Astronomers discover a new type of star covered in helium burning ashes

Astronomers have discovered a puzzling new type of star that is covered in the ashes of burning helium.

Outlining their findings in Monthly Notices of the Royal Astronomical Society, the team of German astronomers who first glimpsed these strange new heavenly bodies say the surfaces of these stars are unlike any other that has ever been observed.

In place of the normal stellar surface of hydrogen and helium, these newly spotted stars are covered in carbon and oxygen – the ashy byproducts of helium burning. Adding to this astronomical riddle, the stars are of a size and temperature that indicates that they’re still burning helium within their cores, which is a stellar property usually only seen in stars of a much more advanced evolutionary stage than these.

“Normally we expect stars with these surface compositions to have already finished burning helium in their cores, and to be on their way to becoming white dwarfs,” explains lead author Professor Klaus Werner from the University of Tübingen.

“These new stars are a severe challenge to our understanding of stellar evolution.”

In an accompanying paper, a separate group of astronomers from the University of La Plata and the Max Planck Institute have risen to the challenge, offering the first possible explanation for how such a star may form – and the posited circumstances are just as exciting as the new stars.

“We believe the stars discovered by our German colleagues might have formed in a very rare kind of stellar merger event between two white dwarf stars,” says Dr Marcelo Miller Bertolami of the Institute for Astrophysics of La Plata, lead author of the second paper.

White dwarfs are stars that have burnt up all their nuclear fuel and shed their outer layers, typically collapsing inwards to become very small and dense. When two of these stars orbit each other closely in a binary system, their mutual orbit tends to collapse inwards as they radiate gravitational waves, and mergers are not uncommon.

But until now, all known white dwarf mergers have resulted in predictable stellar compositions. Astronomers now believe that these surprising new surfaces might be the product of two white dwarfs with very different compositions colliding.

“Usually, white dwarf mergers do not lead to the formation of stars enriched in carbon and oxygen,” explains Miller Bertolami, “but we believe that, for binary systems formed with very specific masses, a carbon and oxygen-rich white dwarf might be disrupted and end up on top of a helium-rich one, leading to the formation of these stars.”

But despite this tentative explanation, no current stellar models can fully account for the surface properties of these new stars.

The team will now work to refine their stellar models to assess the likelihood of their proposed merger situation, hoping to understand not just these strange new stars but also the intricacies of the late evolutionary stages of binary star systems.

Until astronomers develop these more refined models, the origins of the helium-covered stars will remain up for debate.