Billions of white dwarf stars are crystallising, say astronomers

White dwarf star in the process of solidifying

White dwarf star in the process of solidifying. Credit Dr Mark A. Garlick

The pile-up suggests white dwarfs experience a phase change when large amounts of heat dissipate from the stellar core. Thanks to the precise measurements that it is capable of, we have understood the interior of white dwarfs in a way that we never expected.

Astronomers at the University of Warwick have found first direct evidence of white dwarf stars solidifying into crystals and our skies are filled with them. "We believe this is due to the oxygen crystallizing first and then sinking to the core, a process similar to sedimentation on a riverbed on Earth".

It's what happens next that fascinates Pier-Emmanuel Tremblay, a physicist at the University of Warwick in England, whose paper, published this week in the journal Nature, examines what happens to these star corpses, or "stellar embers". If you were keeping track with a thermometer, you would find that the temperature of water stalls at zero degrees Celsius for a bit - the exact time that the H20 molecules are rearranging themselves into the crystal structure of ice.

Tremblay said their studies suggested that white dwarfs stop their cooling by turning from liquid to almost 99 per cent solid over about 1.5 billion years. It is estimated that in some cases these stars have slowed down their aging by as much as 2 billion years, or 15 percent of the age of our galaxy. At some time in their evolution, all white dwarfs will crystallize with more massive ones dying faster than the small ones.

My assumption scientists build on the analysis of more than 15 thousand white dwarfs, distant from the Earth at a distance of 300 light years. As for our Sun, the researcher and his colleagues estimate that it will undergo the crystallization process in around 10 billion years, give or take. Before it begins to solidify and harden gradually, the sun will turn into a red giant, and from that moment it will begin to shrink down and turn into a white dwarf.

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'The sun itself will become a crystal white dwarf in about 10 billion years'. These data revealed an odd "pileup" - an overabundance of white dwarfs with certain colors and brightnesses that can not be explained by the objects' ages or masses.

"Not only do we have evidence of heat release upon solidification, but considerably more energy release is needed to explain the observations", added Tremblay.

After this contraction the star can still create energy by fusing helium to create carbon and oxygen, Tremblay said.

"We saw a pile-up of white dwarfs of certain colors and luminosities that were otherwise not linked together in terms of their evolution", said Tremblay.

"It's exciting how scanning stars across the sky and measuring their properties can lead to evidence of plasma phenomena in matter so dense that cannot be tested in the laboratory".

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