The Andromeda Galaxy

The Andromeda Galaxy (also called M31) is the nearest big galaxy to our own Milky Way. It is a giant spiral galaxy about 2.5 million light years away, holding around 1 trillion stars: roughly twice as many as the Milky Way. Andromeda is one of the most distant objects you can see with the naked eye: when you look at it on a dark night you are seeing light that left 2.5 million years ago, before the first humans even existed. Andromeda is also on a slow collision course with us, due to merge with the Milky Way in about 4.5 billion years.

  • Other namesM31, NGC 224Catalogued by Charles Messier in 1764
  • TypeBarred spiralSimilar to the Milky Way
  • Distance2.5 million light yearsThe most distant naked-eye object for most observers
  • Diameterapprox. 220,000 light yearsAbout twice as wide as the Milky Way
  • Number of starsapprox. 1 trillionAround twice as many as the Milky Way
  • Collision with Milky WayIn approx. 4.5 billion yearsStars will not hit, but discs will merge

Our biggest neighbour

Andromeda is the biggest member of our local family of galaxies, the Local Group. It is a giant barred spiral galaxy, similar in shape to the Milky Way but larger. From here we see Andromeda tilted at about a 12 degree angle, so we get a slightly elongated view that shows off its bright bulge and faint outer spiral arms.

Andromeda has its own family of satellite galaxies, including the bright dwarf M32 and the slightly bigger dwarf M110, both visible right next to it in small telescopes. Like the Milky Way, Andromeda has roughly 50 dwarf companions, some still being discovered today.

How to find Andromeda in the night sky

Andromeda is best seen in the autumn from the Northern Hemisphere. Here is one of the easiest ways to find it:

  • Find the Great Square of Pegasus, a large square of four bright stars high in the autumn sky.
  • From the top-left star of the square (called Alpheratz), step left along two pairs of bright stars in the constellation Andromeda.
  • From the second of those stars (Mirach), look up and slightly to the right. You should spot a faint elongated smudge of light.

That smudge is the central bulge of the Andromeda Galaxy, and you are seeing it across 2.5 million light years of empty space. With binoculars you can see Andromeda clearly even from a town garden. Under really dark skies it stretches across several degrees of sky, around six times wider than the Moon.

Edwin Hubble and the discovery that changed astronomy

For most of human history, what we now call the Andromeda Galaxy was thought to be a small cloud of gas inside our own Milky Way, simply because nobody understood what galaxies were. The famous American astronomer Edwin Hubble changed that in 1923. Using the giant 100-inch telescope on Mount Wilson in California, Hubble spotted a pulsating Cepheid variable star inside Andromeda.

By comparing the Cepheid's true brightness with how dim it looked, Hubble could work out how far away it was. The answer (around 900,000 light years at the time, later revised to 2.5 million) was so much further than the size of the Milky Way that there was only one possible conclusion: Andromeda is a separate galaxy of its own, sitting in deep intergalactic space. Hubble had effectively just discovered that the universe was full of galaxies, not just stars. It is one of the biggest single discoveries in the history of science.

Fact The light from Andromeda you see tonight set out around 2.5 million years ago. At that time, our human ancestor Homo habilis was just starting to use stone tools. Modern humans had not evolved yet. By the time the next photons leave Andromeda this evening and reach Earth, humans 2.5 million years from now (if any are still here) will see Andromeda exactly the way we do now: it has not changed enough in a few million years to look different to the naked eye.

The coming collision

Andromeda is heading towards the Milky Way at about 110 km/s. In roughly 4.5 billion years from now, the two galaxies will begin to merge. The merger itself will take another billion years or so to complete, and the result will be a single, much larger elliptical galaxy that astronomers have already nicknamed "Milkomeda".

Importantly, individual stars will almost never collide. Galaxies are mostly empty space, so even when two of them combine, the chance of any two stars actually hitting each other is virtually zero. What does change is the night sky: stars will be flung onto wild new orbits, the familiar spiral pattern of the Milky Way will be destroyed, and any astronomers around at the time will see a giant blob of stars instead of a band of light across the sky.

Did you know? Until the early 2000s, astronomers thought the Milky Way was bigger than Andromeda. Then better measurements showed Andromeda has more dark matter and more stars (around 1 trillion vs the Milky Way's 100 to 400 billion). On the other hand, the Milky Way appears to be more compact and may actually have more total mass once dark matter is counted. The size question is still being argued.
Deeper dive: how Hubble's discovery ended the "Great Debate"

For most of the 1800s and early 1900s, astronomers had been arguing about what the faint fuzzy patches called "spiral nebulae" actually were. By 1920, this had grown into a famous public argument called the Great Debate, between two leading American astronomers: Harlow Shapley (who thought the spiral nebulae were small clouds inside our own Milky Way) and Heber Curtis (who thought they were separate "island universes" far away from us).

The debate was held at the Smithsonian in Washington, DC, in April 1920. Both sides made good points and the audience went home undecided. Three years later, Edwin Hubble settled the matter for good. By spotting a known type of pulsating star (a Cepheid variable) in the Andromeda "nebula", he could prove it was around a million light years away, far outside the Milky Way. Curtis had been right all along. The spiral nebulae were entire other galaxies.

Hubble's discovery is one of the great paradigm shifts in science. Before 1923, the universe was a single galaxy of stars. After 1923, our galaxy is just one of billions of other galaxies, scattered through a universe so much vaster than anyone had imagined that it took decades for science to catch up. Within a few more years Hubble had also discovered that the universe is expanding, leading directly to the modern Big Bang theory.

For our home galaxy, see the Milky Way. For the shapes, see spiral galaxies and elliptical galaxies. Or start with the basics: what is a galaxy.