All spiral galaxies have a disk, with light enhanced along spiral arms.
Most also have spheroids, although in some galaxies they are rather small. The inner part of the spheroid, where it merges with the disk, is often called the bulge . The outer part is often called the stellar halo . The stellar population of the bulge is intermediate between that of the disk and the halo. If an object is held in a circular orbit by gravity, we can determine the mass required to keep it in that orbit provided we know its speed and orbital radius.