SYDNEY: Astronomers have discovered a completely new class of star based on eight unusual examples of burned-out celestial objects detected in our Milky Way galaxy.
Experts argue that these white dwarf stars might represent an unidentified stage in star development. "They could eventually teach us a lot about how massive stars evolve and die," said Patrick Dufour, an astrophysicist at the University of Arizona, USA, who made the discovery.
Out of fuel
A star undergoes many radical changes throughout its lifespan including the inevitable exhaustion of its fuel source. An exhausted star will evolve into a neutron star, a black hole or a white dwarf – depending on its mass.
Black holes and neutron stars are formed when a large star collapses, while white dwarfs are the product of small- or medium-sized stars like our Sun. Despite their lack of fuel, white dwarfs continue to radiate heat for billions of years.
Once a white dwarf cools down and no longer emits significant heat or light, it will enter what is likely to be its last stage of existence, becoming a dark mass known as a 'black dwarf'. Scientists believe that the universe – despite being 13.7 billion years old – is still too young for any black dwarfs to exist.
Much of what we know about stellar evolution is based on the observation of a star's atmosphere. Eighty per cent of known white dwarfs have a hydrogen-rich atmosphere, while most others have a helium-rich atmosphere.
Stellar atmosphere
A paper published today in the British journal Nature reports the discovery of a new type of white dwarf star distinguished by its carbon-dominated atmosphere. "Nobody ever thought this could exist and it will be a challenge to explain how they form," said lead author Dufour.
The researchers studied data from the Apache Point Observatory in New Mexico and found eight white dwarf stars with unexpected mass, temperature and chemical composition. Using these measurements the team concluded that the eight stars were rich in carbon instead of the usual hydrogen or helium.
The stark contrast between these eight exceptional white dwarfs and those previously studied has intrigued astronomers. The most logical explanation, according to Dufour, is that the unusual atmosphere is caused by an original mass larger than most white dwarfs, but not quite large enough to explode as a supernova like massive stars do.
Lilia Ferrario is a researcher from the Australian National University in Canberra who studies white dwarfs. "It's just not clear what happens when a star is about to die," she said. "This research will be quite helpful particularly in regards to the final stages of star death."
