The Structure of the Milky Way

Here is a diagram of our galaxy from the side.

The Halo

This is roughly spherical. It is mostly very hot but very diffuse gas. The halo contains objects called globular clusters. These are groups of stars very close together. Globular clusters are amongst the oldest objects in the Universe, being about 10 to 15 billion years old. Most of the mass of the galaxy is contained in the halo, mostly in the form of dark matter. The halo also contains individual stars known as halo field stars.

The Disk

This is the flat rotating Frisbee shaped part of the galaxy. As well as lots of gas and dust the disk contains youngish stars including our sun. The disk contains open clusters. these are groups of quite young stars, The Pleiades being an example.

Bulge

There is a greater concentration of stars here. At its heart is the nucleus where the concentration is greatest of all. This is a region of violent and energetic activity which emits huge amounts of infra red, radio and gamma radiation. It is possible that the nucleus contains a massive black hole.