The Structure of the Milky Way Galaxy

The three main parts of the structure of the Milky Way galaxy are the bulge, the disk, and the halo. Above is an edge-on diagram of the Milky Way with each of these noted.

The Disk

The disk is the flat rotating frisbee shaped part of the galaxy. Almost all of the current star formation in the Milky Way takes place in the spiral arms of the disk within a few hundred light-years of the plane of the galaxy. The disk has a more sparsely populated component known as the thick disk, which extends farther from the plane of the galaxy (about 1000 light-years) than the rest of the disk.

The Bulge

The bulge is the dense roughly spherical concentration of stars at the center of disk. The center of the galaxy lies in the bulge, roughly 24,000 light-years from the sun.

The Halo

The halo is the sparsely populated extended region that encompasses the disk and bulge of the galaxy. The brightest members of the halo are large star clusters, known as globular clusters. Individual stars, known as halo field stars, also occupy this region of space.

Sample Volume

This is a rough representation of the region that our sample of RR Lyraes is contained in. This shows that our sample of RR Lyraes should contain members of the disk, thick disk, and halo populations.