The issue of the true shape of our Milky Way is an interesting one because of the problem that we live inside of it and cannot step outside to look back upon it and see its true shape. Through a variety of observations of its constituents; stars, gas clouds and other populations of objects, we feel very certain that it is a spiral-type galaxy with a flat shape and a central nuclear 'bulge' containing billions of old stars. The above photo was taken by the COBE satellite at 3.5 microns of the entire sky and shows just how flat the Milky Way is as viewed from the inside.
The Sun is located 24,000 light years from the nuclear region on the inner edge of what seems to be a spiral arm, or fragment of one. It is a very flat system depending one which tracers of its shape you use. If you use molecular clouds, they tend to move in orbits that seldom stray more than 150 light years above or below the major plane of the Milky Way. Since the Milky Way has a diameter of possibly as much as 100,000 light years, this means that the thickness of the Milky Way is 50/100000 or 5/10000 of its maximum size making it flatter than a sheet of writing paper!
Copyright 1997 Dr. Sten Odenwald
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