Shape and Size
Because we live inside the Milky Way Galaxy, it is hard to know exactly what it looks like.
But astronomers believe the Milky Way Galaxy is a typical spiral galaxy that contains about
100 billion to 400 billion stars. Figure26.17shows what our Galaxy would probably look
like if seen from the outside.
Figure 26.17: This artist’s rendering shows what we currently think the Milky Way Galaxy
would look like seen from above. The Sun and solar system (and you!) are a little more than
halfway out from the center. ( 10 )
Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. The disk
is about 100,000 light-years across and 3,000 light-years thick. Most of the Galaxy’s gas,
dust, young stars, and open clusters are in the disk.
The central bulge is about 12,000 to 16,000 light-years wide and 6,000 to 10,000 light-years
thick. The central bulge contains mostly older stars and globular clusters. Some recent
evidence suggests the bulge might not be spherical, but is instead shaped like a bar. The
bar might be as long as 27,000 light-years long. The disk and bulge are surrounded by a
faint, spherical halo, which also contains old stars and globular clusters. Some astronomers
believe there is a gigantic black hole at the center of the Galaxy.
The Milky Way Galaxy is a big place. If our solar system were the size of your fist, the