How to Find the Galaxy M31
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Andromeda, the Woman in Chains, is an unremarkable fall constellation except as a home to the easiest galaxy to see from Earth.
The constellation Andromeda is simple to find because of the surrounding constellations. The easily visible W shape of Cassiopeia lies just above Andromeda, and the Great Square of Pegasus adjoins Andromeda.
Stars of Andromeda
The brightest star in the Great Square of Pegasus is actually considered part of Andromeda: the magnitude 2.06 star Alpheratz. Alpheratz lies 97 light-years away. Another magnitude 2.06 star, Mirach, lies to the upper left of Alpheratz. Mirach lies 199 light-years away. The third brightest star in Andromeda is magnitude 2.1 Almach, to the upper left of Mirach. Almach lies 354 light-years away.
Finding the Andromeda Galaxy
Although M31, the Andromeda Galaxy, is centrally located in the constellation Andromeda, it is actually easier to find by using the stars of Cassiopeia. The magnitude 3.5 galaxy can be seen as a hazy patch in a dark sky, but for most people binoculars or a telescope is required to find the wide misty patch. Andromeda stretches about three degrees across and one degree wide. It lies at a distance of approximately 2.9 million light-years from Earth, which means that as we view the light from Andromeda, we are seeing it as it was 2.9 million years ago.
To find the Andromeda Galaxy, look for the W shape of Cassiopeia. With the two V shapes that make up the W, use the V shape on the right. Imagine the V as an arrow, pointing into the constellation Andromeda. From the tip of the arrow star to Andromeda is a little over 15 degrees. The arrow of Cassiopeia does not point precisely to Andromeda: You will have to look a bit to the right to find the galaxy M31.
To use a star from Andromeda to find the galaxy, start with the star that is connected to the Great Square of Pegasus. Then look left and find three stars leading away from it: a 3rd magnitude star, Mirach, and Almach. Focus on Mirach, and the look above Mirach. There are two stars in a line above it, the top one dimmer than the bottom. If you can see the highest star without optical aid just to its right for a slightly lighter patch of sky. Put your binoculars or a telescope on it to confirm that you’ve found Andromeda.
Observing M31, the Andromeda Galaxy
M31, at magnitude 3.5, should be easy to see once you have located in your binoculars or telescope. The grayish patch should show an oval shape, characteristic of spiral galaxies. Two other Messier objects are within your field of view when you are looking at Andromeda. M32 and M110 and elliptical galaxies that are companions to the large Andromeda galaxy. They also lie 2.9 million light-years away. M32 is magnitude 8.1 and lies closest to the galaxy, appearing to lie on the larger spiral’s edge. M110 is magnitude 8.5 and lies a bit farther from the large spiral.