Odd Little Stars

By Phil Berardelli
ScienceNOW Daily News
21 November 2007

It seems as though every time astronomers point their telescopes at the night sky, some weird new finding forces them to revamp their theories. And so it is with nine newly discovered white dwarfs. The stars defy their expected chemical makeup and by rights shouldn't even exist. An explanation could open up a new branch of astronomy.

White dwarfs earn their moniker by being quite small, astronomically speaking. They start out as normal stars, but over billions of years, they expand into red giants before exhausting their energy and collapsing into objects not much bigger than Earth. Until this year, all known white dwarfs followed this pattern, and they all boasted atmospheres consisting of either hydrogen or helium, which can be easily identified by the spectral lines of their respective light.

Not so the nine discovered by an international team and reported in tomorrow's issue of Nature. Plucked from millions of stars and galaxies analyzed over the past 7 years by the Sloan Digital Sky Survey, this bunch burns considerably cooler than normal and contains atmospheres made entirely of carbon, with no traces of hydrogen or helium. Astronomers don't have a clue why. Usually, a star produces excess carbon when it is about to shut down the nuclear-fusion cycle that keeps it burning. No fusion means gravitational collapse followed by a supernova explosion that splashes the star all over its galaxy.

So why are these white dwarfs still around? One possibility, notes astronomer and lead author Patrick Dufour of the University of Arizona, Tucson, is that the stars simply might not have grown massive enough--about 10 times heavier than the sun--to explode but are so close to the limit that they might be harboring abnormally high amounts of carbon. The unique chemical signature of the stars may provide clues to what's going on. "It tells us that nature has found a way that we didn't know to make white dwarf stars without the usual hydrogen or helium surface layers," Dufour says.

A whole new class of carbon-dominated white dwarf stars is a "major discovery," says astronomer Pierre Bergeron of the University of Montreal in Canada. It's something that occurs only once in a decade or so, he says, and the underlying process governing these white dwarfs "will launch an entirely new field of research." Astronomer Klaus Werner of the Institute for Astronomy and Astrophysics in T?bingen, Germany, agrees. "There is currently no explanation how such stars can be formed," he says. "It's a real challenge to stellar-evolution theory."