Our neighbours: Andromeda and the Local Group

The light from the Andromeda galaxy left it about 2.5 million years ago.

A small group of galaxies

Galaxies are not scattered evenly. NASA explains that they gather into groups of about 100 or fewer members held together by their own gravity, while larger clusters may contain thousands. The Milky Way belongs to the Local Group, which NASA's galaxies page says holds more than 50 galaxies; an older NASA Hubble page says more than 20, a reminder that the count keeps rising as faint dwarf galaxies are found. Most members are dwarfs with up to a few billion stars. Only a few are large, and the biggest is the Andromeda galaxy, our nearest large neighbour.

Zoom out further and the Local Group sits just off the edge of the Virgo cluster and is part of a much larger structure, the Laniakea supercluster. NASA notes that superclusters are not held together by gravity as a whole. Together with empty voids and long walls of galaxies they make up the cosmic web, the largest pattern of matter in the universe.

The Magellanic Clouds

From the Southern Hemisphere two fuzzy patches are visible to the naked eye: the Large and Small Magellanic Clouds, dwarf galaxies that orbit the Milky Way. JPL gives the Large Magellanic Cloud a distance of about 160,000 light-years and a width about one third of the Milky Way's; if it could be seen in full it would cover as much sky as about 480 full Moons. It is full of star-forming regions, including the Tarantula Nebula. These two clouds also changed astronomy: studying variable stars there, Henrietta Swan Leavitt found that a Cepheid's pulsing period is tied to its true brightness, which made Cepheids a way to measure distances.

How Andromeda became a galaxy

NASA gives Andromeda's distance as about 2.5 million light-years, or 0.8 megaparsecs. That makes it about 25 Milky Way diameters away, and it is the most distant object most people can see with the naked eye. Its light left it about 2.5 million years ago. Like the Milky Way it is a barred spiral, and it is somewhat larger.

Will we collide?

Andromeda is moving towards us. In 2012 an analysis of Hubble observations by Roeland van der Marel and Tony Sohn suggested a direct hit within about 5 billion years. In 2025 a team led by Till Sawala combined newer Hubble data with ESA's Gaia mission, tested 22 uncertain quantities, and ran 100,000 simulations. They found only about a 50% chance that the two galaxies merge within the next 10 billion years, and about a 2% chance of a head-on collision in just 4 to 5 billion years. The Large Magellanic Cloud pulls the Milky Way off course, making a merger a little less likely, while Andromeda's companion M33 pulls the other way. As ESA points out, the Sun will likely burn out in about 5 billion years anyway.