Our big neighbour

The Andromeda Galaxy: a trillion stars, and heading our way.

What Andromeda is, how big and how far, the October night in 1923 when one star and the word VAR! proved it is a galaxy of its own, what Hubble has seen inside it, its odd family of satellites, whether it will hit us, and how to find it in the sky. Every fact is linked to its source, and there is a quiz at the end.

Start with the basics How to find it

A large oval spiral galaxy glowing white and gold, tilted almost edge on against a black starry sky, with dark dust lanes and two small fuzzy companions — The Andromeda Galaxy through an amateur telescope, with its companions M32 and M110.Picture: Adam Evans, CC BY 2.0, via Wikimedia Commons (cropped and resized)

2.5 millionlight-years away, says NASA NASA, M31

1 trillion+stars, says ESA/Hubble ESA/Hubble, 2015

110 kmcloser to us every second Royal Observatory

VAR!the note that changed astronomy, 1923 NASA, V1

The short version

The Andromeda Galaxy, also called M31, is the nearest major galaxy to our own Milky Way. NASA, M31

It is a spiral galaxy in the constellation Andromeda. NASA, M31

It shares various characteristics with the Milky Way. Britannica

So the light we see left Andromeda 2.5 million years ago: we see it as it was then. EarthSky

Sources don’t always agree on the numbers, such as how far away it is or how wide. Where they differ, we show each figure side by side.

A starry night sky above a large telescope dome, with a faint oval smudge of light among the stars — The Andromeda Galaxy in the sky above one of the four Unit Telescopes of ESO’s Very Large Telescope in Chile.Picture: ESO/B. Tafreshi, CC BY 4.0, via Wikimedia Commons (cropped and resized)

The gap, to scale, using NASA’s widths and distance: the galaxies are tiny compared with the space between them (our drawing).Drawing: our own, simplified and not to scale

Explore

The basicsOur nearest big neighbour.

Hubble and VAR!The star that changed the universe.

Inside Andromeda200 million stars in one picture.

SatellitesA lopsided family, and a weirdo.

The coming meetingHead on, or a miss?

How to see itFollow the W of Cassiopeia.

Tile pictures are credited on their pages and on the credits page.

Try the games

A 12-question quiz, a myth-or-fact challenge, and an approach meter that counts how much closer Andromeda has come while you have been reading.

Play now

More from the family: our own galaxy has its own site, milky-way.galaxies.im, and the black holes, including the one at Andromeda’s heart, live at black-holes.galaxies.im.