Andromeda Galaxy: Complete guide and how to see it

The Andromeda Galaxy is the closest major galaxy to our home galaxy the Milky Way and is located over 2 million lightyears from Earth.

It is a spiral galaxy found in the Andromeda constellation and is about 200,000 lightyears wide. It is also known as M31, part of the deep-sky Messier Catalogue.

The Andromeda Galaxy is notable for its bright blue tinge, which is caused by pockets of hot young stars burning bright blue.

How to find the Andromeda Galaxy

The Andromeda Galaxy can be seen in the night sky in the northern hemisphere throughout much of the year, but is best seen in autumn.

What's more, the Andromeda Galaxy can be seen with the naked eye under good conditions, making it the furthest object than can be seen with the human eye.

Use Cassiopeia to find the Andromeda Galaxy

The W-shaped constellation of Cassiopeia is a great tool for locating the Andromeda Galaxy.

This technique of hopping from star to star to help you locate a difficult target is known, unsurprisingly, as star-hopping.

From the UK, Cassiopeia appears circumpolar, meaning it never sets.

Its shape is distinctive and its stars are easy to see, even from a suburban city location.

In the winter months, Cassiopeia can be seen in the north-west. Its W shape makes it easy to find, but you can use a star chart or stargazing or astronomy app to help you.

You can use Cassiopeia to help you locate the Andromeda Galaxy using these steps:

1. Locate Cassiopeia in the night sky
2. At the bottom of the deeper of the two Vs of Cassiopeia is star Schedar
3. Treat this V like an arrow, which points to the Andromeda Galaxy
4. You'll see the Andromeda Galaxy as a faint smudge with the naked eye

Use the Great Square of Pegasus to find the Andromeda Galaxy

Another star-hop you can do to find the Andromeda Galaxy involves the asterism known as the Great Square of Pegasus.

1. Imagine a diagonal from the southwest to northeast corner of the Square
2. Extend it for about the same distance again to reach star Mirach
3. Turn 90º to head northwest for a short distance towards dimmer Mu (m) Andromedae
4. Continue on to Nu (n) Andromedae

For more on how to find it for yourself, read our guide on how to see the Andromeda Galaxy.

The Andromeda Galaxy and the Milky Way are gravitationally interacting, and astronomers have calculated that both galaxies will collide in about 4.5 billion years time. This event is known as the Andromeda-Milky Way collision.

The Great Debate

Observations of the Andromeda Galaxy have led to some of the most important discoveries in the Universe: particularly its part in the Great Debate, in which astronomers Harlow Shapley and Heber Curtis debated about whether other galaxies existed beyond our own.

The matter was settled largely because of the work of Edwin Hubble, after whom the Hubble Space Telescope was named.

Hubble was able to photograph individual stars in the 'Andromeda Nebula' (as it was then known) and in 1923, his observations of Cepheid variable stars led him to conclude that the Andromeda Galaxy must be located beyond our own Galaxy.

Life in the Andromeda Galaxy?

We know there is life in the Milky Way galaxy, in the shape of us!

But what about our nearest major galaxy, the Andromeda Galaxy?

Since life has not yet been confirmed beyond Earth, astronomers cannot say for sure whether there exists life elsewhere in the cosmos, the Andromeda Galaxy included.

But since the 1990s, the field of exoplanet science - the study of planets beyond our Solar System - has exploded, to the point that now astronomers calculate there is on average at least one planet orbiting every star we can see in the night sky.

And considering the Andromeda Galaxy is thought to contain about 1 trillion stars, that means at least 1 trillion planets, some of which will be orbiting close enough to their stars for liquid water to form on the surface.

Since liquid water is a key condition for life as we know it here on Earth, it is an important part of the search for life beyond our planet.

For all we know, galaxies like the Andromeda Galaxy could be teeming with habitable worlds, some of which may even host intelligent life.

But until definitively detected, for now the question as to whether life exists in the Andromeda Galaxy remains unanswered.

Hubble image

In 2015, NASA released an amazing image showing part of the Andromeda Galaxy, captured by the Hubble Space Telescope.

The image is the sharpest and largest ever taken of the galaxy.

The cropped version released by NASA contains 1.5 billion pixels, and over 600HD TV screens would be required to display the whole image.

The Andromeda Galaxy image is the biggest Hubble image ever released and shows over 100 mill stars and thousands of star clusters.

Pictures of the Andromeda Galaxy

Below is a selection of images of the Andromeda Galaxy captured by astrophotographers and BBC Sky at Night Magazine readers.

If you would like to capture images like these, read our guide to deep-sky astrophotography or our tutorial on how to photograph the Andromeda Galaxy.

If you do manage to photograph the Andromeda Galaxy, don't forget to send us your images or share them with us on Facebook, Twitter and Instagram.