How big is Andromeda Galaxy? | Scale of the Universe

The Andromeda Galaxy, also called Messier 31 or M31, is the nearest large galaxy to the Milky Way, about 2.5 million light-years away. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Andromeda's bright disk of stars is about 140,000 light-years across, and fainter stars spread much farther out.

Size

Andromeda is a spiral galaxy: a flat disk of stars around a central bulge. Seen from Earth, its bright disk stretches about six times as wide as the full Moon. It looks like a long oval because it is seen almost edge-on, tilted 77 degrees.

A galaxy has no sharp edge, so astronomers often measure its width out to where its glow fades to a set level of faintness. At that level, the Third Reference Catalogue of Bright Galaxies gives Andromeda a width of about 190 arcminutes, where an arcminute is a sixtieth of a degree. A 2021 study used Cepheids, stars whose regular pulsing reveals their true brightness, to put Andromeda about 2.48 million light-years away. At that distance, the bright disk is about 140,000 light-years across.

The disk does not stop there. In 2005, astronomers measured the motions of more than 2,800 stars around Andromeda and found a faint, lumpy outer disk. It reaches about 130,000 light-years from the center, with a few of its stars nearly twice as far out.

Counting that faint disk, Andromeda is about 260,000 light-years across. NASA gives a width of about 220,000 light-years, and the European Space Agency about 200,000.

Size comparisons

Andromeda's bright disk is about 1.4 times as wide as the disk of the Milky Way, which is about 100,000 light-years across.
If the Milky Way's disk were shrunk to the width of a basketball, 24 centimeters (9.4 inches), Andromeda's bright disk would be about 33 centimeters (13 inches) across and about 6 meters (20 feet) away.
Virgo A, a giant galaxy that is one of the next smaller objects in the explorer, is about the same width when measured to the same faintness.
The Pinwheel Galaxy, one of the next larger objects in the explorer, is about 1.3 times as wide as Andromeda when measured the same way.

Seeing Andromeda

Andromeda is bright enough to see without a telescope from a dark place. The oldest known record of it is in The Book of Fixed Stars, written by the Persian astronomer Abd al-Rahman al-Sufi in the year 964.

Because it is so close, Andromeda covers too much sky for the Hubble Space Telescope to capture in one view. In January 2025, NASA released a picture of it built from over 600 overlapping Hubble images taken over more than 10 years. It shows about 200 million stars, which NASA notes is still only a fraction of the galaxy's total.

How Andromeda grew

The lumps in Andromeda's faint outer disk are almost certainly the remains of smaller galaxies that Andromeda pulled apart. The 2005 study concluded that much of that outer disk was built this way, from captured stars. NASA also describes Andromeda's past as including mergers with smaller galaxies that orbited it.

Andromeda is now moving closer to the Milky Way. Astronomers have long predicted that the two will collide in about 4 to 5 billion years and merge into a single elliptical galaxy. A 2025 study using measurements from the Hubble and Gaia space telescopes also included the pull of two smaller neighbors, the Triangulum Galaxy and the Large Magellanic Cloud. It found about a 50% chance that the two large galaxies do not merge within the next 10 billion years.