Andromeda and The Milky Way

by Jack Holt

Series of images showing predicted merger between
Milky Way galaxy and neighbouring Andromeda Galaxy.

Only two and a half million light years away, the Andromeda galaxy is our closest neighbouring galaxy and it is hurtling towards us, travelling 60 kilometres every second. 

Gravitational attraction pulls our two galaxies together across this vast distance due to their immense mass and size. To put this into perspective, the Milky Way is 150,000 light years across and has a mass of 1 trillion solar masses, the Andromeda galaxy is 200,000 light years across and has a mass of 2 trillion solar masses. A solar mass is roughly the mass of our sun, 330000 earths or 1048 Jupiters. A light year is how far light travels in a year (9.8 million million kilometres). Each of our galaxies rotates around a supermassive black hole at the centre of their spiralling arms. The supermassive black hole at the centre of our galaxy is called Sagittarius A* (pronounced Sagittarius A Star), it has a mass of roughly 4.3 million solar masses, yet it is only 26 million kilometres across making it one of the densest objects in our galaxy. However, neutron stars are far denser, they are the aftermath of a star going supernova and its remaining core imploding as it runs out of fuel. It is estimated that our galaxy could be home to over one billion of them. 

Incomprehensible forces will pull our two galaxies closer and closer together until they eventually collide, causing chaos in both galaxies as the two supermassive black holes begin to orbit one another, getting closer until they cannot escape each other's gravity and form one even more massive black hole. This would mean the complete destruction of our solar system as a new galaxy is formed. Luckily this catastrophic event is over 4 billion years away so there is nothing to worry about. But how do we know this and how do we know that the Andromeda galaxy is hurtling towards us? It is thanks to a phenomenon known as blue shift. This is a measurement taken by astronomers of how much the waves we receive from a distant object are stretched or squashed towards the red or blue end of the spectrum depending on their wavelength. Blue shift means the waves are squashed towards the blue end of the spectrum as they are being pushed towards us , proving to us that it must be travelling towards our galaxy. This is how the doppler effect works, just with light waves instead of sound waves. When a loud sports car or a police car with its sirens on passes by, you can hear an audible difference in pitch. As it travels towards you the sound waves are squashed so the pitch is higher, but when it has gone past you and is travelling away from you the sound waves are stretched so the pitch is lowered. This is exactly the same for light waves except we use a spectrum to measure their change in wavelength rather than their change in pitch. So this is how we know that the Andromeda Galaxy is hurtling towards our home.