The stars could still be burning hydrogen in their outer layers
White dwarfs could be lying about their age by hiding behind a youthful veil of hydrogen, a new study has found.
These small, dense stars represent the final stage in the life cycle of most stars, when they have thrown off their outer layers of hydrogen, leaving only the core behind. The current understanding of white dwarfs is that all fusion within them has stopped and that they are cooling down over time. This creates a strong link between a star’s temperature and its age, which astronomers make use of to date how old stars are. However, a new set of observations made with the Hubble Space Telescope has thrown this relationship into doubt.
“We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity,” says Jianxing Chen from Alma Mater Studiorum Università (AMSU), who led the study. “This was quite a surprise as it’s at odds with what is commonly believed.”
The study compared the white dwarfs in two globular clusters, M13 and M3. The clusters are similar in age and metallicity (how many elements they have other than hydrogen and helium), but have differences in the stellar populations that are expected to go on to become white dwarfs. That makes them the perfect testing grounds to see how these stars cool over time.
“The superb quality of our Hubble observations provided us with a full view of stellar populations in the two globular clusters,” says Chen. “This allowed us to really contrast how stars evolve in M3 and M13.”
Chen and his team were able to see that M13 contained a population of stars that were still burning hydrogen in their outer layers, which slowed their cooling, making them appear more youthful than they actually are.
“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, also from AMSU, who coordinated the study. “We’re now investigating other clusters that are similar to M13 to further constrain the conditions that drive stars to maintain the thin hydrogen envelope which allows them to age slowly.” www.hubblesite.org
IMAGES: ESA/HUBBLE/NASA AND G PIOTTO ET AL