how to…observe the andromeda Galaxy
Star clusters galore rest in Cassiopeia’s ‘W’ shape
However much you dress it up with technical terms and scrawled equations, astronomy is basically the study of things that are very far away. Even the closest objects astronomers study – shooting stars, noctilucent clouds and the northern lights – are tens of kilometres above our heads. The Moon is almost 385,000 kilometres (239,000 miles) away, the Sun 146 million kilometres (91 million miles) away and the closest star is so many kilometres away that its light – travelling at 300,000 kilometres (186,411 miles) every second – takes over four years to reach us…
It stands to reason that out of all the billions of celestial objects out there, one must be the most distant the naked eye can see. Luckily for sky-watchers, this object, which is 2 million light years away, is something spectacular, and not just a faint star: M31, the great Andromeda Galaxy.
M31 is a huge spiral galaxy, larger than our own and containing hundreds of millions of stars. In around 5 billion years’ time our Milky Way will collide with M31, and the two galaxies will merge into one.
Despite shining at magnitude 3.3, M31's large apparent size of 3 x 1 degrees – six times the size of the full Moon – means its light is spread over a wide area, giving it a low surface brightness. Consequently, any trace of light pollution will wash M31 from the sky; you will only see it if you're looking at a truly dark sky.
Once you’ve let your eyes adjust to the darkness you just need to do a bit of ‘star hopping’. First find Cassiopeia, the distinctive W-shaped constellation of the Queen, then drop down to the upside-down ‘Y’ of Perseus (the Hunter) below it. That constellation’s central star, Mirfak, is the starting point for a celestial hopscotch to the east along the line of stars forming Andromeda. Having reached Mirach, the third starry stepping stone along, you should see a small smudge above it – that’s M31.
If it isn’t immediately obvious try using the astronomers’ trick of ‘averted vision’, which means looking just off to one side. This means the galaxy’s faint light will fall on more sensitive parts of your retina and should become visible.
“In 5 billion years our Milky Way will collide with M31”