Unusual giant white dwarf that failed to explode as a supernova offers clues to understanding the universe

The unusual dwarf -- dubbed WDJ0551+4135 -- might be the product of two stars smashing into each other over 1.3 billion years ago


                            Unusual giant white dwarf that failed to explode as a supernova offers clues to understanding the universe
Artist's impression of two white dwarfs in the process of merging (University of Warwick/Mark Garlick)

An oddball giant white dwarf, 150 light-years away from us, holds clues to understanding how stars explode into a supernova, says a new study.

The unusual dwarf -- dubbed WDJ0551+4135 -- might be the product of two dead stars smashing into each other over 1.3 billion years ago. Catching scientists by surprise was its size -- it is gigantic, measuring 1.14  times heavier than the sun, even surpassing that of its counterparts. "The discovery was completely serendipitous," Dr. Mark Hollands, from the University of Warwick Department of Physics, tells MEA WorldWide (MEAWW).

White dwarfs are corpses of low or moderate mass stars. When these stars run out of fuel, they eventually lose their outer layers, ending up with just their cores. The star has something even more unusual: its atmosphere. Using the Gaia spacecraft, the team studied the light emitted out of the dwarf. This spacecraft has been tasked to explore and map out our Milky Way galaxy.

The light holds information about the white dwarf's atmosphere. In it they found high amounts of carbon and hydrogen, together in harmony. While white dwarfs have reserves of carbon, hydrogen, and helium, scientists do not expect to see carbon and hydrogen coexisting in this manner. For comparison, the atmosphere in a burning star like our Sun is primarily made of hydrogen and helium. Carbon comes into the picture much later -- when the star dies. 

"We now know of a new class of white dwarf with carbon-hydrogen atmospheres, and so it would be good to try to find more of these to see if there a largely similar, or if there is a lot of variation between them," explains Dr. Holland.

Dr Holland and his colleagues also looked at the dwarf's age: older stars orbit the Milky Way faster than younger ones, and this object is moving faster than 99% of the other nearby white dwarfs. This suggests that this star is older than it looks.

Building on the data gathered on its composition and size, the team concluded that two average-sized white dwarfs could have orbited each other in the beginning. With time, they began getting closer until they fused into one gigantic white dwarf named WDJ0551+4135.

An artist's illustration of a supernova. (NASA)

What does it tells us about supernova?

White dwarfs that merge together are capable of detonating, triggering a massive explosion. And scientists predict that a star with a mass greater than 1.14 times that of the sun should go supernova. This is because stars as heavy could crumble and explode under its weight. But this white dwarf featured in the study, oddly, has not exploded. Despite its size, the white dwarf continues to survive to this day, explains the study.

Because it did not explode, Holland believes that studying why it failed to blow itself up could help scientists understand more about those that do.

"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," explains Holland.

The study has been published in Nature Astronomy.

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