The Onrushing Galaxy

The Ultimate Collision

To some people, bright city lights are the finest of sights. But the more discerning prefer their celestial counterparts. I think of this when looking at images of the great Andromeda galaxy (M31), visible in the early evening over Britain this spring. It is hard to imagine that this fuzzy blob of light is a galaxy 20 per cent more massive and 40 per cent larger than our own. A thousand years ago, the Arab astronomer Al Sufi called it a ``little cloud." It must have looked slightly smaller to him than it does to us with the naked eye, since M31, now 2.3 million light years away, is rushing towards us at 22 miles per second. It was then therefore about a tenth of a light year further from us than it is now.

And further back in history, it would have been still fainter and smaller. During the last Ice Age, there must have been a time before which it was invisible, even to the most watchful eyes. I remember stories of the days when pictures of deep sky objects like M31 first became available in colour. The picture editor of one American weekly news magazine became highly excited at the prospect of publishing it. ``It looks great," he told an editorial meeting." This Andromeda thing is easily the best of these so called galaxies. But it's edge on. Some of the others are face on. I want a picture of this one face-on." ``That might take some time," said an assistant who knew some science. ``How much time?" barked the impatient editor. ``About seven hundred million years." Let us go forward two billion years into the future. M31 has twice presented itself face-on, and is now once again edge-on. Now this great galaxy is no longer a tiny smudge, but a vast object brighter than a hundred full moons that dominates half the sky. M31 is a ``cannibal’’ galaxy that long ago grew to its present size by swallowing up another galaxy. Now, within another billion years, it is going to grow larger still by devouring ours. Nobody knows what sights to expect when the two galaxies collide. Certainly it will dwarf the spectacle of the brightest city lights. The convulsions will totally change the star patterns we see from Earth, and many more stars will become visible to the naked eye. It is unlikely that any star from M31 will crash into the Sun (the distances between stars being so great), but the Solar System's environment will gradually change for the worse. The amount of interstellar gas and dust will increase, and there will a much greater incidence of supernovae and gamma ray bursters. Many more comets will form in the Oort Cloud, and more of these will swing inwards towards the Sun. Life on Earth-and on other planets circling Sun-like stars that man will surely by that time have colonised-will become much more hazardous. Alternatively, the Sun and its planets may be flung out of the Milky Way altogether. Then we would be lonely indeed! In these conditions the night sky would be so dull that we might prefer the city lights.