Direct evidence of white dwarf crystallisation found

A cutaway showing an artist’s impression of a white dwarf after undergoing crystallisation. Image: University of Warwick/Mark Garlick

Astronomers have found the first direct evidence supporting a five-decade-old idea that white dwarfs, the compact, burned-out remnants of stars like our Sun, solidify into solid spheres of oxygen and carbon crystals as their cores slowly cool in the absence of nuclear fusion. This crystallisation, similar to water changing into ice but at much higher temperatures – about 10 million degrees Celsius – slows down the evolution of white dwarfs, making them appear up to two billion years younger than they actually are.

“This is the first direct evidence that white dwarfs crystallise, or transition from liquid to solid,” said Pier-Emmanuel Tremblay of the University of Warwick. “It was predicted fifty 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.”

White dwarf stars are among the oldest in the universe, the final evolutionary state of suns without enough mass to drive core collapse to neutron star densities. Instead, compact cores roughly the size of a terrestrial planet are left behind, slowly radiating away their stored heat over billions of years.

It is estimated that up to 97 percent of the 100 to 400 billion stars making up the Milky Way will eventually turn into white dwarfs while their much more massive cousins collapse to form neutron stars and, at the extreme end of the scale, black holes. The Sun is expected to become a white dwarf when its core runs out of nuclear fuel in about five billion years.

But it will take another five billion years or so to turn into a solid crystalline sphere, Tremblay says.

A star’s evolutionary path is determined by its mass. Stars like the Sun will end up as white dwarfs in the absence of nuclear fusion to offset the inward pull of gravity. Much more massive stars pass beyond that state, collapsing into neutron stars or black holes. Image: European Space Agency

Using data from the European Space Agency’s Gaia spacecraft, Tremblay’s team selected 15,000 white dwarf within 300 light years of Earth and analysed their luminosities and colours. They found a “pile-up” of stars at specific luminosities and colours that did not correspond to a single mass or age.

Comparing the data with evolutionary models, the researchers saw the pile-up strongly coincided with the period when such stars would be expected to release large amounts of latent heat, slowing down the cooling process.

In the collapsed core of a white dwarf, atoms are packed so tightly that electrons become “unbound,” forming a conductive gas in a fluid of positively charged nuclei. When temperatures in the core drop to around 10 million C, the fluid can begin to solidify.

“Not only do we have evidence of heat release upon solidification, but considerably more energy release is needed to explain the observations,” Tremblay said. “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. This will push the carbon upwards, and that separation will release gravitational energy.”

Tremblay said the team’s research would not have been possible without the ultra-precise data collected by Gaia.

“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. Before Gaia we had 100-200 white dwarfs with precise distances and luminosities – and now we have 200,000. This experiment on ultra-dense matter is something that simply cannot be performed in any laboratory on Earth.”