Andromeda: the galaxy coming our way

How to find it: locate the Great Square of Pegasus. From its north-east corner star, Alpheratz, hop two stars along the chain of Andromeda to Mirach, then turn north past two fainter stars. The galaxy shows as a soft oval about the width of your thumb at arm's length — its full extent, visible only in long-exposure photos, is six times the width of the full Moon.

Zoom in to see that the 'cloud' is made of stars, then switch wavelengths. Andromeda looks like a calm spiral in visible light, but in ultraviolet and infrared it turns out to be ringed.

The light reaching your eye from Andromeda tonight set off about 2.5 million years ago. When it left, the first members of our own genus, Homo, were just beginning to make stone tools in East Africa. Andromeda is the most distant object most people can see without a telescope, which makes it the oldest light most people will ever see.

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 record. For the next nine centuries it was simply a nebula, and in 1920 astronomers still debated in public whether such spirals were clouds inside the Milky Way or "island universes" of their own.

The answer came in October 1923. On a photographic plate from the 100-inch Hooker telescope, Edwin Hubble found a Cepheid variable star in Andromeda, crossed out the "N" for nova he'd first written and marked it "VAR!". Cepheids' brightness reveals their distance, and this one was far beyond the edge of the Milky Way. In one plate, the known universe went from one galaxy to countless billions.

Andromeda is one of the few galaxies moving towards us: its light is blueshifted, and it is closing on the Milky Way at roughly 110 km/s. For more than a decade the standard picture, based on Hubble measurements published in 2012, was a near-certain collision in about 4.5 billion years, merging the two into a giant elliptical galaxy nicknamed "Milkomeda".

A study published in Nature Astronomy in June 2025, led by Till Sawala of the University of Helsinki, reran the problem with newer Hubble and Gaia data and 100,000 simulations that included the pull of the Large Magellanic Cloud and Triangulum. It found roughly a 50–50 chance of a merger within the next 10 billion years, and only about a 2% chance of a direct collision in the next 4–5 billion years. The Large Magellanic Cloud tugs the Milky Way slightly off course, making a hit less likely; Triangulum's mass makes it more likely.

Either way, even a full merger would barely touch the stars themselves — the gaps between them are so vast that direct stellar collisions would be vanishingly rare. And the Sun will have grown into a red giant by then, a far bigger concern for Earth.

On 16 January 2025 NASA and ESA released a Hubble mosaic of Andromeda assembled from roughly 600 separate fields and more than 1,000 orbits of observing time, spread over a decade. At about 2.5 billion pixels it is the largest Hubble image ever made, and it separates more than 200 million individual stars — still only a fraction of the galaxy's estimated trillion.

Measuring each star's colour and brightness lets astronomers work out when it formed. The result is a star-formation history of a whole galaxy: Andromeda seems to have had a burst of new stars about 2–4 billion years ago, possibly when it swallowed a sizeable galaxy, an episode our own Milky Way never went through on that scale.

Andromeda is far bigger than its bright disc. Deep surveys found a Giant Stellar Stream in 2001 — a river of stars torn from a galaxy that fell in — and dozens of faint dwarf galaxies orbiting it. In 2020 Hubble's Project AMIGA used the light of background quasars to detect Andromeda's halo of thin hot gas reaching about 1.3 million light-years out, and in places twice that. Since the Milky Way has a halo too, the two galaxies' outer haloes may already be touching.