Karen Denise Roberts, age 12, of Newport News, Virginia, for her question:

Do galaxies ever collide?

Astronomers tell us that the galaxies can and do collide and there are telescope photographs to prove it. Out there in the vast reaches of space, pairs of enormous star systems sometimes meet in what surely must be the greatest traffic accidents in the universe. However, such an event is not necessarily disastrous. If the stars in those galaxies have populated planets, it is not likely that the people would even notice the stupendous collision.

Our Milky Way, of course, is just one of countless galaxies strewn throughout the infinite universe. One of its nearest neighbors is the Great Nebula in Andromeda    at a safe distance of about 1 1/2 million light years. Each of these cosmic twins measures about 100,000 light years from side to side and contains about 100 billion starry suns, plus stupendous clouds of dusty gases.

This pair is not likely to collide. But if they did, most likely they would not create a cosmic catastrophe. Astronomers suspect that few, if any, of their stars would be disturbed. The distances between them are so great that the two vast spiral nebulas would pass right through each other and their stars would be somewhat like specks of dust floating through a sunbeam. Pictures of remote colliding galaxies do not reveal the star collisions one might expect.

However, not all the material in an average galaxy has condensed to form separate stars. Much of it is dust and gases, strewn around in enormous clouds. As old stars die out, new ones will be created from this material. Galaxies collide at terrific speeds. Friction and heat could send their cosmic gases zooming away into outer space. Eventually the starry systems would separate and move apart    minus their gaseous clouds. As the existing stars subsided, there would be no material to create new ones.

Astronomers base this accident report on a lot of known facts about the universe. Most likely it happens this way, though nobody can be certain of the details. However, photographs reveal no evidence that the widely separated stars collide. But powerful radio emissions come from the scene of the accident. This would be expected if the gaseous clouds do indeed become embroiled in the collision.

Celestial surveys reveal that the countless galaxies; are not distributed evenly through outer space. They tend to congregate in groups of two or three or several hundred. Like everything else in the universe, all of them are on the move. Hence it seems likely that traffic accidents are likely to occur    and a collision between two galaxies may be a fairly common cosmic event.