White Dwarfs can Continue Burning Hydrogen, Even After They’re Dead

White dwarfs are supposed to be dead remnants of stars, doomed to simply fade away into the background. But new observations show that some are able to maintain some semblance of life by wrapping themselves in a layer of fusing hydrogen.

White dwarfs are the dense leftover cores of sunlike stars. They are exposed to the universe when stars build up too much carbon and oxygen in their centers. They then tear themselves apart in a slow, agonizing process that eventually creates a planetary nebula. With all the outer layers stripped, the core remains – a white dwarf.

White dwarfs are inert. They don’t have enough mass to ignite nuclear fusion of their carbon and oxygen, and so they just sit there, slowly radiating away their heat for eons upon eons.

But new research is challenging that simple story.

“We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity,” explained Jianxing Chen of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who led this research. “This was quite a surprise, as it is at odds with what is commonly believed.”

The research team led by Chen compared two globular clusters, M3 and M13, with the Hubble Space Telescope. They especially focused on near-ultraviolet wavelengths, which allowed them to compare more than 700 white dwarfs inside the two clusters.

They found that the white dwarfs in M3 were pretty standard, but that M13 had two different populations: one set of normal white dwarfs, and another that were enveloped by an outer layer of hydrogen.

Combining the results with computer simulations, Chen and his colleagues determined that around 70% of the white dwarfs in M13 had so much hydrogen surrounding them that the shells were actively undergoing fusion. That fusion is able to keep the white dwarfs warmer for far longer than astronomers had previously thought possible.

Astronomers use established cooling time relationships to connect the temperature of a white dwarf to its age, which helps them better understand their origins and environment. The new research makes that relationship a little murkier.

“Our discovery challenges the definition of white dwarfs as we consider a new perspective on the way in which stars get old,” added Francesco Ferraro of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who coordinated the study. “We are now investigating other clusters similar to M13 to further constrain the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly.”