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11 January 2019, 09:13 | Edward Lowe
An artist's illustration of a white dwarf in the process of solidifying. Credit University of Warwick Mark Garlick
"In the future, these objects will be completely dominant".
Tremblay and his colleagues analyzed data gathered by the European Space Agency's Gaia spacecraft, which launched in December 2013 to help researchers construct the best-ever 3D map of the Milky Way. And because white dwarfs are among our cosmos' oldest stellar objects, with predictable life stages, astronomers often use them as "clocks" to date surrounding groups of stars.
Currently, our sun is about half way through the main sequence phase, which means it creates energy by fusing hydrogen into helium in its core. "It is assessed that now and again these stars have slowed down their aging by as much as 2 billion years, or 15 percent of the age of our cosmic system".
After that, the white dwarfs turn into black ones that give no heat. The work verified earlier predictions that such a catalog of white dwarf stars would reveal a number in the slow process of turning from what is essentially a liquid form to a solid state.
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"Before Gaia we had 100-200 white dwarfs with precise distances and luminosities - and now we have 200,000", Tremblay said, giving the satellite the bulk of the credit for the research team's discovery.
"We realised that this was not a distinct population of white dwarfs, but the effect of the cooling and crystallisation predicted 50 years ago".
"It's like taking a hot coal out of a fire and letting it cool off into the night", said JJ Hermes, an astronomer at the University of Boston who worked on the study.
For the first time astronomers have directly observed evidence of white dwarf stars solidifying into giant crystals.
Just like any other fluid that solidifies in cold enough temperatures, so will those liquified nuclei will when temps in the core cool down enough and releases enough for them to crystallize. "It was anticipated fifty years back that we should observe a pile-up in the number of white dwarfs at specific radiances and colors because of crystallization and only now this has been observed".
This pileup, or traffic jam in the data suggests that at around the same point in the cooling process, the stars simply stop getting colder.
The only explanation for this is that these stars have an extra energy source, said Tremblay.
Once the crystal arrangement is in place, the ice will continue to cool at a more or less steady rate until it reaches the same temperature as the environment in the freezer. Red dwarfs, the most common type of star, keep burning until they have transformed all their hydrogen into helium, turning into a white dwarf. We believe this is due to the oxygen crystallising first and then sinking to the core, a process similar to sedimentation on a river bed on Earth.
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