Galaxies Photo Gallery
A galaxy is an enormous system of stars, gas and dust that is gravitationally bound and rotating about a common center. Galaxies come in a number of types including spiral, barred spiral, elliptical, lenticular and irregular (see Hubble Classification of Galaxies). The number of stars in a galaxy can range from ten million in a dwarf to a hundred trillion in a giant.
The Sun is just one of 100 to 400 billion stars in the Milky Way Galaxy. A classic spiral galaxy, the Milky Way is a flat disk of stars that rotates once about its axis in 250 million years. It is approximately 100,000 light-years in diameter, and 1,000 light-years thick. The Milky Way is part of the Local Group of some 30 galaxies including the Andromeda Galaxy (M31) and the Triangulum Galaxy (M33). The observable universe contains an extimated 200 billion galaxies.
The 18th-century French astronomer Charles Messier recorded 40 galaxies (24 spiral, 8 elliptical, 4 barred, and 4 lenticular) in his Messier Catalog of deep sky objects. See Galaxy (Wikipedia) for more information.
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