Keith Clinard, age 12, of High Point, North Carolina, for his question:

How dense is Sirius

Now is a good time to see the brightest star that ever appears in the night skies above the earth. It is Sirius, alias the Dog Star, in the winter constellation Canis Major. Taurus, with a glowering red star in his eye, rises early. Behind him comes the dazzling rectangle of Orion, the Celestial Hunter, and sparkling behind his heels comes Sirius.

Search the sky tonight for the brightest  star. But watch out, or you may pick a planet. Tonight, giant Jupiter in the eastern part of the sky is more than twice as bright as the brightest star. And Venus, far over in the west is more than twice as bright as Jupiter. You can tell that these two are planets because they shine with steady, unblinking light. A real star is a twinkler and Sirius, brightest of them all, sparkles like a dazzling diamond. Look for it this evening in giant Jupiter's part of the sky.

Seen from north of the equator, Sirius is the closest star to our Solar System. It is almost twice as big as the sun and more than twice as massive. With all that extra mass, you would expect Sirius to be a very dense star. But it is not.

The densest part of a star is the fiery core where particles are packed together under terrific pressure. It may be 20 or more times more dense than the rest of the star. This is taken into account when we figure the mean density of the whole star.

If we take the mean density of the sun to be 1.0, the mean density of Sirius is 0.3. We can translate this comparison into more understandable everyday terms. But first we must transport imaginary samples from each star to the earth, where they can be compared under the same gravity conditions,. A cupful of average material from the sun would weigh a little more than three cupfuls of average material from Sirius. The sample cupful from the sun would weigh 1 1/2 times as much as a cupful of ordinary water. The Sirius sample would equal the weight of a little less than half a cupful of water.

Sirius actually is a double star. The Dog Star we see is the big, bright feather weight. Its companion is a white dwarf star called, naturally, the Pup. It is almost as massive as the sun and all this material is crammed into the volume of a planet. The density of the Pup is 25,000 times greater than that of the sun and a pint of its material placed on earth would weigh around 20 tons.

Different stars vary in brilliance and size, color and temperature. But the greatest differences are in star densities. The material in a red giant star may be thinner than the gases in a light bulb. The density of big, red Antares is 20,000 times.less than ordinary air. Sirius and many other average stars range from somewhat less to somewhat more than the density of water. The densities of some of the white dwafrs are three and maybe four times greater than the density of the heavy little Pup.