Of all the lights in the heavens, one stands alone. Looking at the night sky, we stare at a family of stars all akin to our own, all a part of the Milky Way Galaxy. However, nestled within the stars of the constellation Andromeda is a faint patch of light from far beyond.
At roughly 2.5 million light years distant, the Andromeda Galaxy, or M31, is one of the most distant objects visible to the unaided eye. But it is also our nearest galactic neighbor, a two-armed spiral similar in makeup to our own Milky Way. Observing M31 from the outside in, astronomers have learned about the makeup and formation of our own galaxy.
Observed since the dawn of humanity, the light we see left M31 while dinosaurs walked the earth and is the collective glow of billions upon billions of stars. The galaxy has been observed since ancient times, but its designation as M31 dates to the 1774 publication of French astronomer Charles Messier’s catalog of the 45 greatest naked-eye marvels of the heavens.
Spanning almost 250,000 light years, M31 is more than twice the size of our Milky Way. And traveling at 200 miles a second, it is on a collision course with our own galaxy in some 2.5 billion years, when the two will merge into one giant elliptical galaxy.
To find this looming menace, look to the northeast for the royal family, King Cepheus, with his papal-crown shape; Queen Cassiopeia, shaped like an M or W depending on the tilt of your head; and their daughter the princess Andromeda. Of Andromeda’s stars, the brightest is the second-magnitude Alpheratz, which joins with three stars of Pegasus to the south to form the Great Square of Pegasus. Higher, marking the jeweled belt at the princess’s waist, shines M31.