Small galaxies may contain fewer than a million stars while very large galaxies may contain more than one trillion stars. Galaxies can be up to 100,000 light-years in diameter. Elliptical galaxies show very little structure and are flat to spherical in shape. Spiral galaxies are flat and generally have a bright nucleus, and arms that come out of the nucleus and form a giant pinwheel shape. Barred spiral galaxies are also flat and have arms that extend sideways in a short straight line before curving into a spiral shape.
Irregular galaxies are usually small and have no particular shape.
The Milky Way is our Galaxy. Some astronomers estimate that the Milky Way has more than 100 billion stars. The Milky Way is a member of a group of about 20 galaxies know as the Local Group. The stars that occur in the Milky Way are classified as Population I stars or Population II stars. Population I stars occur primarily in the spiral arms of the galaxy and include stars ranging in age from a few hundred thousand to 10 billion years old.