Dean Adamyk, age 13, of Schenectady, New York, for his question:

Can two galaxies coIlide?

The average galaxy is an enormous system of perhaps a hundred billion stars. Its stars are separated by tens and hundreds of light years. As a rule, it is separated from its teeming galactic neighbors by at least a hundred thousand light years. Nevertheless, on very rare occasions, a couple of galaxies do come close enough to collide. We actually have some very clear telescope photographs that show two enormous galaxies in collision.

This cosmic collision must be stupendous, or so one would think. Astronomers can only guess what really happens    and their guesses are somewhat surprising. Most of a galaxy is empty space and even in the crowded center the starry populations are separated by vast distances. When two such systems collide, some astronomers suspect there is still plenty of unoccupied space between all of their stars. Of course, nobody knows for sure, but two galaxies may collide without causing any collisions between their individual stars.