Also known as M31, the Andromeda galaxy is a majestic spiral featuring up to one trillion stars and is a similar size to the Milky Way.
At a distance of 2.5 million light-years, it is so close to us the galaxy appears as a cigar-shaped smudge of light high in the autumn sky.
If its gaseous halo could be viewed with the naked eye, it would be about three times the width of the Big Dipper.
This would easily be the biggest feature on the night-time sky.
Through a program called Absorption Map of Ionised Gas in Andromeda (Project AMIGA), the study examined the light from 43 quasars.