Globular Clusters are ball shaped groups of 100,000-1,000,000 (one-hundred thousand - one-million) stars, which are some of the oldest stars in our galaxy (more info about Globular Clusters can be found in The Lives of the Stars slide show). The following two pictures are examples of Globular Clusters:
Globular Clusters are a part of our Milky Way Galaxy, however they
are not found in the flat disk of our Galaxy like the Sun is, instead
they orbit the center of the Galaxy in a spherical "halo".
Since Globular Clusters orbit around the center of our Galaxy, we can
use them to find the center and size of our Galaxy.
Below is a diagram roughly showing what our Milky Way would look like
from above the disk of the Galaxy (a face-on spiral). It would
have a central bulge (which is actually slightly elongated - a
barred spiral), and many spiral arms. The Sun is
located about 2/3rds of the way out from the center in one of the
spiral arms.
Below is a diagram roughly showing what our Milky Way would look like if viewed along the disk of the Galaxy (an edge-on spiral). The Globular Clusters orbit around the center of the Galaxy, thus if we can find the center of all of the Globular Clusters, we have found the center of our Galaxy.
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