Blogtrek

Star Collision

I have seen some terrifying, apocalyptic images as of late: the burning towers of Planeattack, the destruction of cities in the movies Armageddon and Independence Day, and in numerous artist renditions of the times billions of years in the future when the sun heats up and becomes a red giant, turning the Earth into a lava ball. I saw some more horrifying images yet in the current issue of Scientific American. The 2002 November issue shows what looks like a bright shining yellow lava asteroid about to collide with a dark, reddish Earth. But it was no asteroid. It was the Sun, distorted into the shape of an elongated bird egg by the approach of a white dwarf star from nowhere. Stars are so far apart that the chance of them colliding is remote, but maybe not that remote, as they attract to each other through gravity. The article says that if a white dwarf star, as massive as the Sun but in the volume of an Earth, were to collide with the Sun, it would ignite the entire Sun and blow it up leaving behind a nebula. There is a painting in this article that shows this. These two paintings are among the most striking I have ever seen. They do leave out a few points; for example, the mere approach of the white dwarf would have swung the planets into weird orbits or flung them out altogether, and the Sun is pictured as a normal-colored yellow object, whereas if this were to really happen, the Sun would have been blazingly bright, as it is now, and no reasonable picture could reflect this.

The article does give an "arithmetic" table for saying what happens if stars of different types were to collide. For example, if two red giants collide, they would throw off their outer envelopes and become a pair of white dwarfs. If two main sequence stars (such as the Sun) were to collide, the result would be a bigger main sequence star, and so forth. This gives an arithmetic of sorts. White dwarf + black hole = black hole (with disk). Red supergiant + neutron star = white dwarf + neutron star (with disk) and so forth. This operation is clearly commutative, since, for example, the Sun and a white dwarf are the white dwarf and the Sun. Is it associative? I thought at first yes, since there are ranks of objects and the same things always happen to them; for example, a red giant encountering any kind of star will turn into a white dwarf. However, (red giant + main sequence) + main sequence is not the same as red giant + (main sequence + main sequence), as in the former, one of the main sequence stars would get annihilated by a white dwarf, but in the second this does not happen.