Science: Dimmest Dwarf

Colleagues sometimes tease Astronomer Willem J. Luyten of the University of Minnesota by calling him a “stellar mortician” because of his passionate interest in dying stars. Luyten does not mind the ribbing; the faint pinpoints of light that he studies are the end products of stellar evolution and hold many secrets of the universe. Recently, Astronomer Luyten found the dimmest star yet: a minuscule “white dwarf that emits 50,000 times less light than the sun, yet probably contains an equal or greater mass. “This one,” he says, “looks to be at the end of the line.”

The Companion. The first white dwarf was found when mid-19th century astronomers noticed that Sirius, the brightest star in the sky, wobbles slightly, and theorized that it revolves around another star too close and dim to be seen separately. Later astronomers, using more sophisticated telescopes to eliminate the glare, finally picked out the other star nestling close to Sirius, and gradually accumulated some surprising information about it.

Studying the Companion’s orbit around Sirius, they proved that its mass is 96% of the sun’s, yet it gives 400 times less light. At first they thought that it was an average, sun-sized star that gives less light because of low temperature. But by 1915 astronomers were able to prove that its surface is really hotter than the sun’s and gives three times as much light per square inch. If a star’s surface is bright but the star as a whole gives off little light, then the only possible conclusion is that the star must be small. The Companion turned out to be only about 25,000 miles in diameter, and into this comparatively modest volume the star’s whole sunlike mass had to be crammed. The astronomers’ amazing conclusion: the Companion of Sirius is made of material that weighs 2,000 lbs. per cubic inch.

Degenerate Matter. Though seemingly incredible, these figures for the Companion have withstood all attacks, and astronomers, particularly Dr. Luyten, have since found many white dwarfs even smaller and denser. The current theory is that they are stars that have burned nearly all their hydrogen, turning it by nuclear fusion into helium and heavier elements. With the hydrogen gone, the star contracts. As its mass concentrates into a smaller volume, its gravitational field increases in power, eventually growing strong enough to compress the material near the star’s center into “degenerate” matter whose electrons and nuclei have been pushed close together.

Dr. Luyten does not know definitely the size or mass of his latest white dwarf, but he believes that it weighs at least ten tons, or 20,000 lbs., per cubic inch. It could conceivably weigh as much as 1,000 tons per cubic inch, in which case a chunk of star no bigger than a grapefruit would weigh more than the 84,000-ton Queen Elizabeth.

The ultimate fate of a white dwarf, says Dr. Luyten, is to grow slowly dimmer and smaller. After billions of years, its light will change from white to yellow, then to red. Eventually it will die, and the product will be a black dwarf: a cold sphere of degenerate matter weighing as much as the sun, but smaller than most planets and giving no light at all.