Billions of white dwarf stars are crystallising, say astronomers

January 10, 2019

For the first time astronomers have directly observed evidence of white dwarf stars solidifying 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.

Stars like our own sun which aren't large enough to collapse into black holes will become white dwarfs once they reach the end of their life-cycle.

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

The discovery, led by Dr Pier-Emmanuel Tremblay and published in the journal Nature, means that white dwarf stars could be potentially billions of years older than previously thought.

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.

The team in Warwick studied 15,000 white dwarf candidates within around 300 light years of Earth and measured their colour and luminosity for clues about their composition.

What they found surprised them.

There was a "pile-up", or "an excess in the number of stars at specific colours and luminosities that do not correspond to any single mass or age".

These excess corresponded to a particular time in the evolution in a star in which they are expected to slow their cooling process and thus age more slowly.

Dr Tremblay said: "This is the first direct evidence that white dwarfs crystallise, or transition from liquid to solid.

"It was predicted 50 years ago that we should observe a pile-up in the number of white dwarfs at certain luminosities and colours due to crystallisation and only now this has been observed.

"All white dwarfs will crystallise at some point in their evolution, although more massive white dwarfs go through the process sooner.

"This means that billions of white dwarfs in our galaxy have already completed the process and are essentially crystal spheres in the sky.

"The sun itself will become a crystal white dwarf in about 10 billion years," added Dr Tremblay, reminding us that these crystals - although a new discovery - are the usual end for stars.

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