White dwarfs are slowly cooling stars that have abandoned their outer layer during the end of their life. 98% of all stars in the Universe ultimately end up as white dwarfs. Studying these cooling stages helps astronomers understand not only white dwarfs but also their earlier stages as well.
Based on the observations from the NASA/ESA Hubble Space Telescope, an international group of astronomers have discovered the first evidence that white dwarfs can slow down their rate of aging by burning hydrogen on their surface. The evidence suggests that white dwarfs can still undergo stable thermonuclear activity.
Jianxing Chen of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who led this research, said, “This was quite a surprise, as it is at odds with what is commonly believed.”
To understand the physics reinforcing white dwarf evolution, astronomers compared cooling white dwarfs in two globular clusters M3 and M13.
Chen said, “The superb quality of our Hubble observations provided us with a full view of the stellar populations of the two globular clusters. This allowed us to contrast how stars evolve in M3 and M13.”
The observation of clusters at near-ultraviolet wavelengths allowed astronomers to compare more than 700 white dwarfs. They found that M3 contains standard white dwarf, which is cooling stellar cores. On the other hand, M13 contains two populaces of white dwarfs: standard white diminutive people and those that have managed to hold on to an external envelope of hydrogen, allowing them to burn for longer and cool more slowly.
When the team compared their results with computer simulations, it was found that almost 70% of white dwarfs in M13 are burning hydrogen on their surfaces. This slows down the rate at which these hydrogen-burning white dwarfs are cooling.
Francesco Ferraro of the Alma Mater Studiorum Università di Bologna said, “Our discovery challenges the definition of white dwarfs as we consider a new perspective on how stars get old. We are now investigating other clusters similar to M13 to constrain further the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly.”
- Chen, J., Ferraro, F.R., Cadelano, M. et al. Slowly cooling white dwarfs in M13 from stable hydrogen burning. Nat Astron (2021). DOI: 10.1038/s41550-021-01445-6