Oxygen on a planet might be a sign of life, but in two peculiar white dwarf stars it could indicate a narrow escape from a violent death. Their oxygen content marks them as failed stellar bombs – the remnants of stars that almost went supernova.

The new stars are among thousands of white dwarfs picked up by the Sloane Digital Sky Survey. Like all white dwarfs they are the dead, cooling cores left behind by mainstream stars, and are mainly made of helium. Usually the second most plentiful ingredient is carbon – but when a group of astronomers led by Boris Gänsicke at the University of Warwick, UK, analysed the spectrum of light from these two white dwarfs, they found that the objects hold far more oxygen than carbon.

“It’s extreme – these things look very different from any white dwarfs we’ve seen before,” says team member Danny Steeghs.

Creating so much oxygen requires a nuclear furnace fiercer than that needed for a carbon-rich mixture, so the stars that spawned these white dwarfs must have been hot and massive. Simulations suggest that they must have been almost too big to end their days gently – any larger, and they would have grown a core so massive and dense that it would inevitably have collapsed, releasing enough energy to blow the rest of the star apart in a supernova explosion.


The critical mass needed to create such a supernova is thought to be between 7 and 10 times that of the sun. These almost-bombs might help astrophysicists to pin down the threshold more precisely, and Gänsicke hopes to use the Very Large Telescope in Chile’s Atacama Desert to get a clearer spectrum and reveal their chemistry in more detail. “It will give the theoreticians something to work with,” he says.

Journal reference: Science, DOI: 10.1126/science.1180228