The outer disc of Andromeda (above) shows a wide variety of stars, including many Sun-like ones and older variables. Image credit: NASA, ESA and T.M. Brown (STScI), of the stars in Andromeda’s ...

Some of Andromeda's dwarf galaxies are particularly puzzling, though. Dwarf galaxies around the Milky Way, for example, either stopped forming stars billions of years ago, ...