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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're 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 approximate shape of the Milky Way Galaxy and the position within it of our solar system.
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 ). 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 which start at the Galaxy's center. The distance between our spiral 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.
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.
X-ray image of Milky Way taken by Chandra X-ray Observatory
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