The Milky Way, or Galaxy, is the whole concourse of stars and other bodies which can be seen stretched across the heavens. It includes our own sun and its planets, as well as all stars visible to the naked eye. But the name is commonly restricted to the luminous band or belt where most classes of stars are concentrated.
The spiral arms of the Milky Way are rich in hot, bright stars, interstellar clouds of gas (mainly hydrogen) and dust. The first evidence of spiral arms was obtained in 1951 by the American astronomer W.W. Morgan, who identified three.
Our own system of sun and planets appears to be situated towards the inner edge of one of the arms, which is about 1,300 light-years away. The Andromeda nebula, a vast mixture of gaseous and solid matter, is visible as a small luminous patch in our sky. But it is comparable in size to the Milky Way and seems remarkably similar to our own galaxy.
The Palomar telescope, 200 inches in diameter, situated on Mount Palomar in California, has perhaps 1,000 million galaxies within the scope of its vision.
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