A galactic halo is a large, relatively dust-free, spherical region surrounding a spiral galaxy, such as our own. The inner, visible part of the halo, which has roughly the same diameter as the galactic disk, is occupied by Population II objects, including globular clusters and old, individual stars. Beyond this is a much larger region, called the dark halo or extended halo.

 

Dark halo

 

Artwork showing the approximate extent of the dark matter halo around a large spiral galaxy such as our own. Credit: Jose Wudka.

 

The dark halo of a galaxy, such as the Milky Way, contains large amounts of dark matter. Although it emits no light or other forms electromagnetic radiation the presence of the dark halo s revealed by its gravitational effect on the galaxy's rotation. The nature of galactic dark matter is still undetermined.

 

In the case of the Milky Way system, the dark halo outweighs the rest of the Galaxy by a factor of approximately 20 to 1. Similar halos of nonluminous matter appear to surround and envelope other large galaxies.