The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kuklos (meaning milky way)) is our native galaxy. It appears as a hazy band of white light across the celestial sphere, formed by stars within the disc of its namesake galaxy. It is also simply known as the Galaxy, as our Solar System is a part of it. The Milky Way appears brightest in the direction of Sagittarius, where the galactic center lies.
Relative to the celestial equator, the Milky Way passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux. This reflects the fact that the Earth's equatorial plane is highly inclined to the galactic plane, as is the ecliptic (the plane in which the Earth and the other significant planets orbit the Sun). The fact that the Milky Way divides our night sky into two roughly equal hemispheres reflects the fact that the solar system lies close to the galactic plane.
In 2004, a team of astronomers estimated the age of the Galaxy. (The team consisted of Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton.) The team used the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the Beryllium content in two stars in Globular Cluster NGC 6397. This allowed them to deduce the time elapsed between the rise of first generation of stars in the entire Galaxy and the first generation of stars in the Globular Cluster, at 200 million to 300 million years. They added that estimate to an estimated age of the stars in the Globular Cluster: 13,400 ± 800 million years. The sum is their estimated age of the Milky Way Galaxy: 13,600 ± 800 million years.
The Milky Way galaxy is a large spiral galaxy of Hubble type SBbc (loosely wound barred spiral) with a total mass of about 1012 solar masses (M☉), comprising 200-400 billion stars (see ). The galactic disk has a diameter of about 100,000 light-years (see 1 E20 m for a list of comparable distances). The distance from the Sun to the galactic center is about 27,700 light-years.
The stars in the Galaxy's disk rotate around the Galaxy's center, which is suspected to harbour a supermassive black hole. Sagittarius A* is thought to be the most plausible candidate for the location of this supermassive black hole. It takes the solar system about 226 million years to complete one orbit ("galactic year"), and so has completed about 25 orbits during its lifetime. The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days. The orbital speed of stars in the Milky Way does not depend much on the distance to the center: it is always between 200 and 250 km/s for the Sun's neighbours . Hence the orbital period is approximately proportional to the distance from the star to the Galaxy's center (without the power 1.5 which applies in the case of a central mass). The disk has a bulge at the center.
Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees (see ). There are believed to be four major spiral arms and which all start at the Galaxy's center. These are named as follows, according to the image at right:
There are at least two smaller arms or spurs, one of which is:
The Earth's solar system may be found close to the inner rim of this Arm, in the Local Fluff, 8.0±0.5 kpc from the galactic center. The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years (see ).
Outside of these is the Outer Ring or Monoceros Ring , a proposed ring of stars around the Milky Way by astronomers Brian Yanny and Heidi Jo Newberg . This ring conists of gas and stars torn from other galaxies as they merged with our own billions of years ago.
The disk is surrounded by a spheroid halo of old stars and globular clusters.
While the disk contains gas and dust obscuring the view in some wavelengths, the halo does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk.
The galactic neighborhood
The Milky Way, the Andromeda Galaxy and the Triangulum Galaxy are the major members of the Local Group, a group of some 35 closely bound galaxies; The Local Group is part of the Virgo Supercluster.
The Milky Way is orbited by a number of dwarf galaxies in the Local Group. The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light years. The smallest, Carina Dwarf , Draco Dwarf , and Leo II are only 500 light years in diameter. The other dwarfs orbiting our galaxy are the Small Magellanic Cloud; Canis Major Dwarf, the closest; Sagittarius Dwarf Elliptical Galaxy, previously thought to be the closest; Ursa Minor Dwarf ; Sculptor Dwarf, Sextans Dwarf , Fornax Dwarf , and Leo I.
There are numerous legends in many traditions around the world regarding the creation of the Milky Way. In particular, there are two similar ancient Greek stories, that explain the etymology of the name Galaxias (Γαλαξίας) and its association with milk (γάλα). One legend describes the Milky Way as a smear of milk, created when the baby Herakles suckled from the goddess Hera. When Hera realized that the suckling infant was not her own but the illegitimate son of Zeus and another woman, she pushed it away and the spurting milk became the Milky Way.
Another story tells that the milk came from the goddess Rhea, the wife of Cronus, and the suckling infant was Zeus himself. Cronus swallowed his children to ensure his position as head of the Pantheon and sky god, and so Rhea conceived a plan to save her newborn son Zeus: She wrapped a stone in infant's clothes and gave it to Cronus to swallow. Cronus asked her to nurse the child once more before he swallowed it, and the milk that spurted when she pressed her nipple against the rock eventually became the Milky Way.
Older mythology associates the constellation with a herd of dairy cows/cattle, whose milk gives the blue glow, and where each cow is a star. As such, it is intimately associated with legends concerning the constellation of Gemini, which it is in contact with. Firstly, with Gemini, it may form the origin of the myth of Castor and Polydeuces, concerning cattle raiding. Secondly, again with Gemini, but also with other features of the Zodiac sign of Gemini (i.e. Canis Major, Orion, Auriga, and the deserted area now regarded as Camelopardalis), it may form the origin of the myth of the Cattle of Geryon, one of The Twelve Labours of Herakles.
Peoples in Eastern Asia believed that the hazy band of stars were "Silvery River" of the Heaven. Also, Altair and Vega were thought to be lovers, who were bound not to meet each other but on the seventh day of the seventh month, Qi Xi(Tanabata in Japan and Chilseok in Korea), when the magpies form the bridge over the galactic river.
- J. P. Vallée, "The Milky Way's Spiral Arms traced by Magnetic Fields, Dust, Gas and Stars", The Astrophysical Journal, volume 454, pp. 119-124, 1995. Available online through NASA's Astrophysics Data System
- Press release, Canadian Galactic Plane Survey
- Press release, European Southern Observatory
- Sandage, A. & Fouts, G., The Astrophysical Journal, volume 97, p. 74, 1987
- The Milky Way Galaxy, SEDS Messier pages
- Milky Way spiral gets an extra arm New Scientist.com
- MultiWavelength Milky Way NASA site with images and VRML models
- An Atlas of the Universe
- Proposed Ring around the Milky Way