White dwarf star enters its era of crystallization, turning into a ‘cosmic diamond’

By | June 11, 2023

To us, stars can look like carved jewels, shimmering cool against the velvety darkness of the night sky. And for some of them, that might actually be true.

As a certain type of dead star cools, it gradually hardens and crystallizes. Astronomers have found one that does just that in our cosmic backyard, a white dwarf composed mostly of carbon and metallic oxygen just 104 light-years away, whose temperature-mass profile suggests the star’s center is turning into a dense, hard, ‘cosmic diamond’ made up of crystallized carbon and oxygen.

The discovery is detailed in a paper accepted in the Monthly Notices of the Royal Astronomical Society and available on the arXiv prepress website.

“In this work we present the discovery of a new Sirius-like quadruple system 32 parsecs away, composed of a crystallizing white dwarf companion to the already known triple HD 190412,” writes an international team of astronomers led by Alexander Venner of the University of Southern Queensland in Australia.

“By virtue of its association with these main-sequence companions, this is the first crystallizing white dwarf whose total age can be externally limited, a fact we use by attempting to empirically measure a cooling delay caused by core crystallization in the white dwarf.”

All things in the Universe must change. Every star that hangs in the firmament, shining brilliantly with light generated by atomic fusion, will one day run out of fuel for their fires and evolve into something new.

For the vast majority of stars those below about eight times the mass of the Sun, and including the Sun that something is a white dwarf star.

When the fuel runs out, the star’s outer material is sucked into the surrounding space and the remaining core, no longer supported by the outward pressure provided by the fusion, will collapse into an ultradense object, the size of the Earth (or the Moon!) , but with a mass of 1.4 suns.

The matter in white dwarf stars is highly compressed, but is prevented from collapsing further by something called electron degeneracy pressure. No two electrons can occupy identical states, and this prevents the white dwarf from becoming even denser, as seen in a neutron star or black hole.

White dwarfs are dim, but still glow from residual heat. Over time, they cool and are expected to evolve into something called black dwarf stars when they lose all their heat and become a cold lump of crystallized carbon.

Calculations suggest that this process takes a long time, about a quadrillion years (or a trillion years); since the Universe is only about 13.8 billion years old, we don’t expect to find one any time soon.

What we can do is identify signs of crystallization starting in the cores of the white dwarfs we see around us.

During crystallization, the carbon and oxygen atoms inside the white dwarf stop moving freely and form bonds, arranging themselves in a crystal lattice. During this process, energy is released, which is dissipated as heat.

This produces a sort of plateau or slowdown in the cooling of the white dwarfs, which can be seen in the color and brightness of the star, making it appear younger than it actually is.

To accurately measure a star’s brightness, you need to know precisely how far away it is, which has been made much more possible in recent years thanks to the high-precision star mapping conducted by the Gaia mission.

This means that we can now identify crystallizing white dwarfs with much more confidence.

Venner and his colleagues were using Gaia’s data to search for multiple star systems, identifying stars whose association with others may not be clear.

And they found that a recently discovered white dwarf star (remember these things are very faint) was gravitationally bound to what was thought to be a system of three stars, called HD 190412.

The discovery of the white dwarf, now dubbed HD 190412 C, turned the triplet into a quadruple, but there was more to it. Its properties suggest that it is undergoing the crystallization process.

Whether or not that white nano crystal is a diamond is unknown; the density of white dwarfs is about 1 million kilograms per cubic meter, while the density of diamond is about 3,500 kilograms per cubic meter. There are allotropes denser than carbon; on the other hand, there are a lot of diamonds floating out there in space.

The other three stars in the system allowed the team to outwardly limit the age of the white dwarf, something that had never been done before for a crystallizing white dwarf.

The age of the system is approximately 7.3 billion years. The age of the white dwarf appears to be around 4.2 billion years. The discrepancy is 3.1 billion years, suggesting that the rate of crystallization slowed the white dwarf’s cooling rate by about 1 billion years, the researchers say.

By itself, dating isn’t enough to alter our models of white dwarf crystallization, but the discovery and its proximity to Earth suggest that there could be many other similar systems out there that we can exploit to compare this fascinating process.

“We propose that the discovery of this system at just 32 parsecs suggests that Sirius-like systems containing crystallizing white dwarfs are likely to be numerous.

“We conclude that the discovery of the HD 190412 system has opened a new avenue for understanding the crystallization of white dwarfs.”

The research was accepted in Monthly Notices of the Royal Astronomical Societyand is available on arXiv.

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