Star Theory Gets New Spin
The Small Magellanic Cloud features a young cluster, NGC346, with its many hot stars.
Credit: A. Nota (ESA/STScI) et al., ESA, NASA

The spinning of stars was thought to work like a food processor, bringing material from deep inside to the surface and mixing everything in a predictable way. The kitchen-science analogy just broke down.

Instead, turning up the spinning speed can yield a layer cake or marbled effect, a team of scientists finds.

To some extent, all stars are thought to spin. Cooler stars spin the slowest, with the sun rotating at about 4,500 mph (7,200 kph), while the massive hotties can whip around at more than 600,000 mph (1 million kph).??

The rapid rotation is thought to produce currents that move material from a star's core, including fusion-produced elements such as nitrogen, to the surface.

"Current models of star rotation could be compared to a food processor," said lead researcher Ian Hunter of Queen's University of Belfast in Northern Ireland. "As you turn up the speed, the mixing between the layers of a star becomes more thorough and more nitrogen should be visible at the surface."?

However, Hunter found more surface nitrogen than expected on 20 percent of slow spinners and less than expected on 20 percent of rapid rotators. The researchers studied more than 800 young stars in two nearby galaxies, the Large and Small Magellanic Clouds.

"The food processor model doesn't seem to be working, or at least can't explain the whole picture," Hunter said. His results were presented this week at the Royal Astronomy Society's National Astronomy Meeting in Ireland.

The slowpokes may have large magnetic fields that make them rotate slowly while providing a mixing mechanism, said Hunter's colleague Philip Dufton of Queens University. "This is, however, very speculative," Dufton stressed.

The researchers are even more puzzled by the dearth of surface nitrogen in the fast rotators.

"The only idea that we have is that they could be part of a binary system (two stars orbiting each other)," Dufton told, "and that a recent interaction between them has spun up the star. [So] for most of its life the star rotated slowly and no mixing occurred." He notes, the research has yet to find any evidence of such star companions.