How to Find the Andromeda Constellation

The night skies of September and October herald the return of the constellation Andromeda. While not the showiest constellation in the sky, Andromeda harbors a fascinating deep-sky object and is the source of intriguing historical tales.

Finding the Andromeda Constellation

To find constellation Andromeda, first look for the W-shaped constellation Cassiopeia in the northern part of the sky. Andromeda is located directly next to Cassiopeia, and is also connected to a boxy shape of stars that make up the constellation Pegasus. Andromeda is visible to all northern hemisphere viewers and many, but not all, viewers south of the equator. 

The History of Andromeda

In ancient Greece and Rome, the stars of Andromeda were seen in combination with the stars of Pisces to form a goddess of fertility. The Arabic astronomers saw "Al Hut" — a fish. In ancient China, stargazers saw various figures of legend in Andromeda's stars, including a famous general and palaces for their emperors. In the south Pacific, where these constellations are low on the horizon, stargazers saw the stars of Andromeda, Cassiopeia, and Triangulum joined together as a porpoise. 

The Brightest Stars of Andromeda

Andromeda Constellation has four bright stars and numerous dimmer stars. The brightest is called α Andromedae, or Alpheratz. Alpheratz is a binary star located less than 100 light-years away from us. It is shared with Pegasus, although it's not formally part of that constellation

The second-brightest star in Andromeda is called Mirach, or β Andromedae. Mirach is a red giant lying about 200 light-years away, located at the foot of a trio of stars that appear to lead to Andromeda's most famous deep-sky object: the Andromeda Galaxy. 

Deep Sky Objects in Constellation Andromeda

The most famous deep sky object in the northern hemisphere sky is the Andromeda Galaxy, also known as M31. This object is a spiral galaxy that lies about 2.5 million light-years away from us. It is heavily populated with up to 400 billion stars and is thought to have two black holes at its heart. 

The Andromeda Galaxy is the most distant object that can be spotted from Earth with the naked eye. To find it, head out to a dark observation location, then locate the star Mirach. From Mirach, trace a line out to the next stars. M31 will look like a faint smudge of light. The best way to look at it is through binoculars or a telescope, you will be able to make out the oval shape of the galaxy. It will appear to be facing you "edge-on."

In the 1920s, the Andromeda Galaxy was known as the Andromeda Nebula, and for a long time, astronomers thought it was a nebula inside our own galaxy. Then, a young astronomer named Edwin Hubble took a look at it through the 2.5-meter Hooker telescope at Mount Wilson in California. He observed Cepheid variable stars in Andromeda and used Henrietta Leavitt's "period-luminosity" relation to determine their distance. It turned out that the distance was too great for the so-called nebula to be in the Milky Way. The stars had to be located in a different galaxy. It was a discovery that changed astronomy. 

More recently, the orbiting Hubble Space Telescope (named in Hubble's honor) has been studying the Andromeda Galaxy, taking detailed images of its billions of stars. Radio astronomers have mapped sources of radio emissions within the galaxy, and it remains an object of intense observation. 

In the far future, the Milky Way and Andromeda galaxies will collide. The collision will form a massive new galaxy that some have dubbed "Milkdromeda."