London, November 22 : Astronomers have discovered white dwarf stars with pure carbon atmospheres, which they say possibly evolved in a sequence that was unknown before.

The astrologers say that the dwarf stars identified by them may have evolved from stars that are not quite massive enough to explode as supernovae but are just on the borderline.

Scientists have always believed that most white dwarf stars have a core made of carbon and oxygen which is hidden from view by a surrounding atmosphere of hydrogen or helium, but they never expected stars with carbon atmosphere.

"We've found stars with no detectable traces of helium and hydrogen in their atmospheres," Nature magazine quoted University of Arizona Steward Observatory astronomer Patrick Dufour as saying.

"We might actually be observing directly a bare stellar core. We possibly have a window on what used to be the star's nuclear furnace and are seeing the ashes of the nuclear reaction that once took place," he added.

The stars identified are among 10,000 new white dwarf stars that were found in the Sloan Digital Sky Survey (SDSS).

Astronomy Professor James Liebert had identified a few dozens of the newfound white dwarfs as "DQ" white dwarfs in 2003. When observed in optical light, DQ stars appear to be mostly helium and carbon.

Dufour developed a model to analyse the atmospheres of DQ stars as part of his doctoral research at the Universite de Montreal. His model simulated cool DQ stars, stars at temperatures between 5,000 degrees and 12,000 degrees Kelvin.

"When I first started modeling the atmospheres of these hotter DQ stars, my first thought was that these are helium-rich stars with traces of carbon, just like the cooler ones. But as I started analyzing the stars with the higher temperature model, I realized that even if I increased the carbon abundance, the model still didn't agree with the SDSS data," Dufour said.

He revealed that his team had identified eight carbon-dominated atmosphere white dwarf stars among about 200 DQ stars they checked in the Sloan data so far.

According to him, the carbon carbon-atmosphere stars are found only between about 18,000 degrees and 23,000 degrees Kelvin, but the researchers are yet to unravel the mystery as to why so happens.

“These stars are too hot to be explained by the standard convective dredge-up scenario, so there must be another explanation," Dufour said.

Astronomers predicted in 1999 that stars nine or 10 times as massive as our sun would become white dwarfs with oxygen-magnesium-neon cores and mostly carbon-oxygen atmospheres. More massive stars explode as supernovae.

However, scientists are not sure whether stars eight, nine, 10 or 11 times as massive as the sun are required to create supernovae.

"We don't know if these carbon atmosphere stars are the result of nine-or-10 solar mass star evolution, which is a key question," Liebert said.

The UA astronomers plan making new observations of the carbon atmosphere stars at the 6.5-meter MMT Observatory on Mount Hopkins, Ariz., in December to better pinpoint their masses. (ANI)