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 LatinVia Lactea, in turn derived from the GreekGalaxia (gala, galactos means "milk") is a hazy band of whitelight 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 centre 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 axis of rotation is highly inclined to the normal 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 with a total mass about a trillion times that of the Sun, including 200-400 billion stars (see [3]).
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 amount of mass inside Sun's orbit around the galactic centre is 9.0 × 1010 MO.
The stars in the Galaxy's disk rotate around the Galaxy's center, which is suspected to harbor a black hole.
It takes the solar system about 226 million years to complete one orbit.
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
Scutum-Crux arm
Sagittarius arm, or Sagittarius-Carina arm
Orion arm, or Local arm
Perseus arm
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.
