Space shock: Unique ultra-massive white dwarf discovered – ‘makes no sense’

A unique ultra-massive white dwarf star could be key to understanding the structure of the universe, scientists have announced. The star has “unusually high” levels of carbon in its atmosphere.

This cannot be explained through the normal process of a star’s evolution, according to a new study.

We have a composition that we can’t explain through normal stellar evolution

Dr Mark Hollands

The star instead appears to have formed when two white dwarfs merged into each other approximately 1.3 billion years ago.

The white dwarf failed to explode as a supernova, as would normally be expected.

Scientists believe the star, slightly larger than the Sun, could help experts understand how stars merge.

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This marks the first time astronomers have identified a merged white dwarf by using its atmosphere as a clue.

Dr Mark Hollands, from the University of Warwick’s department of physics, and lead author of the study, said: “We have a composition that we can’t explain through normal stellar evolution.

“The only way you can explain it is if it was formed through a merger of two white dwarfs.”

White dwarfs are the remnants of stars that have burnt out all their fuel and shed their outer layers.

Dubbed WDJ0551+4135, the white dwarf was spotted by astronomers trawling data from the European Space Agency’s Gaia telescope that was launched into space in 2013.

Researchers then used the William Herschel Telescope located in the Canary Islands to analyse light emitted by the white dwarf to identify its chemical composition.

They discovered the star’s atmosphere had an unusually high level of carbon.

Dr Hollands said: “This star stood out as something we had never seen before.

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“You might expect to see an outer layer of hydrogen, sometimes mixed with helium, or just a mix of helium and carbon.

“You don’t expect to see this combination of hydrogen and carbon at the same time as there should be a thick layer of helium in between that prohibits that.

“When we looked at it, it didn’t make any sense.”

Most white dwarfs are approximately 0.6 times the mass of our Sun.

The astronomers found this one to have 1.14 solar masses, despite being only about two-thirds the size of the Earth.

Based on its orbit around the Milky Way, astronomers believe the star is older than it looks.

The white dwarf’s age is hard to understand as the merger restarted the cooling process usually used to estimate its age.

Although the merging likely occurred about 1.3 billion years ago, the individual stars could have existed for many billions of years prior.

Dr Hollands added: “Maybe the most exciting aspect of this star is that it must have just about failed to explode as a supernova – these gargantuan explosions are really important in mapping the structure of the Universe, as they can be detected out to very large distances.

“However, there remains much uncertainty about what kind of stellar systems make it to the supernova stage.

“Strange as it may sound, measuring the properties of this ‘failed’ supernova, and future lookalikes, is telling us a lot about the pathways to thermonuclear self-annihilation.”

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