However, white dwarfs burning hydrogen could cause these age estimates to be inaccurate by as much as 1 billion years. This relatively straightforward relationship between age and temperature has led astronomers to use the white dwarf cooling rate as a natural clock to determine the ages of star clusters, particularly globular and open clusters. The evolution of white dwarfs has previously been modelled as a predictable cooling process. This discovery could have consequences for how astronomers measure the ages of stars in the Milky Way. 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.Ĭomparing their results with computer simulations of stellar evolution in M13, the researchers were able to show that roughly 70% of the white dwarfs in M13 are burning hydrogen on their surfaces, slowing down the rate at which they are cooling. They found that M3 contains standard white dwarfs which are simply cooling stellar cores.
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.
"This allowed us to really contrast how stars evolve in M3 and M13."
WHITE DWARF MAGAZINE STAFF FULL
"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 makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool. 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. These two clusters share many physical properties such as age and metallicity but the populations of stars which will eventually give rise to white dwarfs are different. 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. Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages as well. 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. White dwarfs are the slowly cooling stars which have cast off their outer layers during the last stages of their lives. "This was quite a surprise, as it is at odds with what is commonly believed." "We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity," explained Jianxing Chen of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who led this research.
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