White dwarf stars are meant to be dead - but astronomers have just found some that are still alive

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Scientists have uncovered evidence that white dwarf stars - previously believed to be inert - may in fact simply be ageing much more slowly by burning hydrogen on their surface, challenging a major technique astronomers use to determine stellar ages.

Although the prevalent view of these stars is that they have burnt up their hydrogen, new research contradicts that assumption.

Observations by the Hubble Space Telescope suggests that white dwarfs can continue to undergo stable thermonuclear activity, according to the new paper published in Nature Astronomy.

Jianxing Chen, of the University of Bologna and the Italian National Institute for Astrophysics, who led the 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."

Because white dwarf stars are some of the oldest stellar objects in the universe, they offer scientists a good way to estimate the age of neighbouring stars.

But the new discovery could prompt a reassessment of how old some of the stars in the Milky Way are, as it means the cooling rate of a white dwarf is not necessarily the infallible clock it was once assumed to be.

At their core these stars are solid and made of oxygen and carbon due to what is called a phase transition - similar to water turning into ice, only at much higher temperatures.

Scientists have directly observed evidence of white dwarfs cooling into giant crystals.

Researchers at the University of Warwick believe our skies are filled with these enormous crystals, according to observations made with the European Space Agency's Gaia satellite.

Roughly 98% of all of the universe's stars will complete their lifecycles as white dwarfs, including our own sun, while more massive stars will collapse into neutron stars and black holes.

Astronomers have now compared cooling white dwarfs in two massive collections of stars - the globular clusters M3 and M13 - using the Hubble Space Telescope.

Analysing these clusters at near-ultraviolet wavelengths, the team compared more than 700 white dwarfs and found M3 contained standard white dwarfs which are simply cooling stellar cores.

But they found that M13 contains two populations of white dwarfs.

One population is of standard white dwarfs but another group which has somehow managed to hold on to an outer envelope of hydrogen, meaning they burn for longer and cool more slowly.

The researchers compared their results with computer simulations and found that roughly 70% of the white dwarfs in M13 were burning hydrogen in these envelopes on their surfaces.

Francesco Ferraro, also of the University of Bologna and the Italian National Institute for Astrophysics, aid: "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."

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