End of the world: you won't feel a thing.

End of the world: You won't feel a thing

Suppose the world "ended' and wedidn't hear the whimper. According to two astronomers speaking at this week's joint meeting of the American Astronomical Society and the Canadian Astronomical Society in Vancouver, British Columbia, two kinds of end-of-the-world catastrophes could happen to our Milky Way galaxy, and we probably wouldn't feel either, or even both combined.

Mitchell C. Begelman of the Joint Institutefor Laboratory Astrophysics at the University of Colorado in Boulder described what would happen if a quasar should "turn on' in the center of the galaxy; Marshall L. McCall of the University of Toronto predicted a collision between our galaxy and the spiral galaxy M31 in Andromeda in about 4 billion years. The collision could trigger the quasar, which would boil away the interstellar gas clouds in our galaxy, ending the galaxy's ability to make new stars. On the other hand, the collision by itself could tear the galaxy into pieces and possibly send the sun off on a track of its own into intergalactic space.

However, the earth would be unlikelyto feel very much of either cataclysm, the two astronomers agreed. McCall suggested that what we really should worry about is the possibility of a nearby star undergoing a supernova explosion.

Begelman's scenario begins with recentobservations that the centers of many galaxies, including that of our own, seem to contain very massive black holes with up to a billion times the sun's mass. It is now also well accepted that such supermassive black holes are the things that power quasars. Where large amounts of matter happen to be falling into the black hole, friction and gravitational stresses on the infalling matter can generate the radiation characteristic of a quasar, which far outshines all the rest of the surrounding galaxy. In many cases, however, including the center of our own galaxy, these black holes are lying quiescent, waiting for some happening to bring large amounts of matter near them, which would then begin to fall in and so turn on the quasar. A collision with another galaxy could be such a happening.

If the quasar turns on, Begelman says,most of its radiation will come as X-rays. These X-rays will heat the interstellar gas --the material out of which new stars are continually forming--and give the gas enough energy to escape the gravitational holes of the galaxy. Gradually, from the center outward, the interstellar gas would evaporate into intergalactic space, ending the galaxy's ability to make new stars. This would radically change the evolution of our galaxy and seriously alter our surroundings. However, the catastrophe would stop at our atmosphere, Begelman says. Our atmosphere is much thicker than the interstellar gas, and the X-rays would not be able to penetrate or damage the earth's atmosphere. Thus, we would not feel their effect.

Astronomers have believed that theMilky Way and the Andromeda galaxy, M31, form a binary system, bound together by gravity and orbiting their mutual center of gravity. According to this belief, the Milky Way is following an orbit in the shape of a narrow, flat ellipse that takes it away from M31 and will eventually bring it back to a close encounter with M31. On the basis of such a supposed motion, astronomers have calculated both the age of the universe, which is the time since the motion began, and the total mass of the two galaxies.

As a result of his own studies of themotions of several nearby galaxies, McCall says he no longer believes estimates of these two important cosmological numbers that have been calculated in this way. He finds that the binary system of the Milky Way and M31 is not as isolated as some astronomers have thought. Two other nearby galaxies, IC 342 and Maffei 1, strongly influence the motion of the Milky Way, making its orbit not a simple ellipse but a very complicated curve.

The collision with M31 is still in ourfuture. Yet even if the collision tore the galaxy apart and shot the sun off on a career of its own, the planets are so tightly bound to the sun that they would just go along. The sky we see would change drastically, but that would be the only major effect.

The thing to be afraid of, McCall says, isthat a giant star like Betelgeuse should explode into a supernova. The recent supernova 1987A proves that giant stars can explode in this way. At a distance of 300 light-years from Betelgeuse, the earth would get a blast of ultraviolet and X-rays strong enough to burn off the atmospheric ozone layer. Then more ultraviolet, either from the supernova or from the sun, would fry us.

So maybe it would end with a bang afterall.