Could dying stars hold the secret to looking younger? New evidence suggests that white dwarfs could continue to burn hydrogen in the final stages of their lives, causing them to appear more youthful than they actually are. This discovery could have consequences for how astronomers measure the ages of star clusters.

«We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity,» explained Jianxing Chen of the Alma Mater Studiorum Universita 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.»

White dwarfs are the slowly cooling stars which have cast off their outer layers during the last stages of their lives. They are common objects in the cosmos; roughly 98% of all the stars in the Universe will ultimately end up as white dwarfs, including our own Sun [1]. Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages as well.

To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13 [2]. These two clusters share many physical properties such as age and metallicity [3] but the populations of stars which will eventually give rise to white dwarfs are different. In particular, the overall colour of stars at an evolutionary stage known as the Horizontal Branch are bluer in M13, indicating a population of hotter stars. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool.

«The superb quality of our Hubble observations provided us with a full view of the stellar populations of the two globular clusters,» continued Chen. «This allowed us to really contrast how stars evolve in M3 and M13.»

Using Hubble’s Wide Field Camera 3 the team observed M3 and M13 at near-ultraviolet wavelengths, allowing them to compare more than 700 white dwarfs in the two clusters. They found that M3 contains standard white dwarfs which are simply cooling stellar cores. M13, on the other hand, contains two populations of white dwarfs: standard white dwarfs and those which have managed to hold on to an outer envelope of hydrogen, allowing them to burn for longer and hence cool more slowly.

Story Source: Materials provided by ESA/Hubble Information Centre. Note: Content may be edited for style and length.