Milky Way

The Milky Way or Milky Way Galaxy, or simply the Galaxy, is the galaxy that includes the Solar System. Its name describes how the galaxy looks from Earth: a hazy band of light across the night sky. That light comes from stars in other arms of the galaxy. They are so far away that the naked eye cannot tell them apart one by one. For other uses, see Milky Way (disambiguation).

The Milky Way is a barred spiral galaxy. Its D₂₅ isophotal diameter is estimated at about 87,400 light-years across, but it is only about 1,000 light-years thick at the spiral arms. Recent simulations suggest that a dark matter area, which also contains some visible stars, may stretch almost 2 million light-years wide. The Milky Way has several satellite galaxies. It is part of the Local Group of galaxies, which forms part of the Virgo Supercluster. That supercluster is itself a part of the Laniakea Supercluster.

The Milky Way is estimated to contain 100 to 400 billion stars, and at least that many planets. The Solar System is about 27,000 light-years from the Galactic Center, on the inner edge of the Orion Arm, which is one of the spiral-shaped areas of gas and dust. At the center, the galaxy has a bulge and one or more bars. The Galactic Center is a strong radio source called Sagittarius A*, which is a supermassive black hole. The oldest stars in the Milky Way are nearly as old as the universe itself, so they probably formed shortly after the Dark Ages that followed the Big Bang.

Galileo Galilei first saw the band of light as 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. After the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis, observations by Edwin Hubble in 1923 showed that the Milky Way was just one of many galaxies.

In the Babylonian epic poem Enūma Eliš, the Milky Way is created from the tail of an ancient salt water dragon, Tiamat. After Marduk, the main national god of Babylon, defeats her, he places the tail in the sky.

People once thought this story was based on an older Sumerian version, in which Enlil of Nippur defeats Tiamat instead. Today, it is thought that Babylonian writers made up the story to show that Marduk was greater than the Sumerian gods.

Etymology

In Greek mythology, Zeus places his infant son Heracles, born to Alcmene, on Hera's breast while she is asleep, so the baby will drink her divine milk and become immortal. Hera wakes while nursing and realizes she is feeding an unknown baby. She pushes him away, some of her milk spills, and it produces the band of light known as the Milky Way. In another Greek story, Athena gives the abandoned Heracles to Hera to feed, but his forcefulness makes Hera rip him from her breast in pain.

In Western culture, the name "Milky Way" comes from the way the band looks: a dim, "milky" glow arching across the night sky. It translates the Classical Latin via lactea, which comes from the Hellenistic Greek γαλαξίας, short for γαλαξίας κύκλος (galaxías kýklos), meaning "milky circle." The Ancient Greek word γαλαξίας (galaxias), from the word for "milk," is also the root of "galaxy." The Milky Way was one of 11 "circles" the Greeks identified in the sky, along with the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, the Arctic Circle, the Antarctic Circle, and two colure circles passing through both poles. The English name can be traced back to a story by Geoffrey Chaucer from about 1380, called The House of Fame. Many other peoples have their own names for it. In the Uralic, Turkic languages, and Baltic languages, it is called "Birds' Path," because northern peoples saw migratory birds follow it while migrating in the Northern Hemisphere. The Kaurna people of the Adelaide Plains in South Australia called it wodliparri in the Kaurna language, meaning "house river." The Gomeroi people, between New South Wales and Queensland, called it Dhinawan, the giant "Emu in the Sky" that stretches across the night sky. Traveling pilgrims used it as a guide to the holy site at Santiago de Compostela, which is why it is also called "The Road to Santiago." In Sanskrit, it is called "River Ganga of the Sky," a name used in many Indian languages. The Chinese name "Silver River" (銀河) is used across East Asia, including Korea and Vietnam (Ngân hà). In Japanese, the name is "River of Heaven" (天の川, Ama no gawa), and "Silver River" (銀河, romanized ginga) refers to any galaxy. In Chinese, it is also called 天河 (pinyin: Tiānhé). In West and Central Asia and parts of the Balkans, the name is related to the word for "straw." Persians, Pakistanis, and Turks use this name, as do Arabs, who call it Darb at-Tabbānah (Arabic: درب التبانة), meaning "Pathway of the Straw." In Serbo-Croatian, it is called "Kumova slama" (Godfather's Straw) as well as "Mliječni put" (Milky Way). Scandinavian peoples, such as Swedes, call it "Winter Street" (Vintergatan), since it is most clearly visible in winter in the northern hemisphere, especially at high latitudes where the glow of the Sun late at night can hide it in summer.

Appearance

The Milky Way is a hazy band of white light, about 30° wide, that arches across the night sky. Every star you can see with your own eyes is part of the Milky Way Galaxy, but the name "Milky Way" refers only to this band. The light comes from many stars and other material in the galactic plane that are too far away to see one by one. Brighter patches along the band are called star clouds. The most noticeable is the Large Sagittarius Star Cloud, part of the galaxy's central bulge. Dark areas such as the Great Rift and the Coalsack are places where interstellar dust blocks the light of distant stars. People in the southern hemisphere, including the Inca and Australian Aboriginals, saw these dark areas as dark cloud constellations. The part of the sky hidden by the band is called the Zone of Avoidance.

The Milky Way has a fairly low surface brightness, so light pollution or moonlight can make it hard to see. It needs a dark sky, with a limiting magnitude of about +5.1 or better, and it is much clearer in rural areas when the Moon is below the horizon. Because of light pollution, it is hard to see from bright cities and suburbs. Maps show that more than one-third of Earth's people cannot see it from their homes. The visible band passes through 30 constellations. The Galactic Center lies in Sagittarius, where the band is brightest. From there, the band runs to the galactic anticenter in Auriga and divides the sky into two roughly equal hemispheres. The galactic plane is tilted about 60° to the ecliptic and 63° to the celestial equator. To see it well, your eyes need about 30 minutes to adjust to the dark, and the sky should have low light pollution (rated Bortle 4 or lower), little or no moonlight, and little haze. The best season depends on your hemisphere, with May to August best in North America. In the Northern Hemisphere, look south toward Sagittarius and Scorpius. In the Southern Hemisphere, look near Carina and Centaurus.

Astronomical history

See also: Galaxy § Observation history

Long ago, Greek thinker Aristotle (384–322 BC) wrote in his Meteorologica that earlier thinkers, Anaxagoras (c. 500–428 BC) and Democritus (460–370 BC), believed the Milky Way was made of stars hidden from view by Earth's shadow. Aristotle thought it was part of Earth's upper air, and that its milky look came from the air bending light. The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 AD) disagreed, saying it should look different in different places, which it does not. He said the Milky Way is in space, not in the air. This idea was later important in the Muslim world. The Persian astronomer Al-Biruni (973–1048) called it "a collection of countless fragments of the nature of nebulous stars." The Andalusian astronomer Avempace (died 1138) said it was made of many stars, using a conjunction of Jupiter and Mars in 1106 or 1107 as evidence. The Persian astronomer Nasir al-Din al-Tusi (1201–1274) wrote that it was made of very many small, tightly grouped stars. Ibn Qayyim al-Jawziyya (1292–1350) described it as "a myriad of tiny stars packed together."

In 1610, Galileo Galilei used an optical telescope and found that the Milky Way was made of a vast number of faint stars. In 1755, Immanuel Kant suggested that the Milky Way might be a spinning group of many stars, held together by gravitational forces, like the Solar System but much larger. He also thought some nebulae might be separate "galaxies" like our own. William Herschel tried to describe the Milky Way's shape in 1785 by counting stars in different parts of the sky. In 1845, Lord Rosse built a new telescope and found the spiral shape of the Whirlpool Galaxy (M51).

In 1904, Jacobus Kapteyn studied the proper motions of stars and found that they moved in two streams in nearly opposite directions. Later, scientists realized this was the first sign that the Milky Way rotates. Bertil Lindblad and Jan Oort later found this rotation. In 1917, Heber Doust Curtis studied a nova in the Great Andromeda Nebula (Messier object 31) and found 11 more novae. He estimated its distance from these, and he argued that spiral nebulae were separate galaxies. In 1920, the Great Debate took place between Harlow Shapley and Curtis about these questions. The debate was settled by Edwin Hubble in the early 1920s, using the Mount Wilson observatory's 2.5 m (100 in) Hooker telescope. Hubble showed that the Andromeda Nebula is far too distant to be part of the Milky Way.

The ESA spacecraft Gaia measures distances by finding the parallax of about a billion stars, and it is mapping the Milky Way. Data from Gaia has been called "transformational." A study in 2020 found that the galaxy appears to wobble, which might be caused by "torques" or by accreted matter, or by nearby satellite galaxies. In April 2024, early studies and maps of the Milky Way's magnetic fields were reported.

Astrography

See also: Location of Earth

The Sun sits near the inner edge of the Orion Arm, inside the Local Fluff of the Local Bubble. It lies between the Radcliffe wave and the Split structures, which were once called the Gould Belt. Scientists estimate that the Sun is about 27.14 thousand light-years from the Galactic Center, based on the orbits of stars around Sgr A*. Another study found a smaller distance of 25.64 thousand light-years. The next arm out, the Perseus Arm, is about 6,500 light-years away. The Sun, and so the Solar System, is in the Milky Way's galactic habitable zone. There are far more faint stars than bright stars near the Sun. The Sun moves through space at about 220 km/s, and it takes about 240 million years to complete one orbit of the Milky Way.

Main article: Galactic quadrant

A galactic quadrant is one of four circular sectors that the Milky Way is divided into. Scientists use the galactic coordinate system, which places the Sun at the center of the map, to mark these divisions. Quadrants are named with ordinals, such as "1st galactic quadrant" or "third quadrant of the Milky Way." Viewed from the north galactic pole, with 0° as the ray that runs from the Sun through the Galactic Center, the quadrants are:

The galactic longitude (ℓ) increases counter-clockwise when viewed from north of the Galactic Center, which is in the direction of the constellation Coma Berenices. When viewed from south of the Galactic Center, in the constellation Sculptor, it increases clockwise.

General characteristics

The Milky Way is one of the two largest galaxies in the Local Group, along with the Andromeda Galaxy. Its size depends on how it is measured, and different methods give different results. Most of its stars lie within about 26 kiloparsecs (80,000 light-years) across. One 1997 estimate gave its isophotal diameter as 26.8 kiloparsecs (87,400 light-years), which is smaller than the Andromeda Galaxy's. The total mass of the Milky Way is about 0.88 trillion times the mass of the Sun, and much of that mass seems to be dark matter, an invisible form of matter that interacts with ordinary matter through gravity.

The stars and gas in the Milky Way rotate around its center, and most stars move at similar speeds no matter how far they are from the center. If only the visible matter were present, the speed would drop with distance from the center, but the observed curve stays fairly flat. This suggests extra mass that cannot be seen directly, which is attributed to dark matter. The Milky Way is also moving about 630 km/s (1,400,000 mph) relative to the nearby space around it, and it moves toward the Great Attractor and other galaxy clusters.

Contents

The Milky Way contains between 100 and 400 billion stars and at least that many planets. Counting them exactly is hard, because very-low-mass stars are difficult to detect, especially more than 300 light-years (90 pc) from the Sun. For comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (10¹²) stars. The Milky Way may contain ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes. Filling the space between the stars is a disk of gas and dust called the interstellar medium. The gas layer is hundreds of light-years thick in its colder parts and thousands of light-years thick in its warmer parts.

The disk of stars has no sharp edge. Instead, the number of stars decreases gradually with distance from the center. Beyond about 40,000 light-years (13 kpc) from the center, the number of stars per cubic parsec drops much faster. Surrounding the disk is a spherical galactic halo of stars and globular clusters. The halo is limited in size by the orbits of the Large and Small Magellanic Clouds, two Milky Way satellites whose closest approach to the Galactic Center is about 180,000 ly (55 kpc). The Milky Way's overall absolute visual magnitude is estimated to be around −20.9. Studies using gravitational microlensing and planet observations suggest there may be at least as many planets bound to stars as there are stars, and that there are more rogue planets than stars. A January 2013 study of the Kepler-32 star system by the Kepler space observatory estimated at least one planet per star. Scientists have also estimated that as many as 40 billion Earth-sized exoplanets may orbit in the habitable zones of Sun-like stars and red dwarfs in the Milky Way, according to Kepler space telescope data. The nearest exoplanet may be 4.2 light-years away, orbiting Proxima Centauri. Exocomets, comets beyond the Solar System, may also be common. Compared with other distant galaxies, the Milky Way has a below-average amount of neutrino luminosity, which makes it a "neutrino desert."

Structure

The Milky Way has a bar-shaped core surrounded by a flat, wide disk of gas, dust and stars. Its shape is similar to the type Sbc in the Hubble classification. Astronomers began to suggest in the 1960s that it is a barred spiral galaxy, and observations from the Spitzer Space Telescope in 2005 showed that its central bar is larger than people had thought. The Sun is about 25,000 to 28,000 light-years from the Galactic Center, where Sagittarius A* is. Sagittarius A* is a supermassive black hole with a mass of about 4.1 to 4.5 million times the mass of the Sun.

Main articles: Galactic Center and Sagittarius A*

Further information: Spiral galaxy

The disk has four spiral arms, which contain more gas and dust and more star formation than the rest of the disk. Scientists are still not sure what these arms are really like. The disk is surrounded by a halo of old stars and globular clusters, and the halo holds most of the galaxy's dark matter. Stars are not forming in the halo because there is very little cool gas there.

Formation

Main article: Galaxy formation and evolution

The Milky Way began as one or several small areas of slightly denser matter shortly after the Big Bang, 13.61 billion years ago. Some of these areas seeded globular clusters, where the oldest stars still in the galaxy formed. Nearly half of its matter may have come from other distant galaxies, and these stars and clusters now make up its stellar halo. Because of conservation of angular momentum, the spinning gas flattened into a disk, where later stars formed, including the Sun. Since then, the galaxy has grown through galaxy mergers and by taking in gas. It is still gathering material from small galaxies, including the Large and Small Magellanic Clouds, through the Magellanic Stream, and from high-velocity clouds like the Smith Cloud. Simulations suggest it merged with a very large galaxy called the Kraken about 11 billion years ago. The Milky Way and the Andromeda Galaxy also lie in the "green valley" of the galaxy color–magnitude diagram, a region for galaxies moving from making new stars toward having none.

Globular clusters are among the oldest objects in the Milky Way, so they show the galaxy is at least that old. Scientists estimate the ages of stars by measuring radioactive elements such as thorium-232 and uranium-238, a method called nucleocosmochronology. This gives about 12.5 ± 3 billion years for CS 31082-001 and 13.8 ± 4 billion years for BD +17° 3248. Cooling white dwarf stars also help, and the cluster M4 was estimated at 12.7 ± 0.7 billion years. In November 2018, astronomers reported one of the universe's oldest stars, 2MASS J18082002-5104378 B, about 13.5 billion years old and made almost entirely of materials released from the Big Bang. Its discovery in the Milky Way Galaxy suggests the galaxy may be at least 3 billion years older than previously thought. Some halo stars are almost as old as the age of the Universe, such as HE 1523-0901, estimated at 13.2 billion years in 2007, which was measured using the Very Large Telescope to measure spectral lines from elements made by the R-process and their isotopes. Another star, HD 140283, has been estimated at 13.7, 12.2, or 12.0 billion years. Observations using adaptive optics suggest stars in the bulge are about 12.8 billion years old, and the thin disk formed about 8.8 billion years ago, leaving a gap of almost 5 billion years after the halo formed.

Main article: Local Group

The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies. They belong to the Local Group, a group of 50 closely bound galaxies. The Local Group sits in a Local Void and is part of the Local Sheet, which is part of the Virgo Supercluster. Several voids with few galaxies surround the Virgo Supercluster, including the Boötes Void. The Virgo Supercluster is being pulled toward the Great Attractor, which is part of a larger structure called Laniakea.

Two smaller galaxies and several dwarf galaxies orbit the Milky Way. The largest, the Large Magellanic Cloud, has a diameter of 32,200 light-years. The Magellanic Stream, a stream of hydrogen gas, reaches across 100° of the sky from these small galaxies. Some of the dwarf galaxies orbiting the Milky Way include Canis Major Dwarf, Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The smallest are only 500 light-years across, including Carina Dwarf, Draco Dwarf, and Leo II Dwarf. The Milky Way has already absorbed some dwarf galaxies, such as the one that became Omega Centauri. Andromeda is moving toward the Milky Way at 100 to 140 km/s (220,000 to 310,000 mph). In about 4.3 billion years, an Andromeda–Milky Way collision may happen. If it does, stars colliding with each other would be extremely unlikely, and the two galaxies would slowly merge over about six billion years.