Kathy Callahan, age 13, of Omaha, Nebraska, for her question:

Which are the biggest and smallest stars?

Stellar astronomers can use visual and other practical evidence to measure the dimensions of a star with reasonable accuracy. They have measured only a few thousand of the multitudinous majority in this way. Astrophysicists can estimate the dimensions of far more stars from indirect evidence. A star's size is related to its mass and there is a limit to how much mass it can hold together. The smallest known stars may be as massive as the sun and no bigger than asteroids. The largest are millions of times larger than the sun and perhaps only ten times more massive.

Measuring a star by visual and other practical evidence is somewhat like measuring a golf ball from a distance of 100 miles. But the largest star known provided a helping hand to make the task easier. It is Epsilon Aurigae. Epsilon is the fifth letter of the Greek alphabet and the supergiant is the fifth brightest star in the fall constellation Auriga, the Charioteer.

The champ is not the brightest member of its group for several reasons. The thin gases of this infant supergiant emit mostly infrared light, which the human eye does not see. What's more, its light is dimmed by distance and the supergiant tends to get lost in the Milky Way, Measuring its size under these conditions could be impossible, except that Epsilon Aurigae happens to be a pair of orbiting binaries. Once in every 27 years, the smaller partner spends about 18 months eclipsing its partner. This gives astronomers a chance to check their sizes.

The smaller partner has a diameter 300 times wider than the sun. It is a super¬giant in its own right, big enough to fill and spill over the earth's orbit. But the diameter of the enormous champ is known to be 2,000 times wider than the sun's and other evidence suggests it may be more than 3,000 times wider. If it replaced our sun it certainly would spread far beyond the orbit of Jupiter and just might engulf our entire Solar System.

A decade ago, the smallest known records were held by the white dwarf stars. Most of them range from half to four times the size of the earth    though they may be almost as heavy as our massive sun. However, in the past two years the tinier pulsars were discovered. First their intense signals were detected by radio telescopes. Later, ordinary telescopes spotted light from a few of them. Naturally these remote midgets have not been measured by visual methods. But other evidence suggests that they must range from about five to ten miles in width. Most astronomers agree that no stars are likely to be smaller than the pulsars.

These mighty midgets are estimated to be almost as massive as suns. A cup of white dwarf material may equal the weight of 30,000 cups of water. A cup of pulsar material may weigh as much as the water in a moderate sized ocean. At the opposite end of the scale, the gases of the supergiant are almost as thin as a vacuum. The total mass of Epsilon Aurigae is merely ten times greater than the sun's.