The Milky Way or Milky Way Galaxy, or simply the Galaxy, is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galaxy, which are so far away that they cannot be individually distinguished by the naked eye.
The Milky Way is a barred spiral galaxy with a D25 isophotal diameter estimated at , but only about 1,000 light-years thick at the spiral arms (more at the bar). Recent simulations suggest that a dark matter 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 and is part of the Local Group of galaxies, forming part of the Virgo Supercluster which is itself a component of the Laniakea Supercluster.
It is estimated to contain 100–400 billion stars and at least that number of planets. The Solar System is located at a radius of about 27,000 light-years (8.3 kpc) from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The Galactic Center is an intense radio source known as Sagittarius A*, a supermassive black hole with a mass of 4.100 (± 0.034) million solar masses. The oldest stars in the Milky Way are nearly as old as the universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.
Galileo Galilei 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 and Heber Doust Curtis, observations by Edwin Hubble in 1923 showed that the Milky Way was just one of many galaxies.
Mythology
In the Babylonian epic poem Enūma Eliš, the Milky Way is created from the severed tail of the primeval salt water dragon Tiamat, set in the sky by Marduk, the Babylonian national god, after slaying her. This story was once thought to have been based on an older Sumerian version in which Tiamat is instead slain by Enlil of Nippur, but is now thought to be purely an invention of Babylonian propagandists with the intention of showing Marduk as superior to the Sumerian deities.
Etymology In Greek mythology, Zeus places Heracles, his infant son born to Alcmene, on Hera'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 to Hera for feeding, but Heracles' forcefulness causes Hera to rip him from her breast in pain.
In Western culture, the name "Milky Way" derives from its appearance as a dim, unresolved, "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin , in turn derived from the Hellenistic Greek , short for (), meaning "milky circle". The Ancient 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 meridian, the horizon, the equator, the tropics of Cancer and Capricorn, the Arctic Circle and the Antarctic Circle, and two colure circles passing through both poles. The English term can be traced back to a story by Geoffrey Chaucer :
Other common names "Birds' Path" is used in several Uralic and Turkic languages and in the Baltic languages. Northern peoples observed that migratory birds follow the course of the galaxy^ Sauer, EGF (July 1971). "Celestial Rotation and Stellar Orientation in Migratory Warblers". Science 30: 459–461. while migrating in 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. The Kaurna people of the Adelaide Plains of South Australia called the Milky Way wodliparri in the Kaurna language, meaning "house river". The Gomeroi people between New South Wales and Queensland called the Milky Way Dhinawan, the giant "Emu in the Sky" that it stretches across the night sky. The Milky Way was traditionally used as a guide by pilgrims traveling to the holy site at Santiago de 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. The Chinese name "Silver River" () is used throughout East Asia, including Korea and Vietnam (Ngân hà). In Japan and Korea, "Silver River" (; ) refers to any galaxy. The Japanese name for the Milky Way is the , as well as an alternative name in Chinese (). In Vietnamese, "River of Heaven" (Thiên hà) refers to any galaxy. In West Asia, Central Asia, and parts of the Balkans, the name for the Milky Way is related to the word for straw. Today, Persians, Pakistanis, and Turks use it alongside Arabs, known to them as Darb at-Tabbānah (Arabic: درب التبانة) meaning "Pathway of the Straw". It has been suggested that the term was spread by medieval Arabs who in turn borrowed it from Armenians. In Serbo-Croatian it is interchangeably called "Kumova slama" () along the "Mliječni put" (). Scandinavian peoples, such as Swedes, have called the galaxy "Winter Street" (Vintergatan) as the galaxy is most clearly visible during the winter in 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 in the night sky. 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. 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 of the galaxy. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars. Peoples of the southern hemisphere, including the Inca and Australian Aboriginals, identified these regions as dark cloud constellations. The area of sky that the Milky Way obscures is called the Zone of Avoidance.
The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light, such as light pollution or moonlight. The sky needs to be darker than about 20.2 magnitude per square arcsecond 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 areas 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 occupies an area of the sky that includes 30 constellations. The Galactic Center 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.
The galactic plane is inclined by about 60° to the ecliptic (the path of the Sun in the sky). It is tilted at an angle of 63° to the celestial equator.
Naked eye viewing Naked eye viewing of the Milky Way as a band across the sky is based on several considerations:
Because the galaxy is structured as a plane, a high density of stars is seen by looking into the plane, toward the Galactic Center. However, the other stars outside the band are also in the Milky Way. The galactic plane is 60 degrees tilted from the ecliptic of the Earth's orbit of the Sun. Thus, the band appears in the night sky at an angle to the horizon. As with all celestial object viewing, the Earth's orbit around the Sun affects the direction of view. The night sky is only facing the plane's center for a portion of the orbit; thus, the seasonality of viewing varies. The band is always there, but for a portion of the year, it is in the direction of the sun (daytime), so its light is washed out. As with all celestial object viewing, the Milky Way appears to rise over the horizon, rotate higher, and then fall over the horizon. This is caused by Earth's rotation, which brings the band into and out of our line of sight.
Viewing the Milky Way band of light and its Galactic Center requires the following conditions:
Eyes adapted to the dark sky (30 minutes) Low light pollution (Bortle 4 or lower) Low or no moonlight (during a 5-day window of a new moon) Low level of haze and humidity, which may not be obvious Correct season for the hemisphere from which you are viewing (May–August is best in North America) Correct time of night; as with other celestial objects, it appears to rise and fall in the night sky. Correct viewing direction: in the Northern Hemisphere, it is in the south, between Sagittarius and Scorpius. In the southern hemisphere, it is near Carina and Centaurus.
Astronomical history
Ancient, naked eye observations Greek philosopher Aristotle (384–322 BC) states in his Meteorologica that his predecessors Anaxagoras (–428 BC) and Democritus (460–370 BC) had held the Milky Way to be 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 with W. D. Ross, ed., The Works of Aristotle ... (Oxford, England: Clarendon Press, 1931), vol. III, Meteorologica, E. W. Webster, trans., Book 1, Part 8, pp. 39–40 : "(2) Anaxagoras, Democritus, and their schools say that the milky way is the light of certain stars ... shaded by the earth from the sun's 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. He said that the milky appearance of the Milky Way Galaxy is due to the refraction of the Earth's atmosphere. The Neoplatonist philosopher Olympiodorus the Younger (–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, which it does not. In his view, the Milky Way is celestial. This idea would be influential later in the Muslim world.