By measuring the speeds of stars or gas clouds in the other part of the disk of the Galaxy, we can measure how much mass is in the halo. The outer part of the halo cannot be made of main sequence stars, red giants, or supergiants because these emit too much light to account for most of the mass. The center of our galaxy is best observed at infrared and radio wavelengths, since these pass through the dust in the galactic disk. We also observe some gamma-ray radiation from an object located at the very center of the galaxy. A globular star cluster is a subunit of our galaxy which contains up to a few million stars in a region just a few of light years across.
Globular clusters contain only old stars.
Each star in a cluster follows an orbit about the cluster center. The orbits have a random distribution of shapes, planes, and directions-much like the stars in the spheroid of the Galaxy. Close interactions between stars in a globular cluster can kick some stars into the core and others into the halo. This will lead to an increasing density of stars in the core. The interactions can also produce close binary stars.