Astronomers and planetary scientists are guilty of constantly anthropomorphizing space objects and giving in to the temptation to see familiar shapes in the stars. The way we steal the language of what it means to be alive can sometimes get a bit disturbing though. We talk about stars as being alive when they have nuclear reactions going on in their cores, and we call them dead when they don’t have nuclear reactions in their cores.
For about a hundred years, we’ve referred to white dwarfs as dead stars. These moon-sized objects are the leftover cores of Sun-like stars that have run out of fuel for their core’s nuclear reactions.
The thing is that new Hubble Space Telescope images seem to show undead white dwarf stars with nuclear burning on their surfaces. This was discovered while astronomers were looking at the evolution of stars in the globular clusters M3 and M13. Remarkably, 70% of the white dwarfs in M13 appeared to be hotter than they should be, and this difference in temperature was consistent with the stars holding on to some residual hydrogen on their surfaces, where their extreme gravity allowed that hydrogen to undergo nuclear burning, and thus heat up the star. Why is this happening? We can’t actually tell what theories the scientists had because the article is hidden behind a paywall, but the why isn’t actually the important part here.
The important part is this nuclear burning means our understanding of how white dwarfs cool is only right some of the time, and that means we could be completely misjudging the ages of a variety of objects in the galaxy.
White dwarfs were thought to form at a specific temperature and spend the rest of eternity slowly cooling in ways that can be understood thanks to standard thermodynamics. The physics is something a first- or second-year graduate student can do as a homework assignment, and for those white dwarfs, when you measure their temperatures you can calculate their age. Easy peasy if you’re a physics or astronomy grad student at least. If some fraction of white dwarfs aren’t completely dead, but are instead only mostly dead and are actually burning some hydrogen on the surface, those cooling rates don’t work. And when we try and calculate their age, they will appear to be far hotter – and thus younger – than they would be without that burning.
This is a new frustration for astronomers. There are literally undead white dwarfs out there mucking up their age estimations.
Pamela is going to work on getting us this paper and hopes to be writing an epilogue to this story for our website that will explain if these stars are thought to be re-animated or simply not quite dead yet. Keep an eye out on DailySpace.org.
ESA Hubble press release
“Slowly cooling white dwarfs in M13 from stable hydrogen burning,” Jianxing Chen et al., 2021 September 6, Nature Astronomy