Unusual White Dwarf Star With Never-before-seen Features Discovered: 'When We Looked at It, It Didn't Make Any Sense'

Researchers have discovered an unusually massive white dwarf star that may have formed when two white dwarfs merged together.

These objects are the small, dense remnants of stars nearing to the end of their lifecycles, which have shed their outer layers, leaving behind the stellar core. They are extremely dense—according to NASA, a teaspoon of white dwarf material would weigh about 4 billion tons.

Most white dwarfs tend to have a mass around 0.6 times that that of our own sun, yet they are only about the size of Earth.

A newly discovered white dwarf, however, posed a puzzle for astronomers. According to a study published in the journal Nature Astronomy, the star—known as WDJ0551+4135—was 1.14 solar masses, despite being smaller than the Earth. It was found to have an unexpectedly carbon-rich atmosphere, with a composition that puzzled the authors and is older than it appears, the researchers say.

"When we looked at it, it didn't make any sense," Mark Hollands, lead author of the study from the University of Warwick in the U.K., said in a statement.

"We identified an unusual white dwarf with an atmosphere composed of hydrogen and carbon," Hollands told Newsweek. "Normally we would expect these two elements to be separated by a thick layer of helium, implying there isn't very much of this element. No other white dwarf have been found before that looks like this, and our understanding of our how single stars transition into single white dwarfs does not predict anything like this."

"The only viable alternative is that WDJ0551+4135 formed from a merger. This is also supported by the large mass—twice the average—as well as the fast speed of the white dwarf (older stars tend to move faster than older ones) implying WDJ0551+4135 is older than it looks when only considering its temperature," he said.

white dwraf merger
Artist's illustration of two white dwarfs merging. University of Warwick/Mark Garlick

According to the researchers, the two stars may have once been part of a binary system, before eventually being drawn into each others gravitational influence and merging. They say that the latest finding shed new light on the evolution of massive white dwarfs, as well the phenomenon of supernovae—cataclysmic explosions which sometimes occur when white dwarfs collide with other white dwarfs.

"Within the Milky Way we see many examples of double white dwarfs, some of which we think will eventually merge as their orbital separation shrinks," Hollands said. "In some cases the combined mass will be sufficient to trigger runaway nuclear fusion resulting in a supernova, which are observed in distant galaxies, and used to map the universe."

He continued: "Not all should explode though, with the less massive mergers instead coalescing into very heavy white dwarfs. The difficulty is that normally it is not possible to distinguish these massive merged white dwarfs from those formed from very heavy normal stars."

According to the researchers, WDJ0551+4135 was seemingly on the verge of destruction, just about failing to explode as a supernova.

Scientists have long thought that merging white dwarfs only produce a supernova if the resulting star has a mass greater than 1.4 suns, as it will collapse under its own weight. However, in the last decade or so there has been more evidence to suggest mergers can produce supernovas at smaller masses—at around 1.2 solar masses.

Because WDJ0551+4135 weighs in at 1.14 solar masses, the latest results may provide a constraint on the mass at which nuclear burning can start, according to Hollands.