The Magnetic Dwarf in Draco

Stars, like living organisms, have distinct life cycles. They are born, achieve maturity and then die--often in spectacular fashion. Death inevitably comes when a star has finally exhausted its nuclear fuel. As the stellar fires go out, the cooling gases begin to rush inward, falling toward the star's center under the force of its enormous gravity. For one class of stars, each about the mass of the sun, the end product of this gravitational collapse is a small, glowing cinder called a "white dwarf." Not much larger than the earth, it is so densely compressed that a cubic inch of its matter would weigh 1,000 tons or more on a terrestrial scale.

Scientists believe that some white dwarfs contain huge amounts of magnetic energy. They reason that if the original star had even a weak magnetic field, that field would be so squeezed during the collapse that its strength would rise enormously--just as the air pressure in a balloon increases if it is squeezed into a smaller volume. Trouble is, no one has ever been able to detect any significant magnetism in the 200 or so known white dwarfs. This failure has led to a great deal of uncertainty about widely held theories of stellar evolution.

Now, in a highly dramatic way, two University of Oregon scientists have helped dispel some of the doubts. Physicist-Astronomer James C. Kemp and a collaborating graduate student, J.B. Swedlund, have discovered that a white dwarf in the constellation Draco has an extraordinarily strong magnetic field--at least 20 million times more intense than the earth's and easily the strongest ever measured by man.

Swinging in a Circle. Why had astronomers previously failed to discover this giant celestial magnet? For one thing, magnetic stars are relatively rare. More important, even the closest stars are so distant that their magnetic fields can be detected only indirectly. In stars of normal density, this measurement is easy enough: magnetism alters the frequencies of starlight. Its effects can be seen on the star's spectrum as an odd splitting of spectral lines. But in the spectrum of an extremely dense white dwarf, the splitting is all but obscured. Reason: the white dwarf's tightly packed atoms collide so often that the frequencies of their emitted light overlap and blur the spectrum.

Looking for another way to detect the field, Kemp reasoned that magnetism should also influence the polarization, or the direction of the vibrations, of a white dwarf's light. Thus, he suggested, the light should not only vibrate in the ordinary linear way, but also exhibit circular polarization--much like a string that is simultaneously swinging in a circle and moving up and down.

Late last summer Kemp tested his theory at the University of Oregon's new 6,300-ft.-high Pine Mountain Observatory at the edge of the Cascade mountains. Using the observatory's 24-in. reflector and a new instrument that can measure circular polarization, he studied several white dwarfs but failed to detect any significant magnetic radiation. Then, one cloudy night last June, Kemp let his son Gary, 9, set the telescope's cross hairs on the Draco white dwarf. It was a fortunate decision.

When they analyzed the resulting readings, Kemp and Swedlund could scarcely believe their eyes. The figures indicated that the strength of the white dwarf's magnetic field was somewhere between 10 million and 30 million gauss (v. only about one-half gauss for the earth's and 100,000 gauss for the strongest fields detected around ordinary stars). Indeed, if a spaceship ever came within 1,000,000 miles of the star, it would be hopelessly stalled by its magnetic field. Still unconvinced, Kemp and Swedlund considered other factors--stray molecules in interstellar space, for example --that might have distorted the dwarf's light. But repeated observations produced the same results. Finally, Columbia University Astronomers Roger Angel and John Landstreet, told of the strange readings atop Pine Mountain, quickly verified them with more powerful telescopes and slightly different techniques at Arizona's Kitt Peak National Observatory.

There are implications of the discovery that go beyond the white-dwarf theory. Since the discovery of pulsars three years ago, most astronomers have come to agree that the strange signal-emitting objects are in fact neutron stars--dying stars that according to theory have collapsed with such force that all that remains is a ball of neutrons as small as ten miles across. With the help of Kemp's new technique for detecting distant magnetism, astronomers may now be able to substantiate their pulsar theories. If pulsars are indeed neutron stars--objects even more dense than white dwarfs--they should be even more powerful celestial magnets.

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