Lisa Dapolito, ape 11, of Albany, New York, for her question:

How big must a visible star be?

We tend to think that the brightest stars are the biggest ones. But things Up There seem determined to fool our eyes. For example, a dim and barely visible star may be a lame, seething sun. Avery bright star may be a whopping supergiant, big, enough to swallow half our Solar System. Yet it is three times farther away than the one that seems too pale and dim.

One would think that a star has to be a certain size to become visible from the earth. Actually its visibility depends both on its size and its distance. Its brightness is called magnitude and even astronomers need two scales of magnitude. Apparent magnitude compares the visible brightness of a star with the others we see. Absolute magnitude measures how bright they really would be    if they were all at the same distance from the earth.