White dwarf stars are common in the galaxy, but astronomers have found one that doesn’t seem to obey the rules. They think it was born when two smaller white dwarfs merged together

White dwarf stars are common in the galaxy, but astronomers have found one that doesn’t seem to obey the rules. They think it was born when two smaller white dwarfs merged together

By Yaz Ashmawi
When stars collide
University of Warwick/Mark Garlick
A bizarre white dwarf is likely the result of a dramatic merger between two ancient stars.
White dwarfs are stars in the final stage of their life cycle. They begin as average-mass stars just like our sun. As they burn through fuel in their youth, they shed their outer layers and leave small, dense hot cores behind, and these faint stellar embers slowly cool.
These dwarfs are common throughout the universe, but now Mark Hollands at the University of Warwick, UK and his colleagues have discovered a star 150 light years away from Earth with some very peculiar characteristics.
It has a mass of about double the average for a white dwarf, and it also appears to be moving through the Milky Way extremely quickly faster than 99 per cent of its neighbours.
The most surprising thing of all is its outer atmosphere, which has high levels of carbon and very low levels of hydrogen and helium. These arent the kinds of properties you would expect to see when looking at a conventional white dwarf, says Hollands.
All these quirks point to one conclusion: this white dwarf simply cant have been made through normal star formation. Instead, it must be the product of a collision over a billion years ago between two ancient average sized white dwarfs, says Hollands.
The team suggests that these stars started out orbiting one another in a combined binary system but as they aged they drew closer and closer together until they merged into one.
In order to be completely sure, the astronomers will need to carefully observe tiny changes in the surface brightness of the star, which can tell us more about its core. A star which is the result of a merger should have a different central structure than one formed in more ordinary circumstances, says Hollands.
The find could have important implications for our understanding of the structure of the universe. White dwarfs explode into a supernova when they die, and astronomers use these blasts to determine how far away they are.
Over the last decade, there have been renewed questions about what triggers these events says Hollands. The fact that this collision didnt result in an explosion could tell us something about how and why white dwarfs go supernova in the first place, and help us produce more accurate maps of our galactic neighbourhood.
Journal reference: Nature Astronomy, DOI: 10.1038/s41550-020-1028-0
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