Ken Tapping, 27th April, 2016
Astronomers recently discovered a small galaxy that may have collided with ours several hundred million years ago. What’s left of it is now receding into the distance. A collision between galaxies sounds like the ultimate scenario for a disaster movie. What would one really be like?
About 64 million years ago the Earth was hit by a small asteroid. At the time many ecosystems were in trouble. Ammonites and dinosaurs, which were among the dominant life forms at the time, were in decline. The environmental changes caused by the impact were the last straw, and there was a massive extinction, in which about 75% of the species living on Earth at the time vanished. The asteroid was tiny in cosmic terms. A galaxy contains huge amounts of gas and dust, billions of stars and planets, countless asteroids, and a few black holes. Attempts to picture a collision between two such things beggar the imagination. Actually, we don’t need to imagine. Today’s telescopes are revealing many examples of galaxies that are colliding with one another.
Imagine two great spiral galaxies approaching each other on a collision course. As they get closer their outer parts start to be affected by the other galaxy’s gravity, and they start being pulled out of shape. Great streams of stars and gas get torn off. These may be thrown right off into space, or go into new orbits around one or other of the galaxies. If the collisions are head-on, the galaxies might be completely disrupted as they go through each other. Interactions between the gas and dust clouds and the magnetic fields absorb the energy of collision so that in some cases the galaxies are slowed to the point where they can no longer escape from one another, and are doomed to orbit one another, colliding over and over again, until they coalesce into a new, bigger galaxy. This sounds potentially disastrous for anyone living on planets in those galaxies. However it is not. It is probably part of the process by which galaxies.
Galaxies are actually mostly empty space. Those gas and dust clouds are actually more rarified than any vacuum we can achieve in the laboratory, and stars are enormously far apart. Even when systems of billions of stars collide, the chance of two stars hitting, or even passing close to one another, is very small. If our Earth were in a galaxy that is colliding with another one, we would see – over millions of years – the shape of the Milky Way change. Many of the constellations won’t change at all; others might get new shapes or a new star or two. If another star were to pass close to the Solar System, maybe at Pluto’s distance, the orbits of the planets might get changed a bit, which could be disastrous for us. Some planets could get thrown off into space. However, the chances of this are highly unlikely. One consequence of such impacts is positive. The colliding gas clouds will trigger some of them to collapse, forming new stars. Instead of being a disaster, the collision results in the formation of new stars and planets.
We will get a chance to experience this first-hand in about four billion years. The Andromeda Galaxy, which is a bit larger than ours and some 2.5 million light years away, is heading in our direction at 110 km/sec. The collision is expected to result in one huge, combined galaxy. There are some excellent computer animations on-line showing what the collision will probably be like. Lots of stars will be ejected off into space; there is a chance the Sun will be one of them. However, the effect on the Earth and other planets is expected to be negligible. The Sun is due to run out of fuel in about 2.5 billion years, so in four billion years we will either be extinct or observing this from new, colonized worlds, orbiting younger stars.
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