Image: Hubble Space Telescope
A stellar swan song is a sight to behold. Gases and debris often swirl away from the scene in graceful loops and curves, shaping spectacular forms such as the Eagle nebulae, shown right. "Astronomers have been puzzling over these objects for centuries," says Adam Frank of the University of Rochester. "They're these vast cosmic sculptures and we've never known how they're made." In today's issue of Nature, however, Frank and his colleagues describe a new model to explain these heavenly beauties. At its heart are magnetic fields that twist and turn a star's radiating material into wispy, cloud-like shapes.
To create the model, Frank and Eric Blackman, who study planetary nebulae formation, teamed up with John Thomas, Andrew Markiel and Hugh Van Horn, who all specialize in the magnetic characteristics of stars. This paring of expertise paid off. "This paper probably couldn't have been written by any one of us," Thomas comments. "We had a hunch that our two areas were related to this problem, and it took both to figure it out." They began with data that suggests the core of a dying star decouples from its outer layer, like an egg shell separating from its yolk. What they found was that the two components begin to spin at different rates, powering up twisting magnetic fields around them.
The idea that magnetic fields help shape nebulae is not entirely new, but earlier models, based only on a dying star's outer shell, were unable to show how these fields gained sufficient strength to do so. The success of the new model comes from factoring in the core's influences as well. Lending further support to the new model is the observation that the leftover cores, or white dwarfs, tend to spin more slowly than would be expected. The scientists suggest that drag from the fields causes "magnetic breaking," stalling the dwarf's rotation. "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 adds. "This is potentially the kind of unifying concept that one seeks in science."