Bizarre, ultra-massive white dwarf star is likely two that merged together, astronomers say

It's known as WDJ0551+4135 and appeared to be a massive white dwarf. White dwarf stars are essentially the dead remains of stars like our sun, once they've burned through all of their hydrogen fuel and outer layers.

Usually, they're small and lightweight -- about 0.6 times the mass of our sun, which weighs 4.18 nonillion pounds. This one didn't fit the definition. It was 1.14 times the mass of our sun. That's nearly twice the average mass of a white dwarf star.

Astronomers first spotted the star in data collected by the European Space Agency's Gaia space telescope, launched in 2013.

More data was collected by the William Herschel Telescope on the island of La Palma in Spain, which helped the astronomers analyze the light coming from the star. This breakdown helped them spot another oddity: The star's atmosphere contained a lot of carbon.

The study including these findings published Monday in the journal Nature Astronomy.

"This star stood out as something we had never seen before," said Mark Hollands, lead study author and postdoctoral researcher at the University of Warwick's department of physics.

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

Next, they investigated its age.

When it comes to stars, the older ones actually orbit faster than younger ones. They determined that this star was moving faster than 99% of any other white dwarfs close by, making it older than it first appeared.

The astronomers began to question if the unusual, ultra-massive white dwarf was more than one star, especially due to the puzzling amounts of hydrogen and carbon they detected -- likely from two stars, rather than one.

"We're pretty sure of how one star forms one white dwarf, and it shouldn't do this," Hollands said. "The only way you can explain it is if it was formed through a merger of two white dwarfs."

Binary star systems are two stars that orbit around each other, not unlike the twin suns we see in the sky over Tatooine in "Star Wars: A New Hope."

When one star in a binary system dies, its death actually draws the second star closer because the first star is shrinking and their orbits grow smaller. Eventually, the same thing will happen to the second star. Billions of years along in this process, the distance between the stars will shrink until they merge.

But WDJ0551+4135's merger is unlike any other, since these stars would have been the same size as they merged. Typically, mergers occur between stars with different mass.

Predicted white dwarf mergers also have a limit. Anything more than 1.4 times the mass of our sun would be expected to explode in a supernova.

Yet, these stars merged, created an ultra-massive white dwarf instead of exploding. The researchers believe the merger likely occurred 1.3 billion years ago, meaning both stars could have existed for billions of years before their merger

It's only one of a few merged white dwarf star pairs ever known, and the only one with this particular composition. But future observations may reveal more.

"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. "Maybe the most exciting aspect of this star is that it must have just about failed to explode as a supernova."

Hollands explained that supernovae help astronomers map the universe because they can be detected from such great distances. But this failed supernova has its own unique story to tell, revealing how massive white dwarf stars can become and survive to tell the tale.