"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."
The group of scientists designed a model based on a typical star as it approached the last years of its life. Using data that suggests that the core of such a star decouples from the outer shell of the star 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 curves and contours of a planetary nebula.
The idea that magnetic fields play a role in shaping the material thrown off by dying stars has been addressed before, but the researchers had only looked at the activity of the outer shell, concluding that it could not generate fields of the necessary strength. It took a combination of University astronomers working in two different areas to define the new model and show that such strength is possible.
Astrophysicists John H. Thomas, Andrew Markiel and Hugh Van Horn, specialists in understanding the magnetic characteristics of stars, teamed up with astrophysicists Adam Frank and Eric Blackman, experts in planetary nebulae formation. "This paper probably couldn't have been written by any one of us," says 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."
The new model is reinforced by another well-known phenomenon. The leftover core of such 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 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 a skater's spin slows when she extends her 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," says Thomas. "This is potentially the kind of unifying concept that one seeks in science." The implications of the research reach beyond nebulae. The same processes that generate the magnetic fields in dying stars are also at work in our own sun.
"This shows us a new way for our sun to die," says Frank. Sunspots, solar storms and coronal mass ejections that endanger power grids and satellites are all directly caused by magnetic fields generated in the sun. When our sun runs out of fuel in a scant four or five billion years, we may be treated to front-row seats as its insides knot up and it sheds its skin in graceful arcs wider than our solar system.