Waltz with Detonations

Except for varying brilliance, all sources of stellar light look much the same to the naked eye. But seen through the subtle, prying instruments of modern astronomy, those distant points of light expand into a bewildering variety of stars. Among the strangest are the dwarf novas, described by Astronomer Robert P. Kraft of Mount Wilson and Palomar observatories in the Astrophysical Journal.

Dwarf novas are dim stars that have the strange habit of flaring up at irregular intervals--increasing their brightness almost 100-fold. Astronomers have often speculated about these periodic changes, but until Dr. Kraft used the great 200-inch Palomar telescope to follow 20 dwarf novas through many bright and dim cycles, no one was sure what caused them. Using telescope and spectrograph, Dr. Kraft kept track of the novas' changing temperature, light and motion. After 30 months he was able to prove that at least seven of them are double stars. The two bodies whirl around each other every few hours, moving up to 165 miles per second, 24 times the speed needed to fling a rocket free of the earth's gravitation.

One of each pair of stars, Dr. Kraft thinks, is probably a white dwarf: a star that has burned so thoroughly that it now consists chiefly of "degenerate" matter, denser than anything known on earth. This remarkable stuff weighs thousands of pounds per cubic inch. The nova's degenerate core is extremely hot, but its surface is covered with a thin, rather cool layer of normal matter. The other star of each pair is all normal matter, mostly hydrogen, and just about the same weight and size as the sun. In many cases, Dr. Kraft is sure the two stars are almost in contact, the white dwarf dancing just above the surface of its big, fluffy partner.

As the larger star whirls through its tight orbit, it spins hydrogen off its surface. Some of this gas is attracted by the white dwarf's intense gravitation. When the layer thickens, some of the hydrogen is forced down into contact with the star's degenerate core, which is as hot as the heart of an exploding H-bomb. Suddenly a nuclear reaction races through the hydrogen, turning it into helium and releasing a vast amount of energy. The little dwarf star flares up. many times brighter than its great partner. Once the crisis is over the stars waltz peacefully through space once more, waiting for the dwarf to accumulate hydrogen for another bright detonation.

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