Moon-sized crystal revealed in star’s heart

(New Scientist) A crystal the size of our Moon has been revealed hidden at the centre of a dying star, after an analysis of pulsations in the star’s brightness. The measurement confirms indirect evidence that the cores of all such dying stars – called white dwarfs – solidify as they cool. The discovery may improve age estimates of our galaxy, of which white dwarfs provide an important constraint.

Astronomers led by Travis Metcalfe of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, studied the white dwarf BPM 37093, the only known pulsating white dwarf in our galaxy thought likely to have produced a crystal core.

They used a worldwide network of telescopes to keep the white dwarf constantly in sight for about two weeks, and then studied how the star dimmed and brightened by about one per cent every five to 10 minutes.

The pulsations, caused by convection in the outer non-solid part of the star, are the only way to determine directly if the core has solidified. As the star cools and the crystal core grows, the period of the pulsations gets smaller.

Researchers compared models of the crystal core to the observations to determine that 90 per cent of the white dwarf’s mass, similar to that of the Sun, appears to be a giant, single crystal.

Stripped electrons

But the crystal, which has been likened to a diamond, is in fact unlike any known on Earth. The pressure inside the white dwarf is a million million times the pressure that produces diamonds. This pressure strips electrons from the atoms, leaving the nuclei to form a crystal lattice surrounded by a sea of electrons.

The researchers tested two models, one involving a pure carbon core and the other a pure oxygen core – the latter fitted the observations best. This agrees with theory, which suggests the core is roughly equal parts carbon and oxygen, but with more oxygen concentrated at the crystal’s centre because it crystallises more readily.

White dwarfs, unlike younger stars, are no longer undergoing nuclear fusion. So their predicted cooling curves are more or less understood, making them useful clocks. But the process of forming a solid crystal releases heat into the white dwarf, muddying predictions by one billion to two billion years.

This uncertainty propagates into age estimates for white dwarfs and our galaxy. “Improving our understanding of objects like this is crucial,” says Martin Barstow, an astrophysicist at the University of Leicester, UK.

“We need to know when and how crystals form to calibrate cooling methods, so the age is as accurate as possible,” Metcalfe told New Scientist.