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March 2 (UPI) -- An unusual white dwarf star was formed by a merger between two smaller white dwarfs, according to a new analysis by an international team of astronomers. The ultra-massive white dwarf, WDJ0551+4135, is located 150 light years from Earth. It was first identified by the European Space Agency's Gaia telescope survey. Astronomers keen on studying especially large white dwarfs conducted a followup survey of the star using the William Herschel Telescope.
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Their observations -- detailed Monday in the journal Nature Astronomy -- revealed an unusual envelope surrounding the star, an atmosphere rich in carbon.
"This star stood out as something we had never seen before. You might expect to see an outer layer of hydrogen, sometimes mixed with helium, or just a mix of helium and carbon," lead study author Mark Hollands, an astrophysicist at the University of Warwick in Britain, said in a news release. "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."
White dwarfs are stars that were once like our sun, but have since shed their layers and contracted. Most white dwarfs are half as massive as our sun. The newly discovered white dwarf weighs 1.14 solar masses, but boasts a diameter two-thirds the size of Earth's.
Older white dwarfs orbit the Milky Way faster than younger white dwarfs, and this particular white dwarf is orbiting our galaxy extremely fast, despite not looking all that old.
The aging star's peculiarities forced astronomers to come up with another explanation for its origin.
"We have a composition that we can't explain through normal stellar evolution, a mass twice the average for a white dwarf, and a kinematic age older than that inferred from cooling," Hollands said. "We're pretty sure of how one star forms one white dwarf and it shouldn't do this. The only way you can explain it is if it was formed through a merger of two white dwarfs."
Models have previously predicted the merging of two white dwarfs, but most predicted one white dwarf to be larger than the other. Scientists estimate this particular white dwarf was created by the merging of two similarly sized white dwarfs.
As one star expanded, prior to contracting, it began swallowing up the second. Gravitational waves produced by interaction between the two stars caused their orbits to shrink and the two eventually became one.
The merging process restarts the cooling process, which may explain why the massive star is much older than it appears. Astronomers estimated the merger began roughly 1.3 billion years ago.
"There aren't that many white dwarfs this massive, although there are more than you would expect to see which implies that some of them were probably formed by mergers," Hollands said. "In the future, we may be able to use a technique called asteroseismology to learn about the white dwarf's core composition from its stellar pulsations, which would be an independent method confirming this star formed from a merger."
Scientists were surprised that such a massive merger between two aging stars didn't result in a supernova. By investigating why the duo didn't die an explosive death, researchers may be able to gain new insights into how and why certain stellar systems make it to the supernova stage.
