Steven Goulet, age 12, of Port Coquitlam, B.C., Canada, for his question:
WHAT IS A WHITE DWARF STAR?
On a clear night, we can count about 2,000 shining stars. But this is a mere handful of the total population. There are about 100 billion stars in our galaxy and billions more in other galaxies strewn throughout the universe. And this multitude of stars comes in different types, sizes and colors. Some are called white dwarfs.
Our sun is an average‑type star, about the same size and color as millions of others. Some of the non‑average types are whopping giants and supergiants. Others are dwarf stars, no bigger than some of the planets that orbit our average sun.
The first white dwarf star to be discovered belongs to Sirius, the dazzling Dog Star that follows Orion over the winter sky. The search began because Sirius has a slight wobble which suggests that it is pulled by the gravity of a nearby heavenly body.
In 1862 the Dog Star was found to have a small companion star. Naturally it was called the Pup. Though small, the Pup is white hot and much denser than an average star. Since then more than 100 other white dwarf type stars have been found, each one belonging to a much larger star.
Sirius is the brightest star in our skies. Its light takes almost nine years to reach us, which makes it one of the sun's closest neighbors. It is almost twice as big and twice as hot as our sun, and it sheds 30 times more light. The Pup is hotter and almost as heavy as the sun, yet not much bigger than the earth.
Though the big star Sirius is more than twice as massive as the little Pup, it is made of much lighter materials. A cubic meter of material in the Pup weighs 1,500 tons‑‑and a thimbleful is too heavy to lift. This great density is possible when electrons are stripped from gaseous atoms and the nuclei packed close together.
Sirius and the Pup are close enough to belong together and orbit around each other. However, the big star moves only slightly while its small companion orbits around a large oval orbit. Some astronomers suspect that a white dwarf is an old star that gradually collapsed, though nobody is sure.
We now find that astronomers of long ago referred to our sparkling white Sirius as a red star. Perhaps in those days it was outshone by a huge red companion. If this was so, the Pup may have collapsed in the past few thousand years. When the mystery is explained, we may know more about the history of all the white dwarf‑type stars.