Messier 31 or the Andromeda Galaxy, located in the constellation of Andromeda contains more than one trillion stars. Also visible are Messier 110 above the galaxy, and Messier 32 just below the galaxy disk. Two of the 14 satellite galaxies around Messier 31. Until 1923 it was debated whether such objects were nebula located in our galaxy or separate galaxies like our own Milky Way galaxy. In 1923 Edwin Hubble identified Cepheid variable stars in Messier 31. Cepheid variables have a well defined brightness to variability period that allows astronomers to measure distance to them by knowing their actual brightness. Their distance placed them well outside of our galaxy.
Messier 31 is actually approaching us, and is expected to merge with our galaxy in 4 billion plus years in the future.