The Milky Way is the name given to the luminous belt of stars which encircles the heaven, nearly in great circle. The great circle of the galaxy crosses the celestial equator at an angle of about 62 degrees.
Its diameter from edge to edge is computed between 200,000 to 300,000 light years. The galactic plane is the plane of the great circle passing as nearly as possible through the center of this belt. Both the width and the brightness of the Milky Way vary greatly from one point to another.
The Milky Way is composed of a great number of faint stars which are not individually visible to the naked eye. It is an irregular structure and consists of numerous groups of stars called stars-clouds. The brightness region containing the densest constellation of stars is in the southern sky.
The star clouds are all very remote, some of them are at distances as great as 30,000 light years; other star clouds which are hidden from view by nearer clouds are at presumably still more distant.
The average brightness of the Milky Way is about twice that of the remaining portion of the sky. On account of the presence of faint permanent aurora and to the zodiacal light, the sky on a moonless night is not dark. If the sources of general illumination were absent, the Milky Way would appear much brighter.
It has been established that the galaxy is rotating. The time of one rotation, in the neighborhood of the sun, is calculated to be about 225 million years.
Milky way and other galaxies
The Milky Way is merely one of many other galaxies. They are often termed as spiral nebulae, because when seen, broadside-on, these galaxies show a typical, spiral structure. The great Nebula is Andromeda. It is visible to the naked eye as a faint, diffuse patch of light.
It contains numerous star clouds, bright nebulae. Like our milky way it is slowly rotating. The distance of great nebulae in Andromeda was found to be 753,000 light years, i.e., it is for outside of our galaxy.