How to Find the Andromeda Constellation

Northern hemisphere autumn skies
Look for Andromeda in the northern hemisphere autumn skies looking north.

Carolyn Collins Petersen 

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 constellation Andromeda.
Andromeda constellation is attached to Pegasus the flying horse in the sky. The constellation Pisces lies at her feet. Each one of these constellations has deep-sky objects nearby for observers to search out. Carolyn Collins Petersen 

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

IAU chart showing Andromeda constellation.
The official IAU chart outlines the area that contains Andromeda constellation. It also shows nearby deep-sky objects. IAU/Sky & Telescope 

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."

At 2.5 million light-years, the Andromeda Galaxy is the closest spiral galaxy to the Milky Way. The term "light-year" was invented to handle the immense distances between objects in the universe. Later, "parsec" was developed for truly huge distances. Adam Evans/Wikimedia Commons.

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. 

Andromeda and the Milky Way colliding, as seen from the surface of a planet inside our galaxy.
Andromeda and the Milky Way colliding, as seen from the surface of a planet inside our galaxy. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

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."