BEIJNG, Nov. 22 (Xinhuanet) -- Scientists found eight white dwarfs located in the Milk Way galaxy between about 1,000 and 2,000 light years from Earth, which represent a previously unknown category of stars, media reported Thursday.

Patrick Dufour, astrophysicist Arizona University of U.S, said previously known white dwarfs have fallen into two categories: those with a hydrogen-rich atmosphere and those with a helium-rich atmosphere. But Dufour and three other scientists, writing in the journal Nature, described eight white dwarfs that break the mold by possessing carbon atmospheres.

The researchers think they may have formed from stars much more massive than the sun but not quite massive enough to explode as a supernova.

"It was totally unexpected because all of the white dwarfs we knew so far were either hydrogen-rich or helium-rich. So this is a completely new kind of star," Dufour said.

The scientists have spotted at least a couple of other white dwarfs that might fit into this new category, Dufour added. All of them were among about 10,000 new white dwarfs recently identified in an extensive survey effort.

After blowing off their outer layers, white dwarfs typically leave behind a core of carbon and oxygen that is cloaked by a surrounding atmosphere of hydrogen or helium. The eight newly described ones have atmospheres primarily of carbon, with little or no trace of hydrogen or helium.

"It will be a challenge to try to explain how they form and what does this tell us about stellar evolution," Dufour said.

They might have evolved from a star similar to a unique one called H1504+65, the researchers said. Astronomers think this star violently expelled all its hydrogen and nearly all its helium, leaving behind a stellar nucleus with a surface half carbon and half oxygen.

(Agencies)