- This article is about the galaxy called the Milky Way. For the candy bar of the same name, see Milky Way (candy).
The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia (gala, galactos means "milk")) is 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 Sun's equator and the ecliptic (the plane in which the Earth and the other significant planets orbit the Sun). The galactic pole lies at right ascension 12 h 51,42 m, declination 27° 7,8' (epoch 2000.0; this is a conventional value adopted by the IAU in 1959). 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.
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 agreed 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, 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  . Hence the orbital period is approximatedly 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.
There are believed to be four major spiral arms and at least two smaller ones which all start at the Galaxy's center. These are named as follows, counting outwards from the centre along a radius through our solar system:
- Norma Arm, or 3 kpc Arm
- Scutum-Crux Arm, or Centaurus Arm
- Sagittarius Arm, or Sagittarius-Carina Arm
- Orion Arm, or Local Arm - labeled "0"; a minor spiral arm. The Earth's solar system may be found close to the inner rim of the Local Arm, in the Local Bubble, 8.0 ± 0.5 kpc from the galactic center.
- Perseus Arm
- Cygnus Arm, or Outer Arm
The distance between the local arm and the next arm out, the Perseus arm, is about 6,500 light-years (see ). Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with pitch approximately 12 degrees (see ).
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.
X-ray image of Milky Way taken by Chandra X-ray Observatory
The galactic neighborhood
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, Censored page, 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 Hercules 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.
- J. P. Valleé: "The Milky Way's Spiral Arms traced by Magnetic Fields, Dust, Gas and Stars", The Astrophysical Journal, volume 454 (1995), pp.: 119-124, 1995. Available online through NASA's Astrophysics Data System http://adswww.harvard.edu
- Press release Canadian Galactic Plane Survey, http://www.ras.ucalgary.ca/CGPS/press/aas00/pr/pr_14012000/pr_14012000map1.html
- Sandage, A. & Fouts, G. 1987, AJ, 97, 74
- The Milky Way Galaxy , SEDS Messier pages
- Milky Way spiral gets an extra arm New Scientist.com