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Winter-Spring 2001
Vol. 63, No. 2-3

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HEAVENLY LIGHT SHOW EXPLAINED

The cosmic paintbrush that creates some of the most dazzling images in the night sky may have been found by a team of Rochester astrophysicists, a paper in the January 25 issue of Nature suggests.

A magnetic dynamo similar to the kind that produces storms on our sun appears to shape planetary nebulae, the wispy clouds of light that radiate from some dying stars.

The Rochester team has shown that as some stars die and expel their outer layers, they generate massive magnetic fields that twist the radiating material into beautiful and distinctive shapes.

"Astronomers have been puzzling over these objects for centuries," says Adam Frank, associate professor of physics and astronomy. "They're these vast cosmic sculptures and we've never known how they're made."

Seeking to solve the puzzle, the team designed a model based on a typical star approaching the last years of its life. Using data suggesting that the core of such a star separates from its outer shell like a yolk spinning inside an egg, the researchers found that magnetic fields power up and twist as the two spin at different rates. As matter is blown off the dying star, it roughly follows these bent magnetic lines, creating the majestic, gauzy curves and contours of a planetary nebula.

The idea that magnetic fields play a role in this phenomenon has been addressed before, but researchers had looked only at the activity of the outer shell, concluding that it could not generate fields of the necessary strength. It took a combination of astronomers working in two different areas to define the new model and show that such strength is possible.

"This paper probably couldn't have been written by any one of us," says John Thomas, professor of mechanical and aerospace sciences and of astronomy. "We had a hunch that our two areas were related to this problem, and it took both to figure it out."

Joining Thomas in the collaboration were Andrew Markiel and Hugh Van Horn, who, like Thomas, are specialists in understanding the magnetic characteristics of stars, and Adam Frank and Eric Blackman, experts in planetary nebulae formation.

The new model is reinforced by another well-known phenomenon. The leftover core of these dying stars, called a white dwarf, is known to spin more slowly than scientists have thought it should.

The Nature paper suggests that the core is slowed by "magnetic braking"-a sort of drag that's produced by the magnetic fields twisting up like a wrung towel that gets harder and harder to twist. As this is happening, the surface of the white dwarf is thrown out into space along those magnetic lines, slowing its rotation further-much as skaters' spins slow when they extend their arms.

"The dynamo-generated magnetic field that we've proposed may explain many other phenomena of planetary nebulae, such as the launching of the stellar wind," Thomas notes.

"This is potentially the kind of unifying concept that one seeks in science."

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