Jill Schumm, age 11, of Indianapolis, Ind., for her question:

WHAT IS A DOUBLE STAR SYSTEM?

Since before the dawn of history, people have looked up to admire Sirius,    the Dog Star. This sparkler adorns our winter skies, and it is the brightest star we ever see from the earth. Naturally everybody assumed that Sirius is a single star. Imagine the astonishment when it was found to be a double star system, includinga whopper and a midget.

Actually we see just one star because the midget is too small for our eyes to behold. It is a white dwarf star, and astronomers named it the Pup. The two stars are a system because the gravitational pull between them causes them to orbit around each other. The Pup makes a fairly wide orbit, but Sirius barely wobbles because the big star is so massive. Since the Pup was discovered, astronomers have found many other double and triple star systems. In each case, the orbiting partners are locked together by gravity.