Ellen Thompson, age 12, of Waynesboro, Virginia, for her question:

Have scientists found other universes besides our own?

So far as we know, there is no end to the infinite reaches of space. If by chance we found the outside edge, our next question would be: "What's on the other side?" Obviously our human minds cannot grasp the concept of endless infinity. But once in a while, every thinking person is tempted to think and wonder about it. This is a good exercise to stretch the brain    so long as we remember that we could not grasp the answer, even if we found it.

This is a tricky question because the word "universe" ran away with itself. Originally it meant everything in our world, plus all the things we knew about in the sky above. As scientists probed deeper into the space that surrounds the earth, they discovered that there are vast reaches beyond the range of our eyes. The old idea of the visible universe exploded, so to speak, beyond the range of human imagination. But people were used to the old word "universe" and they tried to keep it, even though it no longer fitted the known facts.

This led to a lot of confusion. The old word was given several extra meanings to bring it up to date. Several new terms were dubbed in to clarify the situation. Meantime we learned that our old visible universe is a huge system of about 100 billion stars, isolated in vast oceans of outer space. Thousands of similar star systems were discovered, separated by enormous distances from our universe and from each other. Obviously we need some precise new terms to explain this expanded picture.

Some people refer to our own star system as an Island Universe or the Milky Way Galaxy and some still call it the universe. Maybe the simplest term for it is the Galaxy, spelled with a capital letter. The countless star systems beyond it may be called other island universes, nebulae or nebulas. The simplest term seems to be galaxies, spelled without a capital letter to distinguish them from our own Galaxy.

This leaves us without a replacement for the old universe, which was intended to include everything on earth and all the known items beyond it. We may use the term Universe, with a capital letter, or express this greatest of imaginable concepts with the word cosmos. In any case, nothing has been or will be found beyond it. The cosmic Universe includes all we know plus everything we don't know.

The thousands of known galaxies, or universes, are catalogued by letters, numbers and sometimes clues to the constellations where they may be located in our skies. Our Galaxy is a wheeling lens with spiraling arms. It is called a spiral nebula. Many outer galaxies are spiral nebula types. Others are S shaped or barred. Some are hazy spheres in which, perhaps, the gases are still unborn stars. All of them are included in the one and only Universe, or cosmos.