way, in contrast to the
elliptical orbit In astrodynamics or celestial mechanics, an elliptical orbit or eccentric orbit is an orbit with an orbital eccentricity, eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. Some or ...
of a planet around a star. Although the disk contains dust that obscures the view in some wavelengths, the halo component does not. Active
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, sta ...
takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little cool gas to collapse into stars.
Open cluster An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...
s are also located primarily in the disk. Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much farther than previously thought, the possibility of the disk of the Milky Way extending farther is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the
Cygnus Arm The Norma Arm is a minor spiral arm of the Milky Way extending from and around its central hub region. The inner portion of the Arm is called the ''Norma Arm'' in narrow meaning. The outer end of it is identified either with the Cygnus Arm (not ...
and of a similar extension of the Scutum–Centaurus Arm. With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Upon the 2004 discovery of a ring of galactic debris in an in-plane orbit around the Milky Way, it was initially believed that the debris was the remnant of a system dubbed the
Canis Major Dwarf Galaxy The Canis Major Overdensity (''CMa Overdensity'') or Canis Major Dwarf Galaxy (''CMa Dwarf'') is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major. The supposed small gal ...
. Other scholars believed it to be due to the Galactic warp, a view which has been supported by more recent evidence as of 2021. 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 began in 2000 a ...
of the northern sky shows a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a
dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...
is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about away.


Gaseous halo

In addition to the stellar halo, the
Chandra X-ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...
,
XMM-Newton ''XMM-Newton'', also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second corners ...
, and ''Suzaku'' have provided evidence that there is also a gaseous halo containing a large amount of hot gas. This halo extends for hundreds of thousands of light-years, much farther than the stellar halo and close to the distance of the Large and Small
Magellanic Clouds The Magellanic Clouds (''Magellanic system'' or ''Nubeculae Magellani'') are two irregular dwarf galaxies in the southern celestial hemisphere. Orbiting the Milky Way galaxy, these satellite galaxies are members of the Local Group. Because both ...
. The mass of this hot halo is nearly equivalent to the mass of the Milky Way itself. The temperature of this halo gas is between 1 and 2.5 million K (1.8 and 4.5 million °F). Observations of distant galaxies indicate that the Universe had about one-sixth as much
baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle that contains an odd number of valence quarks, conventionally three. proton, Protons and neutron, neutrons are examples of baryons; because baryons are ...
ic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way. If the finding that the mass of the halo is comparable to the mass of the Milky Way is confirmed, it could be the identity of the missing baryons around the Milky Way.


Galactic rotation

The stars and gas in the Milky Way rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 200 and 220 km/s. Hence the
orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
of the typical star is approximately proportional to the length of the path traveled. This is unlike the situation in the Solar System, where two-body gravitational dynamics dominate, and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation. If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotational speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter. The rotation curve of the Milky Way agrees with the universal rotation curve of spiral galaxies, the best evidence for the existence of
dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...
in galaxies. Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve.


Formation


History

The Milky Way began as one or several small overdensities in the mass distribution in the
Universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...
shortly after the
Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...
13.61 billion years ago. Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. Nearly half the matter in the Milky Way may have come from other distant galaxies. These stars and clusters now comprise the stellar halo of the Milky Way. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to
conservation of angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk. Since the first stars began to form, the Milky Way has grown through both
galaxy merger Galaxy mergers can occur when two (or more) Galaxy, galaxies collide. They are the most violent type of Interacting galaxy, galaxy interaction. The Gravitation, gravitational interactions between galaxies and the friction between the gas and Cosmi ...
s (particularly early in the Milky Way's growth) and accretion of gas directly from the Galactic halo. The Milky Way is currently accreting material from several small galaxies, including two of its largest satellite galaxies, the
Large Large means of great size. Large may also refer to: Mathematics * Arbitrarily large, a phrase in mathematics * Large cardinal, a property of certain transfinite numbers * Large category, a category with a proper class of objects and morphisms (o ...
and
Small Small means of insignificant size Size in general is the Magnitude (mathematics), magnitude or dimensions of a thing. More specifically, ''geometrical size'' (or ''spatial size'') can refer to three geometrical measures: length, area, or ...
Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in
high-velocity cloud High-velocity clouds (HVCs) are large accumulations of gas with an unusually rapid motion relative to their surroundings. They can be found throughout the galactic halo of the Milky Way. Their bulk motions in the local standard of rest have veloc ...
s like the Smith Cloud. Cosmological simulations indicate that, 11 billion years ago, it merged with a particularly large galaxy that has been labeled the Kraken. Properties of the Milky Way such as stellar mass,
angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
, and
metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...
in its outermost regions suggest it has undergone no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies. Its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies. According to recent studies, the Milky Way as well as the Andromeda Galaxy lie in what in the galaxy color–magnitude diagram is known as the "green valley", a region populated by galaxies in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star-formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both the Milky Way and the Andromeda Galaxy. Measurements of other galaxies similar to the Milky Way suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies.


Age and cosmological history

Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way. The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived
radioactive element A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...
s such as
thorium-232 Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half life of 14.05 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its de ...
and
uranium-238 Uranium-238 ( or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it i ...
, then comparing the results to estimates of their original abundance, a technique called
nucleocosmochronology Nucleocosmochronology, or nuclear cosmochronology, is a technique used to determine timescales for astrophysics, astrophysical objects and events based on observed ratios of radioactive heavy elements and their decay products. It is similar in ma ...
. These yield values of about for CS 31082-001 and for
BD +17° 3248 BD, Bd or bd may refer to: In arts and entertainment * B. D. (Doonesbury), a major character in the ''Doonesbury'' comic strip * ''Bande dessinée'' (or "bédé"), a French term for comics * Bass drum, in sheet music notation * Brahe Djäkna ...
. Once a
white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...
is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as . Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years. In November 2018, astronomers reported the discovery of one of the oldest stars in the universe. About 13.5 billion-years-old, 2MASS J18082002-5104378 B is a tiny ultra metal-poor (UMP) star made almost entirely of materials released from the
Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...
, and is possibly one of the first stars. The discovery of the star in the Milky Way
Galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
suggests that the galaxy may be at least 3 billion years older than previously thought. Several individual stars have been found in the Milky Way's halo with measured ages very close to the 13.80-billion-year
age of the Universe In physical cosmology, the age of the universe is the cosmological time, time elapsed since the Big Bang: 13.79 billion years. Astronomers have two different approaches to determine the age of the universe. One is based on a particle physics ...
. In 2007, a star in the galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way. This estimate was made using the UV-Visual Echelle Spectrograph of the
Very Large Telescope The Very Large Telescope (VLT) is an astronomical facility operated since 1998 by the European Southern Observatory, located on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each equipped with ...
to measure the relative strengths of
spectral line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency ...
s caused by the presence of
thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...
and other elements created by the
R-process In nuclear astrophysics, the rapid neutron-capture process, also known as the ''r''-process, is a set of nuclear reactions that is responsible for nucleosynthesis, the creation of approximately half of the Atomic nucleus, atomic nuclei Heavy meta ...
. The line strengths yield abundances of different elemental
isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
s, from which an estimate of the age of the star can be derived using
nucleocosmochronology Nucleocosmochronology, or nuclear cosmochronology, is a technique used to determine timescales for astrophysics, astrophysical objects and events based on observed ratios of radioactive heavy elements and their decay products. It is similar in ma ...
. Another star,
HD 140283 HD 140283 (also known as the Methuselah star) is a metal-poor subgiant star about 200 light years away from the Earth in the constellation Libra, near the boundary with Ophiuchus in the Milky Way galaxy. Its apparent magnitud ...
, has been estimated at either 13.7 ± 0.7 billion years, 12.2 ± 0.6 billion years, or 12.0 ± 0.5 billion years. According to observations utilizing
adaptive optics Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in Astronomy, astronomical telescopes and laser communication systems to remove the effects of Astronomical seeing, atmo ...
to correct for Earth's atmospheric distortion, stars in the galaxy's bulge date to about 12.8 billion years old. The age of stars in the galactic
thin disk The thin disk is a structural component of spiral and S0-type galaxies, composed of stars, gas and dust. It is the main non-centre (e.g. galactic bulge) density of such matter. That of the Milky Way is thought to have a scale height of around i ...
has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the
galactic halo A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of a galaxy comprise its halo: * the stellar halo * the galactic corona (hot gas, i.e. a plas ...
and the thin disk. Recent analysis of the chemical signatures of thousands of stars suggests that stellar formation might have dropped by an order of magnitude at the time of disk formation, 10 to 8 billion years ago, when interstellar gas was too hot to form new stars at the same rate as before. The satellite galaxies surrounding the Milky Way are not randomly distributed but seem to be the result of a breakup of some larger system producing a ring structure 500,000 light-years in diameter and 50,000 light-years wide. Close encounters between galaxies, like that expected in 4 billion years with the Andromeda Galaxy, can rip off huge tails of gas, which, over time can coalesce to form dwarf galaxies in a ring at an arbitrary angle to the main disc.


Intergalactic neighbourhood

The Milky Way and the
Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...
are a
binary system A binary system is a system of two astronomical bodies of the same kind that are comparable in size. Definitions vary, but typically require the center of mass to be located outside of either object. (See animated examples.) The most common ki ...
of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the
Local Group The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a " dumbbell" shape; the Milky Way ...
, surrounded by a Local Void, itself being part of the Local Sheet and in turn the
Virgo Supercluster The Local Supercluster (LSC or LS), or Virgo Supercluster is a formerly defined supercluster containing the Virgo Cluster and Local Group, which itself contains the Milky Way and Andromeda galaxies, as well as others. At least 100 galaxy group ...
. Surrounding the Virgo Supercluster are a number of
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, ...
, devoid of many galaxies, the Microscopium Void to the "north", the Sculptor Void to the "left", the
Boötes Void file:boovoid.png, 259x259px, A map of the Boötes Void The Boötes Void ( ) (colloquially referred to as the Great Nothing) is a roughly Sphere, spherical region of Interstellar medium, space in the vicinity of the constellation Boötes. It contain ...
to the "right" and the Canes-Major Void to the "south". These voids change shape over time, creating filamentous structures of galaxies. The Virgo Supercluster, for instance, is being drawn towards the
Great Attractor The Great Attractor is a region of gravitational attraction in intergalactic space and the apparent central gravitational point of the Laniakea Supercluster of galaxies that includes the Milky Way galaxy, as well as about 100,000 other galaxi ...
, which in turn forms part of a greater structure, called Laniakea. Two smaller galaxies and a number of
dwarf galaxies A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...
in the Local Group orbit the Milky Way. The largest of these is the
Large Magellanic Cloud The Large Magellanic Cloud (LMC) is a dwarf galaxy and satellite galaxy of the Milky Way. At a distance of around , the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal Galaxy, Sagittarius Dwarf ...
with a diameter of 32,200 light-years. It has a close companion, the
Small Magellanic Cloud The Small Magellanic Cloud (SMC) is a dwarf galaxy near the Milky Way. Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about , and contains several hundred million stars. It has a total mass of approximately 7 bill ...
. The Magellanic Stream is a stream of neutral
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
gas extending from these two small galaxies across 100° of the sky. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way. Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf,
Sculptor Dwarf The Sculptor Dwarf Galaxy (also known as Sculptor Dwarf Elliptical Galaxy or the Sculptor Dwarf Spheroidal Galaxy, and formerly as the Sculptor System) is a dwarf spheroidal galaxy that is a satellite of the Milky Way. The galaxy lies within the ...
,
Sextans Dwarf The Sextans Dwarf Spheroidal is a dwarf spheroidal galaxy that was discovered in 1990 by Mike Irwin as the 8th satellite of the Milky Way, located in the constellation of Sextans. It is also an elliptical galaxy, and displays a redshift because ...
, Fornax Dwarf, and Leo I Dwarf. The smallest dwarf galaxies of the Milky Way are only 500 light-years in diameter. These include Carina Dwarf,
Draco Dwarf The Draco Dwarf is a spheroidal galaxy which was discovered by Albert George Wilson of Lowell Observatory in 1954 on photographic plates of the National Geographic Society's Palomar Observatory Sky Survey (POSS). It is part of the Local Gr ...
, and Leo II Dwarf. There may still be undetected dwarf galaxies that are dynamically bound to the Milky Way, which is supported by the detection of nine new satellites of the Milky Way in a relatively small patch of the night sky in 2015. There are some dwarf galaxies that have already been absorbed by the Milky Way, such as the progenitor of
Omega Centauri Omega Centauri (ω Cen, NGC 5139, or Caldwell 80) is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of , it is the largest known globular clus ...
. In 2005 with further confirmation in 2012 researchers reported that most satellite galaxies of the Milky Way lie in a very large disk and orbit in the same direction. This came as a surprise: according to standard cosmology, the satellite galaxies should form in dark matter halos, and they should be widely distributed and moving in random directions. This discrepancy is still not explained. In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they orbit the Milky Way, causing vibrations when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, in a computer model, the movement of these two galaxies creates a dark matter wake that amplifies their influence on the larger Milky Way. Current measurements suggest the Andromeda Galaxy is approaching the Milky Way at . In 4.3 billion years, there may be an
Andromeda–Milky Way collision The Andromeda–Milky Way collision is a galactic collision that may occur in about 4.5 billion years between the two largest galaxies in the Local Group—the Milky Way (which contains the Solar System and Earth) and the Andromeda Galaxy. The ...
, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, the chance of individual stars colliding with each other is extremely low, but instead the two galaxies will merge to form a single
elliptical galaxy An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main galaxy morphological classification, classes of galaxy described by Edwin Hubble in his Hub ...
or perhaps a large
disk galaxy Disc or disk may refer to: * Disk (mathematics), a two dimensional shape, the interior of a circle * Disk storage * Optical disc * Floppy disk Music * Disc (band), an American experimental music band * ''Disk'' (album), a 1995 EP by Moby Other ...
over the course of about six billion years.


Velocity

Although
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory of the relationship between Spacetime, space and time. In Albert Einstein's 1905 paper, Annus Mirabilis papers#Special relativity, "On the Ele ...
states that there is no "preferred"
inertial frame of reference In classical physics and special relativity, an inertial frame of reference (also called an inertial space or a Galilean reference frame) is a frame of reference in which objects exhibit inertia: they remain at rest or in uniform motion relative ...
in space with which to compare the Milky Way, the Milky Way does have a velocity with respect to cosmological
frames of reference In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system, whose origin, orientation, and scale have been specified in physical space. It is based on a set of reference points, defined as geometric ...
. One such frame of reference is the
Hubble flow Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther a galaxy is from the Earth, the faster ...
, the apparent motions of galaxy clusters due to the
expansion of space The expansion of the universe is the increase in distance between gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion, so it does not mean that the universe expands "into" anything or that space ex ...
. Individual galaxies, including the Milky Way, have peculiar velocities relative to the average flow. Thus, to compare the Milky Way to the Hubble flow, one must consider a volume large enough so that the expansion of the Universe dominates over local, random motions. A large enough volume means that the mean motion of galaxies within this volume is equal to the Hubble flow. Astronomers believe the Milky Way is moving at approximately with respect to this local co-moving frame of reference. The Milky Way is moving in the general direction of the
Great Attractor The Great Attractor is a region of gravitational attraction in intergalactic space and the apparent central gravitational point of the Laniakea Supercluster of galaxies that includes the Milky Way galaxy, as well as about 100,000 other galaxi ...
and other
galaxy cluster A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. Clusters consist of galax ...
s, including the
Shapley Supercluster The Shapley Supercluster or Shapley Concentration (SCl 124) is the largest concentration of galaxies in our universe that forms a gravitationally interacting unit, thereby pulling itself together instead of expanding with the universe. It appears ...
, behind it. The Local Group, a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda Galaxy, is part of a
supercluster A supercluster is a large group of smaller galaxy clusters or galaxy groups; they are among the largest known structures in the universe. The Milky Way is part of the Local Group galaxy group (which contains more than 54 galaxies), which in tu ...
called the Local Supercluster, centered near the
Virgo Cluster The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the Virgo constellation. Comprising approximately 1,300 (and possibly up to 2,000) member galaxies, the cluster forms the heart of the larger ...
: although they are moving away from each other at as part of the Hubble flow, this velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster. Another reference frame is provided by the
cosmic microwave background The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...
(CMB), in which the CMB temperature is least distorted by Doppler shift (zero dipole moment). The Milky Way is moving at with respect to this frame, toward 10.5 right ascension, −24° declination (
J2000 In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to ...
epoch, near the center of Hydra). This motion is observed by satellites such as the
Cosmic Background Explorer The Cosmic Background Explorer (COBE ), also referred to as Explorer 66, was a NASA satellite dedicated to cosmology, which operated from 1989 to 1993. Its goals were to investigate the cosmic microwave background radiation (CMB or CMBR) of th ...
(COBE) and the
Wilkinson Microwave Anisotropy Probe The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic mic ...
(WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.


See also

* Baade's Window *
Galactic astronomy Galactic astronomy is the study of the Milky Way galaxy and all its contents. This is in contrast to extragalactic astronomy, which is the study of everything outside our galaxy, including all other galaxies. Galactic astronomy should not be con ...
*
Galactic Center GeV excess The Galactic Center GeV Excess (GCE) is an unexpected surplus of gamma-ray radiation in the center of the Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance ...
* Oort constants


Notes


References


Further reading

* * * * Plait, Phil, "The Milky Way's Secrets: Our galaxy's night-sky spectacle sparked
scientific revolution The Scientific Revolution was a series of events that marked the emergence of History of science, modern science during the early modern period, when developments in History of mathematics#Mathematics during the Scientific Revolution, mathemati ...
s", ''
Scientific American ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it, with more than 150 Nobel Pri ...
'', vol. 329, no. 4 (November 2023), pp. 86–87.


External links


Milky Way – IRAS (infrared) survey
– wikisky.org
Milky Way – H-Alpha survey
– wikisky.org

– Images and
VRML VRML (Virtual Reality Modeling Language, pronounced ''vermal'' or by its initials, originally—before 1995—known as the Virtual Reality Markup Language) is a standard file format for representing 3-dimensional (3D) interactive vector graph ...
models (NASA)
Milky Way – Panorama (9 billion pixels)
.

– SEDS Messier website
Milky Way
– Infrared Images
Milky Way
– Mosaic of galactic plane (March 19, 2021)

{{Portalbar, Star, Solar System, Science, History of science, Physics, Technology, Geophysics Articles containing video clips Astronomical objects known since antiquity Barred spiral galaxies