About 150 light-years away from us, there’s a star that just doesn’t add up.
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.”