This article is about the galaxy called the Milky Way. For the candy bar of the same name, see Milky Way candy bar.

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, which is also simply called the Galaxy as we are in 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 plane of orbit is highly inclined to the galactic plane. 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 with a total mass of about 10¹² solar masses (M_☉), comprising 200-400 billion stars (see [3]). The galactic disk has a diameter of about 100,000 light-years (see for a list of comparable distances). The distance from the Sun to the galactic center is about 27,700 light-years. The amount of mass interior to the Sun's orbit around the galactic centre is M_☉.

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 closer a star is to the Galaxy's center, the shorter is its orbital period. 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
• 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 [2]). Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with pitch approximately 12 degrees (see [1]).

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 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. Canis Major is a dwarf galaxy, which is being pulled into the Milky Way.

Mythology

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.

References

1. 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
2. Press release Canadian Galactic Plane Survey, http://www.ras.ucalgary.ca/CGPS/press/aas00/pr/pr_14012000/pr_14012000map1.html
3. Sandage, A. & Fouts, G. 1987, AJ, 97, 74

External link:

1. The Milky Way Galaxy, SEDS Messier pages
2. Milky Way spiral gets an extra arm New Scientist.com