The Milky Way and Andromeda galaxy are on a collision course!

Forget global warming baking the Earth - how about something even bigger, like the destruction of the entire Milky Way galaxy? Celia Escamilla Rivera looks at new evidence that predicts a cataclysmic fate for our own galaxy.

Although the Universe is expanding, making galaxies move away from each other, collisions between galaxies that are located in close proximity to each other can still happen. This shouldn't come as too much of a surprise; close pairs of galaxies are bound by the gravity of the dark matter surrounding them, drawing them together and resisting the expansion. As it happens, our own galaxy, the Milky Way, finds itself in a close pair – and in 4 billion years from now, a gigantic collision will take place between it and its neighbour the Andromeda galaxy (also known as M31).

In the 1920's, Edwin Hubble and others observed that most galaxies appear to be rushing away from us, which led to the discovery that the Universe is expanding. Andromeda is an exception to the rule, however, and is moving toward the Milky Way at about 250,000 miles per hour. (That's fast enough to get you from Earth to the Moon in one hour!) But while it has been known since then that M31 is moving in our general direction, we haven't been able to tell whether it's coming straight at us, or will glance off at an angle. Thanks to recent, painstaking measurements of the motion of Andromeda, taken using the Hubble Space Telescope, the issue has now been decided.

After almost a century of speculation, Roeland Van der Marel and Sangmo Tony Sohn, astronomers at the Space Telescope Science Institute in Baltimore, Maryland, have announced a clear picture of how events will unfold over the coming billions of years. They confirm that M31 is currently 2.5 million light-years away, that it is inexorably falling toward the Milky Way under the mutual pull of gravity between the two galaxies, and that we are on a collision course.

From Earth, the encounter of both galaxies will be spectacular. (That is, if the human species survives long enough to see it!) For us here and now, however, advanced computer simulations based on the Hubble measurements will have to suffice (see here for a video). These have shown that both systems will eventually reshape into a single elliptical galaxy, similar to those commonly observed in the local universe. It is also likely that the merger will trigger an intense phase of creation of new stars, and that the super-massive black holes at the centres of the two galaxies will merge. Although the shapes of the galaxies will be profoundly distorted during the collision, however, individual stars are unlikely to collide with each other because the space between them will still be huge.

The scientists believe that the gravitational disturbance could cause the entire Solar System to change its position – the simulations show that it will probably be tossed much farther from the core of the galaxy than it is today. To make matters more complicated, M31’s small companion, the Triangulum galaxy, M33, will join in the collision and perhaps later merge with the Andromeda/Milky Way pair. There is even a small chance that M33 will hit the Milky Way first. Either way, it will take another two billion years after the collision for both agglomerations of stars to merge completely into a single galaxy.

So why has it taken the better part of a century to figure this out? Telling whether the galaxy is moving towards us or away from us is the easy part – that can be measured using the Doppler effect, where light is stretched or compressed depending on the relative motion between two objects, leading to a change in wavelength of the light that we observe. It is the transverse (sideways) motion that causes problems, however. In order to measure this, we must determine how the positions of the stars of Andromeda change with time. Since they are so far away, any change is very slight, and so we must take measurements over an extended period of time to have any hope of detecting the effect. Furthermore, some "fixed background" must be used in order to accurately fix the positions of the stars. Distant galaxies which lie behind Andromeda must therefore be detected so that they can be used as a reference. Neither of these make for easy measurements. To begin with, one must identify which stars belong to Andromeda, and which are actually galaxies in the background. Over the course of the period of many years in between measurements, however, a telescope's cameras are likely to change slightly, introducing subtle differences in how they capture images over time. Those images can give a confused picture of the stars, making it difficult to determine whether we observe motion or not.

Taking into account these issues, Van der Marel, Sohn and collaborators took an image from 2002 and then another from 2010 to see how much the positions of the stars had shifted over that time, and then used this information to figure out the motion of the galaxy. With this measurement, plus some careful calculations, they found an exact time for the collision: 4.107 billion years from now.

We'll have something else to worry about by that point, though. "In 4 billion years, the nuclear fuel in the Sun will have begun to run out and our star will have begun to increase in size", said Van der Marel. Because of the natural evolution of the Sun, its temperature will increase more and more, and in a few billion years will be so hot that life as we know it on Earth will not be possible. But we're talking about the distant future, in billions of years, so perhaps that global warming issue is a bit more pressing after all...

Image credit: NASA; ESA; A. Feild and R. van der Marel, STScI (link).