Because of Earth’s position in one of the Milky Way’s minor spiral arms, it is difficult to see other portions of the galaxy. We can’t easily observe immigration events of groups of stars, for example, which would provide a record of galactic mergers and other cosmic history. The Milky Way’s nearest neighbor is the Andromeda galaxy. Its proximity to Earth provides telescopes with a relatively unobscured view. To learn more about the immigration history of stars in the Andromeda galaxy, Arjun Dey of NSF’s NOIRLab in Tucson, Arizona, and colleagues analyzed the spectra of some 11 400 stars collected by the Dark Energy Spectroscopic Instrument on the Nicholas U. Mayall 4-Meter Telescope at Kitt Peak National Observatory.
Superposed on this optical image of the sky are the motions of the individual stars in Andromeda. The color coding ranges from red, which indicates those moving away from us, to blue, for...
