Caption

The stellar halo is a common but not well-studied feature of galaxies. This loose collection of stars extends 15 to 20 times beyond the radius of the brightest part of the galaxy, which is what we’re used to seeing in telescope images. The stars comprising a halo are some of the oldest in a galaxy.

The halo of each large galaxy is thought to have been built up through a galactic cannibalism process involving smaller galaxies. What makes galactic halos particularly interesting is the long orbital timescale involved, which means that there is very long persistence of memory of past cannibalism.

One of the few galaxies with a well-studied stellar halo is our neighbor, Andromeda, depicted here in the graphic. The reason Andromeda’s halo can be investigated so thoroughly is simply a matter of distance, both being close enough and bright enough that we can see the full picture with our current class of telescopes.

The stellar halo is illustrated here with exaggerated brightness and density to show how far it extends. In reality, the full stellar halo is too dim and sparse to examine without painstaking hours of observation time on the sharpest resolution telescopes available. When the Nancy Grace Roman Space Telescope launches, it will be able to use its wide field of view to comprehensively image many more stellar halos of more distant galaxies.