Weird floating crystals can stop stars ageing for billions of years

Some white dwarfs seem to stop ageing for billions of years, and this may be due to the behaviour of unusual ice crystals that heat up the stars.

Some white dwarf stars may be frozen in time because of strange floating crystals. The crystals seem to keep the stars from cooling as they age, so the white dwarfs look relatively young for billions of years.

Some massive white dwarf stars seem to stop ageing for billions of years. Now we may know why

NASA, ESA, P. McGill (Univ. of California, Santa Cruz and University of Cambridge), K. Sahu (STScI), J. Depasquale (STScI)

White dwarfs form when stars similar to our sun burn through all of their fuel and blow off their outer layers. This leaves behind a hot core, which cools over time and eventually freezes over. In 2019, astronomers found that some massive white dwarfs don’t seem to cool consistently, instead maintaining a constant temperature for 8 billion years or even longer.

Simon Blouin at the University of Victoria in Canada and his colleagues simulated the dynamics inside massive white dwarfs to figure out what could be causing this. The culprit could be a peculiar effect of freezing fluids with multiple different compositions, in which heavier elements get pushed out of the ice crystals as they form.

The same thing happens when sea water freezes, producing ice 10 times less salty than the liquid around it, says Blouin. In the white dwarfs, ice crystals of carbon and oxygen form and leave behind neon and other relatively heavy elements in the plasma.

Without those heavy elements, the crystals tend to float up towards the star’s surface, displacing fluid as they move, and eventually melting. This process generates heat, which keeps the entire white dwarf warm. The cycle continues for billions of years until the star’s centre contains nearly all heavy elements and its exterior has mostly light ones, at which point the whole thing slowly freezes.

“We think that there’s something similar going on in all white dwarfs, but probably at a smaller magnitude,” says Blouin. “Instead of stopping cooling for many billions of years, they maybe stop for 1 billion years.”

This could make it harder to measure the ages of cosmic objects. These long pauses in ageing could make it difficult to confirm how old any particular white dwarf is – which means some populations of stars may be much older than we thought.

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