WHAT WE KNOW ABOUT ANDROMEDA

A cloud of stars

In 1864, photographic pioneer William Huggins collected light from Andromeda and other nebulae and split it into rainbow-like spectra.

This showed that Messier 31 was a concentrated group of stars rather than a cloud of interstellar gas. William Huggins’ private observatory, Tulse Hill, London

A spiral ‘nebula’

Hubble took its catalogue number, Messier 31, from French astronomer Charles Messier, who listed star clusters and cloud-like nebulae in his catalogue of 1771. In 1850, astronomer William Parsons used his giant telescope to sketch Andromeda’s shape. The Great Leviathan telescope at Birr Castle, Ireland

Approaching Earth

The compression of Andromeda’s starlight confirmed the galaxy’s motion towards us as early as 1912, even before it was known to be an independent galaxy. The Clark Refractor at Lowell Observatory, Arizona

Double core

In 1993, astronomers discovered that Andromeda’s central core has two distinct concentrations of stars. The odd appearance is now thought to be a result of stars jostling together in certain regions as they orbit the galaxy’s central black hole.

Hubble Space Telescope

Bound for collision

In 2012, astronomers announced the results of a decade-long effort to measure Andromeda’s lateral (sideways) motion in the sky, confirming that it is not enough to avoid the Milky Way and our two galaxies are doomed to collide.

Hubble Space Telescope

A galaxy beyond our own

In 1925, Edwin Hubble identified Cepheid variable stars in images of Andromeda – a type of star whose variations indicate its true brightness. This revealed Andromeda’s true, enormous distance, and that Messier 31 is a galaxy in its own right. Hooker Telescope at Mount Wilson Observatory, California