How far away is the Andromeda Galaxy?

About 2.5 million light-years, or roughly 24 million trillion km. That makes it the nearest large spiral galaxy to the Milky Way.

Can you see Andromeda with the naked eye?

Yes, from a dark site. It appears as a faint, fuzzy oval in the constellation Andromeda and is easiest to find in autumn from the Northern Hemisphere.

When will Andromeda collide with the Milky Way?

It might not. Older forecasts gave about 4 to 5 billion years, but a 2025 study found only about a 50% chance of a merger in the next 10 billion years. Even if it happens, individual stars would almost never hit each other.

How many stars are in the Andromeda Galaxy?

Roughly a trillion, probably more than twice as many as the Milky Way. The figure is an estimate based on the galaxy's brightness and mass.

Who discovered that Andromeda is a galaxy?

Edwin Hubble, who found Cepheid variable stars in it in 1923 and used them to measure its distance. His results, presented in 1925, showed it lies far outside the Milky Way.

Our nearest large neighbour, a trillion-star spiral

Andromeda Galaxy

Spiral galaxy (M31) · ≈ 2.5 million ly

The Andromeda Galaxy, also known as Messier 31 or M31, is the nearest large galaxy to the Milky Way. It lies about 2.5 million light-years away in the constellation Andromeda, and it is the most distant object most people can see with the naked eye, appearing as a faint, elongated smudge on dark autumn nights. It is a spiral galaxy somewhat larger than our own, holding perhaps a trillion stars.

Andromeda played a central part in one of the biggest discoveries in astronomy. In the 1920s Edwin Hubble used stars within it to show that it lies far outside the Milky Way, proving that the universe contains other galaxies. Today it is approaching us, and the two galaxies may one day merge.

Key facts

Distance: ≈ 2.5 million light-yearsAbout 765 kiloparsecs
Type: Spiral galaxyClassified SA(s)b
Diameter: ≈ 150,000–220,000 light-yearsDepends on how far out the faint disc is traced
Number of stars: ≈ 1 trillion
Total mass: ≈ 1–1.5 trillion M☉Similar to the Milky Way; mostly dark matter
Apparent magnitude: ≈ 3.4Visible to the naked eye under dark skies
Size on the sky: ≈ 3°About six full Moons side by side
Approach speed: ≈ 110 km/sTowards the Milky Way
Constellation: Andromeda

A spiral like our own

Andromeda is a large spiral galaxy with a bright central bulge, a broad disc of stars and spiral arms. We see it tilted about 77 degrees from face-on, which is why it looks like a long oval rather than a pinwheel. In infrared light, its dust forms a prominent ring roughly 30,000 light-years from the centre, where most of its star formation is happening.

In many ways it is a sibling of the Milky Way. The two galaxies have broadly similar total masses, though Andromeda appears to have more stars and a wider disc. It forms new stars more slowly than our galaxy, and its population is on average somewhat older.

The central black hole

Andromeda's core hosts a supermassive black hole of roughly 100 million solar masses, about 30 times heavier than Sagittarius A*. The Hubble Space Telescope revealed in the 1990s that the core appears double, an effect astronomers explain as an elongated ring of old stars orbiting the black hole.

From 'little cloud' to island universe

The Persian astronomer Abd al-Rahman al-Sufi described Andromeda as a 'small cloud' in his Book of Fixed Stars around 964 CE, the earliest known written record. Charles Messier added it to his catalogue in 1764 as object 31. For centuries it was called the Andromeda Nebula, and many astronomers believed it was a cloud of gas within the Milky Way, perhaps a forming solar system.

In 1920 astronomers argued in public over whether spiral nebulae were nearby clouds or distant 'island universes'. The question was settled by Edwin Hubble using the 2.5-metre Hooker Telescope at Mount Wilson. In October 1923 he identified a Cepheid variable star in Andromeda. Because a Cepheid's pulsation period reveals its true brightness, he could calculate its distance, which placed it far beyond the edge of the Milky Way. His results were presented publicly at the start of 1925.

A distance revised

Hubble's first estimate was about 900,000 light-years, less than half the modern figure. In the 1950s Walter Baade found that there are two types of Cepheid with different brightnesses, which roughly doubled the distance. The modern value of about 2.5 million light-years comes from several independent methods.

Satellites and a violent past

Like the Milky Way, Andromeda is surrounded by satellite galaxies, including the bright compact elliptical M32 and the larger M110, both visible in small telescopes, along with dozens of fainter dwarfs. A long, faint band called the Giant Stellar Stream loops through its halo, made of stars torn from a smaller galaxy that fell in.

Several lines of evidence suggest Andromeda absorbed a fairly large galaxy roughly 2 to 3 billion years ago. That event may have thickened its disc and triggered a burst of star formation. Andromeda, the Milky Way and the Triangulum Galaxy are the three largest members of the Local Group.

Will Andromeda collide with the Milky Way?

Andromeda is moving towards the Milky Way at about 110 km/s. Measurements of its sideways motion by the Hubble Space Telescope, published in 2012, suggested a nearly head-on collision in about 4 to 5 billion years, eventually merging the two spirals into one large elliptical galaxy. The prediction became one of the best-known forecasts in astronomy.

A 2025 study combining Hubble and Gaia data, and including the pull of the Large Magellanic Cloud and the Triangulum Galaxy, found the outcome is far less certain. It estimated roughly a 50% chance of a merger in the next 10 billion years and only a few per cent chance within the next 5 billion years. If the galaxies do merge, stars are so far apart that hardly any would collide.

Seeing Andromeda for yourself

In the Northern Hemisphere, Andromeda is best seen in autumn, away from city lights. To the naked eye it looks like a faint oval glow, really just the bright central bulge. Binoculars show it stretching further, and long-exposure photographs reveal a disc several times wider than the full Moon.

The light you see left Andromeda about 2.5 million years ago, around the time the earliest members of our own genus were appearing in Africa. In 2025 NASA released a Hubble mosaic of the galaxy made from hundreds of images and showing around 200 million individual stars.

Surprising facts about Andromeda Galaxy

Andromeda is the most distant object that most people can see without a telescope.
On the sky it is about six times wider than the full Moon, but it is too faint for its full extent to be seen without long exposures.
Its central bulge appears to have two nuclei, although only one contains the black hole.
In 1885 a supernova appeared in Andromeda, the first ever seen in another galaxy, though nobody realised at the time that it was outside the Milky Way.
Andromeda is moving towards us even though almost every other galaxy in the universe is moving away.

Frequently asked questions

How far away is the Andromeda Galaxy?: About 2.5 million light-years, or roughly 24 million trillion km. That makes it the nearest large spiral galaxy to the Milky Way.
Can you see Andromeda with the naked eye?: Yes, from a dark site. It appears as a faint, fuzzy oval in the constellation Andromeda and is easiest to find in autumn from the Northern Hemisphere.
When will Andromeda collide with the Milky Way?: It might not. Older forecasts gave about 4 to 5 billion years, but a 2025 study found only about a 50% chance of a merger in the next 10 billion years. Even if it happens, individual stars would almost never hit each other.
How many stars are in the Andromeda Galaxy?: Roughly a trillion, probably more than twice as many as the Milky Way. The figure is an estimate based on the galaxy's brightness and mass.
Who discovered that Andromeda is a galaxy?: Edwin Hubble, who found Cepheid variable stars in it in 1923 and used them to measure its distance. His results, presented in 1925, showed it lies far outside the Milky Way.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources