Astronomers have discovered an ultramassive white dwarf, piecing together its origins as a result of a collision between two smaller white dwarf stars.

A team of astronomers from the University of Warwick led by Mark Hollands, have discovered an ultramassive white dwarf with a unique carbon-rich atmosphere never before observed around such a body. The researchers have concluded that the star — located around 150 light-years from Earth — is the product of a collision between two-parent white dwarfs which both narrowly escaped complete destruction. The finding represents the first time that the origins of such a merged white dwarf have been identified by the composition of its atmosphere and with the aid of some other important clues.

“We found a white dwarf with a unique surface composition of hydrogen and carbon. This star stood out as something we had never seen before,” Dr Hollands, from Warwick’s Astronomy and Astrophysics group and author on a paper detailing the findings published in the journal Nature Astronomy, says. “This is not possible to explain from the stellar evolution of a single star, as we would also expect to see a lot of helium.”

Artist’s impression of two white dwarfs in the process of merging. Depending on the combined mass, the system may explode in a thermonuclear supernova, or coalesce into a single heavy white dwarf, as with WDJ0551+4135. ( University of Warwick/Mark Garlick)

Hollands is alluding to the fact that astronomers expect to see an outer layer of hydrogen, sometimes mixed with helium, or just a mix of helium and carbon when they observe white dwarfs. “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.”

After ruling out single star evolution as the genesis of the massive white dwarf — designated WDJ0551+4135 — Hollands says the team was forced to conclude that it formed as the result of a merger between two white dwarfs.

The discovery of the merger process birthing a massive white dwarf has the potential to raise new questions regarding the evolution of such stars and on our understanding of just how common supernovae are within the Milky Way. But first, the team’s suspicions had to be confirmed, and that would require more clues to be collected.