Donald Wight, age 9, of Algonac, Michigan, for his question:

What exactly is a double star?

The large and small sparklers in the velvety night sky look for all the world like single stars. But a telescope enlarges the picture and shows'more of the details. Even a beginner's telescope reveals that certain stars are really twoin one. The distance between them is so small that our eyes cannot separate them. However, there is one double star you can see    if your eyes are keen and you look in the right place at the right time.

Find the Big Dipper and trace the three stars in its curved handle. The star in the middle of the curve is Mizar. If the night is clear and you are away from city lights and smog, you may be able to see a double star with your own eyes. Stare at Mizar and you may see that it actually is a pair of very close stars. It was one of the first double stars to be noticed and Mizar still serves as a good test of a person's eyesight.     

After the telescope was invented, astronomers noticed that many stars are really close pairs. For a time they thought that the twosomes appeared close together because they happened to be almost in the same direction. Later they noticed that certain pairs move around each other, somewhat like swinging partners in a hoedown. Obviously a double star must be a related system of two stars. And this proved to be true.     

More and more doubles were found and their movements were studied and compared. Some pairs were two large stars, others were one large and one small star. Some were as close as Pluto is to the sun, others were separated by several light years.

But all of them swung around each other like dancing partners. In some cases, only the small star seemed to orbit around a larger star. But astronomers knew that this was impossible because of certain laws that rule the stars. The orbiting patterns must be governed by gravity and each partner has a certain quota of this pulling power. The quota depends on the mass, or amount of material packed into a star's size. A more massive star has more gravity to pull and swing around its less massive partner. But junior has enough pulling power to move its less bulky partner, too.

So a pair of double stars actually orbit around each other, though as a rule one orbit is much bigger than the other. The rules say that they must swing around a certain point. This is nearer the heavier star because it is related to their two masses.

Astronomers detected a slight wobble in Sirius, the dazzling Dog Star. This must be a small orbit and sure enough, telescopes revealed that Sirius has a tiny companion. This planet sized star naturally, was named the Pup. Such a lot of double stars have been found that astronomers suspect that single stars, such as our sun, may be rather rare.