Astronomers discover a new type of star, a hot subdwarf covered in helium-burning ash

Artist's impression of a rare kind of stellar merger event between two white dwarf stars. (Nicole Reindl)

Astronomers studying stellar evolution have stumbled on an unexpected discovery: a new type of star that seems to break the mold of star formation.

Most stars have surfaces made up of hydrogen and helium, but these new stars observed by German astronomers are covered in carbon and oxygen, the ashes left over from helium fusion, something a press release called “an exotic composition for a star.”

While carbon and oxygen can be brought to the surface by a sudden, brief resurgence of explosive helium fusion, our current understanding of that process doesn’t work for these new stars.

That’s because the size and temperature of these new stars indicates that helium is still burning steadily at the core, according to a paper published in the Monthly Notices of the Royal Astronomical Society.

The stars were spotted as part of a program searching for short-lived, hot stars called subdwarfs, to understand the final stages of a star’s life.

Researchers expected to largely detect stars that had already completed their helium fusion in their core and were in the process of turning into a white dwarf — a small, dense, dim star left behind after a medium-sized star has ballooned into a red giant and gone supernova.

So these hot subdwarfs covered in the “helium-burning ash” were an unexpected discovery, and constitute a “new spectroscopic class.”

This new combination of factors could mean that the star was born through a very unique process.

“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,” said Miller Bertolami of the Institute for Astrophysics of La Plata in a press release. Bertolami is lead author of a second paper published alongside the first, which offers a potential explanation for the strange stars.

White dwarfs can collide when they exist in close binary systems due to shrinking orbits. But a hot subdwarf of this nature would be a new outcome.

“Usually, white dwarf mergers do not lead to the formation of stars enriched in carbon and oxygen,” Bertolami explained. “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.”

The second paper states that they ran simulations to see if this merger would work, and that simulations found that such a merger could create a star with similar characteristics.

But although it provides a hypothesis for what may have led to this type of star, our current models can’t be fully sure. Researchers stated that better models looking at how mergers between stars work will help pinpoint the true origin of these new stars.

“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,” Klaus Werner, a professor at the University of Tubingen and head of the team that discovered this new type of star. “These new stars are a severe challenge to our understanding of stellar evolution.”