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 it 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 about 87,400 light-years across. At the spiral arms, it is only about 1,000 light-years thick. 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 is part of the Virgo Supercluster. That supercluster is also 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. It sits on the inner edge of the Orion Arm, 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. 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 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 made 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 came from 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 put his baby son Heracles, whose mother was Alcmene, on Hera's chest while she slept. The baby drank her divine milk so he would become immortal. Hera woke up and found she was feeding a baby she did not know. She pulled him away, and some of her milk spilled across the sky, creating the band of light called the Milky Way. In another Greek story, Athena brought the baby Heracles to Hera to feed, but he drank so eagerly that Hera pulled him away.

In Western culture, the name "Milky Way" comes from how the band looks: a soft, "milky" glow stretching across the night sky. It comes from 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), which comes from the word for "milk," is also the root of the word "galaxy." The Greeks named 11 "circles" in the sky, and the Milky Way was one of them. The others included the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, the Arctic Circle, the Antarctic Circle, and two colure circles that pass through both poles. The English name can be traced back to a poem 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 people in the north saw migratory birds follow it while they flew north and south. The Kaurna people of the Adelaide Plains in South Australia called it wodliparri in the Kaurna language, which means "house river." The Gomeroi people, who live between New South Wales and Queensland, called it Dhinawan, the giant "Emu in the Sky" that stretches across the night sky. Travelers called 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 in Korea and Vietnam (Ngân hà). In Japanese, it is called "River of Heaven" (天の川, Ama no gawa), while "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 the Swedes, call it "Winter Street" (Vintergatan), because it is easiest to see in winter in the northern hemisphere. In summer, the glow of the Sun late at night can hide it, especially far north.

Appearance

The Milky Way is a hazy band of white light, about 30° wide, that stretches 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. They are too far away to see one by one. Brighter patches along the band are called star clouds. The most noticeable one is the Large Sagittarius Star Cloud, which is 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 is faint, so light pollution or moonlight can make it hard to see. You need a dark sky, with a limiting magnitude of about +5.1 or better. It is much clearer in rural areas when the Moon is below the horizon. It is hard to see from bright cities and suburbs because of light pollution. 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, wait about 30 minutes for your eyes to adjust to the dark. The sky should have little light pollution (rated Bortle 4 or lower), little or no moonlight, and little haze. The best season depends on your hemisphere. In North America, May to August is best. 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, the 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 that were 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. He said it should look different in different places, but 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. He used 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 stars that were close together. 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 huge 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. He compared it to the Solar System, but much larger. He also thought some nebulae might be separate "galaxies" like our own. In 1785, William Herschel tried to describe the Milky Way's shape 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. He 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 confirmed this rotation. In 1917, Heber Doust Curtis studied a nova in the Great Andromeda Nebula (Messier object 31) and found 11 more novae. He used them to estimate its distance, and he argued that spiral nebulae were separate galaxies. In 1920, the Great Debate took place between Harlow Shapley and Curtis about these questions. Edwin Hubble settled the debate in the early 1920s. He used 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. It is also mapping the Milky Way. Scientists have called the data from Gaia "transformational." A study in 2020 found that the galaxy appears to wobble. This might be caused by "torques," 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 is near the inner edge of the Orion Arm. It sits inside the Local Fluff, which is part of the Local Bubble. Nearby are 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. This estimate comes from how stars move around Sgr A*. Another study found a smaller distance, about 25.64 thousand light-years. The next arm out, the Perseus Arm, is about 6,500 light-years away. The Sun, and the Solar System with it, is in the Milky Way's galactic habitable zone. Near the Sun, there are many more faint stars than bright ones. The Sun moves through space at about 220 km/s. One trip around the Milky Way takes about 240 million years.

Main article: Galactic quadrant

A galactic quadrant is one of four circular sections that the Milky Way is divided into. Scientists use the galactic coordinate system to mark these divisions. This system puts the Sun at the center of the map. Quadrants are named with ordinal numbers, such as "1st galactic quadrant" or "third quadrant of the Milky Way." When viewed from the north galactic pole, the starting line (0°) is the ray that runs from the Sun through the Galactic Center.

The galactic longitude (ℓ) increases counter-clockwise when viewed from north of the Galactic Center. From that side, the Galactic Center is in the direction of the constellation Coma Berenices. When viewed from south of the Galactic Center, in the constellation Sculptor, the longitude 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. Most of its stars lie within about 80,000 light-years across. One estimate from 1997 gave a smaller size for the Milky Way than for the Andromeda Galaxy. The Milky Way has a very large mass. Much of that mass seems to be dark matter, an invisible kind of matter that can be felt only through gravity.

The stars and gas in the Milky Way travel around its center. Most stars move at similar speeds, no matter how far they are from the center. Scientists think there must be extra mass that we cannot see, which is likely dark matter. The Milky Way is also moving through space toward the Great Attractor and other galaxy clusters.

The Milky Way has between 100 and 400 billion stars. It probably has at least as many planets. Counting them exactly is hard, because very small, dim stars are difficult to detect, especially far from the Sun. For comparison, the nearby Andromeda Galaxy has an estimated one trillion (10¹²) stars. The Milky Way may have ten billion white dwarfs, a billion neutron stars, and a hundred million black holes. Between the stars is a disk of gas and dust called the interstellar medium. This 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 does not have a sharp edge. Instead, the number of stars slowly gets smaller the farther you go from the center. Surrounding the disk is a large, round cloud of stars called a galactic halo. It also contains globular clusters. The size of the halo is limited by the orbits of the Large and Small Magellanic Clouds, two smaller galaxies that travel alongside the Milky Way. Their closest approach to the center of the galaxy is about 180,000 light-years away. The Milky Way's overall absolute visual magnitude is estimated to be around −20.9.

Studies using gravitational microlensing and observations of planets suggest there may be at least as many planets tied to stars as there are stars. There may even be more rogue planets than stars. A January 2013 study of the Kepler-32 star system, using the Kepler space observatory, estimated at least one planet per star. Scientists think up to 40 billion Earth-sized exoplanets may orbit in the habitable zones of Sun-like stars and red dwarfs in the Milky Way. This estimate comes from Kepler space telescope data. The nearest exoplanet may be 4.2 light-years away, orbiting Proxima Centauri. Exocomets, which are comets beyond our Solar System, may also be common. Compared with other distant galaxies, the Milky Way gives off fewer neutrinos, which makes it a "neutrino desert."

Structure

The Milky Way has a bar-shaped center surrounded by a flat, wide disk of gas, dust and stars. Its shape is similar to the type Sbc in the Hubble classification. In the 1960s, astronomers began to suggest that it is a barred spiral galaxy. In 2005, observations from the Spitzer Space Telescope showed that its central bar is bigger 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. It is about 4.1 to 4.5 million times as heavy as the mass of the Sun.

Main articles: Galactic Center and Sagittarius A*

Further information: Spiral galaxy

The disk has four spiral arms. These arms have more gas and dust, and more star formation, than the rest of the disk. Scientists are still not sure what the arms are really like. The disk is surrounded by a halo of old stars and globular clusters. The halo holds most of the galaxy's dark matter. New stars do not form 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 patches of slightly denser matter soon after the Big Bang, about 13.61 billion years ago. Some of these patches formed globular clusters, which contain the oldest stars still in the galaxy. Nearly half of its matter may have come from other distant galaxies. Those stars and clusters now make up its stellar halo. Spinning gas flattened into a disk because of conservation of angular momentum. Later stars formed in this disk, 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, such as the Large and Small Magellanic Clouds, through the Magellanic Stream. It also takes in gas 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 both lie in the "green valley" of the galaxy color–magnitude diagram. This is a region for galaxies that are slowing down their production of new stars.

Globular clusters are among the oldest objects in the Milky Way, so they show that the galaxy is at least that old. Scientists estimate the ages of stars by measuring radioactive elements, such as thorium-232 and uranium-238. This method is called nucleocosmochronology. It 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 with age estimates. 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. It is about 13.5 billion years old and made almost entirely of materials released from the Big Bang. Its discovery 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. One example is HE 1523-0901, estimated at 13.2 billion years in 2007. Scientists measured it with the Very Large Telescope by studying 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. 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 form 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, is about 32,200 light-years across. The Magellanic Stream is a stream of hydrogen gas that stretches 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 about 500 light-years across. These include 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. The Andromeda Galaxy 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. The two galaxies would slowly merge over about six billion years.