A recent article published in Nature Astronomy details evidence that some white dwarfs stay warmer for longer due to the continued nuclear burning (i.e. nuclear fusion) of hydrogen on their surfaces:
By analysing high-resolution photometric data of two very similar Galactic globular clusters (M3 and M13), we find a clear-cut and unexpected overabundance of bright WDs in M13. Theoretical models suggest that, consistent with the horizontal branch morphology, this overabundance is due to a slowing down of the cooling process in ~70% of the WDs in M13, caused by stable thermonuclear burning in their residual hydrogen-rich envelope.Slowly cooling white dwarfs in M13 from stable hydrogen burning, Nature Astronomy (open access PDF)
White dwarfs are the remains of dead stars and are formed once nuclear fusion ends in the star’s core. We normally think of white dwarfs as being relatively inert, with little or no nuclear fusion happening as the star gradually cools.
Now, by looking at the brightness of white dwarfs in M13 (pictured above) the authors of this paper have found that a lot of them are brighter (and so hotter) than expected. This seems to be because many of the white dwarfs are blanketed in a layer of hydrogen which is still undergoing nuclear fusion. The cooling rate of white dwarfs is used to estimate the age of globular clusters, like M13, so this discovery could have implications for how those calculations are done.
Featured image credit: Sid Leach/Adam Block/Mount Lemmon SkyCenter