Why care about stellar halos?
Current theories of galaxy formation contend that larger galaxies are built up through the merging of smaller systems over the lifetime of the universe. Stellar halos are sparse places, with long dynamical times, and the imprints of galaxy collisions (in the form of tidal streams) can be be identified for billions of years after a collision. This makes them ideal starting points for studying the applicability of current models of galaxy formation to the actual physical universe. Why study Andromeda? Andromeda, like our own Milky Way, is a large spiral galaxy. To study our own stellar halo’s global properties, astronomers must contend with our position within the galaxy, which requires disentangling the various constituent parts (thin/thick disk, bulge) and extremely large surveys covering vast portions of the sky. At only 780 kpc distant, Andromeda is far enough away to give us a convenient global view, while close enough that we can take spectra of individual stars. Its disk is also h
