Rare white dwarf will reveal secrets of star evolution

Typical white dwarfs, like this one, have an atmosphere dominated by hydrogen and helium, light gases that float to the top and block the star from view (NASA, ESA, H. Bond (STScI) and M. Barstow (University of Leicester))

Astronomers will be able to gain major insights into the evolution of stars after discovering the first dead star in the universe that is not concealed by a thick layer of hydrogen and helium.

When stars run out of fuel and start to die, they collapse inwards, making them considerably hotter and boiling away enormous quantities of gases.

This leaves just the hot, dense, core of the star – known as a white dwarf. These typically have an atmosphere dominated by hydrogen and helium – gases which, being lighter than heavier elements such as oxygen and carbon, float to the top of the star’s atmosphere and block it from the view of scientists.

But researchers from Brazil and Germany have discovered a white dwarf that contains no hydrogen or helium whatsoever. This will enable astronomers to study the exposed core of the star unimpeded by these gases, providing a unique view on the lives of stellar bodies in their final stages of existence. It could change the way scientists think about the evolution of stars and what happens when these stellar objects die.

“We only found one, so it is a rare event. This star will provide a rare observational test of the evolutionary path towards white dwarfs,” said Professor Kepler de Souza Oliveira Filho of the Federal University of Rio Grande do Sul in Brazil, who led the study.

The white dwarf – known as Dox – is situated about 1,200 light-years away in the constellation Draco and has an atmosphere that is 99.9 per cent oxygen.

The researchers are unsure why Dox differs from other dead stars, but they do have a couple of theories.

One possibility is that the gravitational pull of a nearby star may have stripped Dox of its lighter elements, such as hydrogen and helium.

Alternatively, the star may have had one final rally of carbon burning, sending out a huge pulse of burning gas and plasma into space, taking the lighter elements with it.

Dox was discovered as part of a project to map the entire universe with the help of a 2.5-metre wide-angle optical telescope.

The star is the only one of the 32,000 white dwarfs that have been discovered with an atmosphere of this composition, according to the research, which was published in the journal Science.