This article appeared in the June 2020 issue of Discover magazine as "Welcome to the Neighborhood." Subscribe for more stories like these.
A century ago, most astronomers believed our galaxy was the whole universe. That changed abruptly in 1923, when American astronomer Edwin Hubble discovered Cepheid variable stars in the large “spiral nebula” in the constellation Andromeda. Because the periods of these stars correlate with their luminosities, Hubble could deduce their distances. He concluded that the stars — and the spiral nebula to which they belong — must lie far beyond the Milky Way.
That spiral galaxy, now known as the Andromeda Galaxy (M31), shares many characteristics with our own. Both are barred spiral galaxies with hundreds of billions of stars and disks that span more than 100,000 light-years. And even though astronomers long thought M31 outweighed the Milky Way by two or three times, recent studies hint that the two may have similar masses.
These two giants — cosmic neighbors 2.5 million light-years apart — anchor our Local Group of galaxies. This gravitationally bound collection spans about 10 million light-years and contains more than 54 members. At least, that’s how many astronomers have detected so far. Finding small, dim galaxies taxes even Earth’s biggest telescopes, so quite a few more likely lurk in the shadows of their big brothers. Finding these low-luminosity companions and studying how they interact will help astronomers understand the evolution and ultimate fate of our home galaxy.
