A cosmic collision
Ken Tapping, November 21, 2012
Precise measurements made with the Hubble Space Telescope and other instruments leave no doubt about it. Our galaxy, The Milky Way, is heading for an almost head-on collision with the Great Galaxy in Andromeda. Both galaxies are very roughly the same size, containing billions of stars and a huge amount of gas and dust. Cosmic collisions do not get much larger than this.
The two galaxies are currently about 2.5 million light years apart and approaching each other at about 300 km/sec. They are gravitationally tugging at each other, which will increase the speed as they get closer together. The collision will happen in just under four billion years. Since our Sun will have become a red giant star by then and fried any of our descendents still hanging around on Earth, this is not something we really need to worry about.
A collision between galaxies hitting at such a speed conjures up a vision of unimaginable destruction. In truth the situation is nowhere nearly as bad as that. Galaxies are mostly nearly-empty space. Stars are so far apart that it is very unlikely any of them will collide. The galaxies will more or less slide through each other. The items that will collide will be those huge but tenuous clouds of cosmic gas and dust between the stars. However, the main consequence of those collisions will be the destabilization of those clouds, making them much more likely to collapse, forming new stars. So the collision will result in a burst of new star creation, not destruction.
An astronomer observing from another galaxy will see the two galaxies coalesce into one big galaxy. The two spiral galaxies will become one large elliptical galaxy. There is evidence of this. Firstly we are observing collisions in their various stages between other galaxies, and secondly we have the dramatic results coming from computer simulations.
In principle these calculations are very simple. We use the equations derived by Isaac Newton. If he were around today, he would understand exactly what is going on. We are simply looking at the gravitational attractions bodies exert on each other.
The problem is that there are billions of bodies upon which we have to apply those simple equations. To really see what goes on when galaxies collide we have to calculate for each star the effect of the gravitational attraction of the billions of other stars. We then update the position, speed and direction in which each star is moving and then to do the whole thing again, over and over. This involves billions of billions of calculations. Fortunately supercomputers that can this.
At first we just see the Andromeda Galaxy getting closer. Then around four billion years from now we see the Milky Way starting to be pulled out of shape by the gravity of the approaching galaxy. By about seven billion years both galaxies will be completely disrupted. The bits will fall back together so that around seven billion years from now, The Milky Way and Andromeda Galaxies will be one, huge, elliptical ball of stars. In a way it’s a shame we won’t be around to see all this.
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