Carolyn Burnett, age 12, of Barre, Vt., for her question:

EXCLUDING THE SUN, WHICH IS THE BRIGHTEST STAR IN THE HEAVENS?

Sirius, often called the Dog Star, is the brightest star in the heavens excluding the sun. It is one of the stars nearest to earth, but it is still so far that its light takes nearly nine years to reach the earth.

The bright Sirius or Dog Star can be seen by looking along an imaginary line pointing southward through Orion's belt. It is the head of the constellation Canis Major, the Great Dog.

Sirius is a star of the first magnitude and radiates about 30 times as much light as the sun. Astronomers discovered its companion star in 1862. This star's gravity explained the fact that Sirius seemed to move in a wavy line. The two stars move around each other as they travel through space. The companion star is one of the most remarkable stars in the sky because the material in it is 50 ,000 times as heavy as water.

Material from the companion star would weigh about 1,500 tons per cubic foot if brought to the earth's surface.

Sirius' companion was the first "white dwarf" star discovered. Many more white dwarfs have since been found, some with densities 500,000 times that of water.