Million stars of crystal

For the first time astronomers find evidence for the crystallization of old white dwarfs

When white dwarfs have cooled down enough, their interior is crystallized - they become cosmic crystal balls. © University of Warwick / Mark Garlick
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Cosmic Crystal Balls: There could be millions of white dwarfs in our Milky Way made entirely of crystallized material. These star remnants have cooled down so far that carbon and oxygen have solidified inside. Evidence for this long-postulated phase transition has now been discovered by astronomers in the survey of 15, 000 white dwarfs in our galaxy, as reported in the journal "Nature".

White dwarfs are the remnants of low-mass stars like our sun. When these stars consume their fusion fuel, they first become the red giant and then they eject their outer shell in several explosions. What remains is the star core, in which carbon and oxygen ions are compressed into a plasma of extremely high density. As long as the white dwarfs are still young and hot, this plasma is a kind of liquid.

When the star core freezes

But that does not stay that way: According to theory, the continued cooling of the white dwarfs at a certain point leads to the phase transition. The carbon and oxygen ions in the star nucleus crystallize - the interior of the white dwarfs freezes. At the same time, heat of crystallization is released during this phase change, which temporarily stops or at least slows down the cooling of the white dwarf.

"It was predicted 50 years ago that we would have to observe an accumulation of white dwarfs with certain brightness and color, " says lead author Pier-Emmanuel Tremblay of the University of Warwick. In other words, considering the temperature distribution of white dwarfs in the near cosm, there must be a hump in the curve caused by crystallization. "However, there was no direct evidence of this effect so far, " say the researchers.

Jam in the temperature diagram

Now, however, Tremblay and his team have shown for the first time that there is this hump, and thus White Dwarfs actually crystallize. This was made possible only by the current star catalog of the European Gaia satellite. It contains data on around 1.7 billion Milky Way stars, including the brightness and distance data of tens of thousands of white dwarfs. For their study, the researchers analyzed 15, 000 of these star nuclei and created for the first time a comprehensive mass-temperature diagram. display

And indeed: In this so-called Hertzsprung-Russell diagram, a conspicuous "jam" of white dwarfs could be seen. Many star remains seemed to be in a certain temperature range, depending on their mass. "This mass-dependent congestion in the diagram results from the fact that the White Dwarves spend more time in this phase through the release of their latent heat during crystallization, " explain the astronomers. For some of these star remnants, the further cooling process is thereby delayed by up to two billion years.

Millions of cosmic crystal balls

"This is the first direct proof that white dwarfs crystallize and thus pass from the liquid to the solid state, " says Tremblay. "That also means that millions of White Dwarfs in our galaxy have already completed this process and become crystal balls." In fact, astronomers discovered a White Dwarf some years ago that came from Diamond crystallized carbon seems to exist.

Even our sun could become such a crystal ball of carbon and oxygen in about ten billion years. As the astronomers determined using a model, the oxygen will first crystallize and sink into the core of the white dwarf. By contrast, the lighter carbon accumulates in the outer region of the star's core. (Nature, 2019; doi: 10.1038 / s41586-018-0791-x)

Source: University of Warwick

- Nadja Podbregar