Pedagoguery

When Edwin Hubble started measuring the speed that galaxies are traveling toward or away from us, he discovered that the vast majority of galaxies are traveling away. The few exceptions are all close by, indicating that they are part of a gravitationally bound group called the Local Group. The Local Group consists of two large galaxies, the Milky Way and the Andromeda galaxies, a couple of medium-sized galaxies, and a host of tiny galaxies. The Local Group is not static, however. Several of the smaller galaxies are being ripped apart and consumed by the larger members. And, most notably, we and the Andromeda galaxy are on a collision course.

The Andromeda galaxy is the furthest object visible to the naked eye at just over 2.5 million light years distance. It is approaching us at a speed of around 75 miles per second, so it is inevitable that there will be a collision. The question is, when? The answer is that it depends. It makes a big difference whether we are approaching each other head-on, or if there is a significant transverse component to its velocity. The radial motion is very easy to measure – simply take a spectrum of the galaxy, identify a common absorption line, and measure the shift (blue-shift, in this case). The transverse component is much harder to measure, because it involves measuring a minute sideways shift in comparison to background galaxies.

It is estimated that there is a transverse component to Andromeda's motion of between 0 and 150 km per second, which equates to 0 to 42 microarcseconds per year. That's 42 millionths of 1/3600 of a degree. At the maximum rate, it would take over 1.7 billion years for it to shift the width of the full moon in the sky. However, upcoming space missions, such as the European Space Agency's Gaia mission might be able to detect this motion.

If our course to Andromeda is a direct one, the collision will take place in about 3 billion years. The Sun will still be a main sequence star at that time, since it is estimated that it will be about 5 billion years from now before the Sun swells into a red giant. If our two galaxies are in a more eccentric mutual orbit, then the ultimate collision will still occur, it will just be much more distant in the future.

Galaxy collisions are messy. We see many examples in our telescopes, and they become more common as we look further back into the past. Since galaxies are not solid entities, but gravitationally bound collections of stars and gas it is impossible to say with any precision exactly what will happen, but using the example of collisions we can see, we can get a good idea of the generalities. What is likely to happen is this. First of all, as we approach Andromeda, our velocity towards the galaxy will increase. Both galaxies will tend to get stretched out in a line toward each other, but that line will get distorted by the mutual motion of the galaxies. The result will be a long tidal tail from each galaxy tracing the orbit. Gravitational stretching and compression will ignite a burst star formation in the gas clouds of both galaxies. In the event of a relatively direct collision, the star formation will be quite intense, as gas clouds from both galaxies collide. Collisions between stars will be rare, but chances are good that stars will pass close enough to each other to disrupt planetary systems. The result will be some planets thrown into new orbits, some thrown into their parent stars, and some thrown completely out of their original solar system.

Even in the event of a head-on collision, the collision will not be resolved in a single event. The galaxies' motions will take them apart and together again many times, with each time resulting in a net loss of orbital energy as the more energetic stars are thrown clear. Whether our sun will be one of those, or whether the Earth will get affected by a near miss with another star, are unknown. It's even possible that the Earth could get fried by a nearby supernova explosion due to star formation. What is certain is that the eventual end state of the collision will be an elliptical galaxy, which will eventually absorb all of the galaxies in the local group. By then, it is likely that the accelerated expansion of the universe will have taken all other galaxies so far away that they would be beyond our horizon, and our local galaxy would be all alone.

Next time, supernovae.