Hubble discovers hydrogen-burning white dwarfs enjoying slow ageing

Astronomers have discovered the first evidence that white dwarfs can slow down their rate of ageing by burning hydrogen on their surface.

The finding challenges the prevalent view of white dwarfs – stars that have burned up all of the hydrogen they once used as nuclear fuel – as inert, slowly cooling stars.

The observations come from the Nasa/ESA Hubble Space Telescope.

Jianxing Chen of the Alma Mater Studiorum, University of Bologna, and the Italian National Institute for Astrophysics, who led this research, said: “We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity.

“This was quite a surprise, as it is at odds with what is commonly believed.”

White dwarfs are the slowly cooling stars that have cast off their outer layers during the last stages of their lives.

They are common in the cosmos, and roughly 98% of all the stars in the universe will ultimately end up as white dwarfs, including the sun.

Studying the 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.

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 M3 contains standard white dwarfs which are simply cooling stellar cores.

While M13 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 therefore cool more slowly.

Comparing their results with computer simulations, 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 suggest the discovery could have consequences for how astronomers measure the ages of stars in the Milky Way.

Previously the evolution of white dwarfs has been modelled as a predictable cooling process.

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.

However, white dwarfs burning hydrogen could cause these age estimates to be inaccurate by as much as one billion years.

Francesco Ferraro of the Alma Mater Studiorum, University of Bologna, and the Italian National Institute for Astrophysics, who co-ordinated the study, said: “Our discovery challenges the definition of white dwarfs as we consider a new perspective on the way in which stars get old.

“We are now investigating other clusters similar to M13 to further constrain the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly.”

The findings are published in Nature Astronomy.