Michael Witalis, age 12, of Stratford, Connecticut, for his question:

What exactly are double stars?

Double stars are twin stars, swinging around each other like partners at a heavenly iceshow. The pairs are much closer to each other than they are to the other stars in the universe. And they are bound together by forces that also govern their swinging motions.

If your eyes are as sharp as they should be, you can see a double star without the aid of a telescope. Choose a clear night and an observation post where the celestial view is not blurred by smog or city lights. Then find the Big Dipper and locate the next to the.last star in the handle. This is Mizar Alcor. It looks like a big bright star, shining like all the other stars in solitary splendor. But if you focus your eyes with concentration, you can see a pair of stars. Mizar is the larger star  Alcor its smaller partner.

Mizar and Alcor were observed long ago. Modern astronomy has delved deeper into its nature, and high powered telescopes have revealed that the double star is by no means unique. Some 23,000 others have been spotted. Even larger star groups are not unique. Many stars that appear to be loners in the sky are actually triplets or quadruplets or even quintuplets. Astronomers refer to them as multiple stars, and the double stars are called binaries.

Cosmic distances, of course, are so immense that only a few of these partners can be seen without powerful telescopes. Some pairs are closer than Mercury and the sun. Mizar and Alcor are separated by 30,000 million miles, about five times wider than our Solar System. Most binary systems have one large and one much smaller star.

Double stars are held together by their mutual gravitation, and this invisible force also governs their orbital motions. The determining factor is the mass, or matter, of each star. It compels them to revolve around their center of gravity, midway between their combined masses. This point is closer to the more massive star. Hence its orbit is smaller and it revolves more slowly. Its less massive partner is farther from the key orbital center    and it revolves faster with a much wider orbit. From the earth, the massive partner of a double star may seem merely to sway, remaining almost motionless. Meantime its light weight partner races around a wide orbital ellipse. In some cases, one star passes directly between us and its partner. With each revolution, it hides part of the other star. In regular periods, the double star we see as one, grows brighter and dimmer. A pair of this sort is called an eclipsing binary.

One of the most interesting doubles is Sirius, the Dog Star. In the 1840s, astronomers detected a swaying motion, like a small orbit. In 1862, the new 18 inch telescope spotted the dwarf companion of Sirius    the Pup. Bripht Sirius is 2.3 times as massive as the sun. The little Pup is 1.0 times as massive as the sun. As Sirius traces its small orbit, the Pup takes about 50 earth years to revolve around its huge ellipse.