Galaxies - Their are number of nebulae that have been identified as extra galactic,
it has been discovered that these small cloudy patches are groups of stars, in many ways resembling the group of stars of which the sun is a part. Each of these stars constitutes an island universe as separated from others as individual stars in one group,this group is called a galaxy. In a galaxy the stars tend to be in groups,these are called star clouds, arranged in long spiral arms. The spiral nature is due to the revolution of the stars about the center of the galaxy. This is due to the inner ones revolving more rapidly than the outer ones.
Galaxies which have been discovered seem to congregate in groups, similar to stars in a galaxy. The average estimate of the size of a galaxy is that it is about 100,000 light years in diameter, 15,000 light years thick at the center, and 5,000 light years thick near the outer edge, and that it contains 100,000,000,000 stars.
