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Extremely magnetic degenerate dwarf.

Extremely magnetic degenerate dwarf

White dwarfs can be fascinating. They are sometimes degenerate, and they are often shifted well to the red. Yet some of them can be very magnetic.

These white dwarfs do not lurk in dark alleys; they are a class of stars found in the distant reaches of our galaxy. White dwarfs are a unique venue for astrophysics. Matter there behaves in ways unattainable on earth. One instance of such behavior is the almost unbelievable magnetic field of 700 million gauss (MG), recently found in the white dwarf PG 1031+234 by a group of astronomers led by Gary D. Schmidt of the University of Arizona's Steward Observatory in Tucson. Schmidt told SCIENCE NEWS that he has never seen a theoretical calculation of the upper limit on a white dwarf's magnetic field, but he expects that 700 MG approaches the maximum possible strength for such a field.

The earth's field, which is approximately 1 gauss, is more typical of magnetic fields found in nature. The most powerful long-lasting magnetic fields in terrestrial laboratories go to a quarter-million gauss, and perhaps a little higher than that. "The physical structure of matter as we experience it does not permit the existence of fields like those we have found," says Schmidt. In other words, it pays to be degenerate, but even among magnetic white dwarfs, where conditions are extreme, fields tend to run no higher than tens of millions of gauss. Only one other star has a known magnetic field in this extreme range -- another degenerate white dwarf, catalogued as Grw +70 degrees 8247, which has about 300 MG.

An account of the determination of PG 1031 + 234's magnetic field will appear in the Oct. 1, 1986 ASTROPHYSICAL JOURNAL. Joining Schmidt in the work were Steven C. West and James Liebert of the Steward Observatory, Richard F. Green of Kitt Peak National Observatory in Tucson and H. S. Stockman of the Space Telescope Science Institute in Baltimore.

PG 1031+234 is unique in another important respect: It rotates, and does so very fast for a white dwarf or any other kind of star, going around once in three hours and 24 minutes. Rotation allows astronomers to study the structure of the star's field from many different perspectives. Grw+70 degrees 8247 does not rotate.

The structure of PG 1031+234's magnetic field is also somewhat complicated. There is a basic dipole structure similar to the earth's magnetic field, although the white dwarf's dipole is more widely angled to its rotation axis than is the case on earth. Added to the dipole is a large magnetic spot, a place where magnetic field lines emerge but continue straight out into space rather than bending around to another pole as a dipole field does. The spot is similar to the magnetic spots seen on the sun but is a great many times as strong.

Both the rotation and magnetic field appear to be relics of PG 1031+234's youth, eons before it became a white dwarf. White dwarf status comes at the end of a star's life. It results from a tremendous collapse under the influence of the star's own gravitational self-attraction. A star the size of the sun can collapse to the size of the earth.

Such a collapse causes degeneracy, which in physicists' language has nothing to do with morals or ethics. It merely means that a large number of things are jammed together into only a few available energy states. To put it another way, the familiar structures of atoms with their well-defined shells of orbiting electrons are crushed out of existence. What remains is a collection of closely packed nuclei swimming in a "sea" of detached electrons.

Rotations and magnetic fields that already exist can be conserved through such a collapse. The collapse will greatly increase the strength of the magnetic field and the speed of the rotation. Thus the extreme values present in PG 1031+234 could have arisen from relatively ordinary values in a relatively ordinary progenitor star. --D. E. Thomsen
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Title Annotation:white dwarf stars
Author:Thomsen, Dietrick E.
Publication:Science News
Date:Apr 19, 1986
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