Stars are celestial bodies that have fascinated humans for centuries. They appear as cut jewels, glittering coldly against the velvet darkness of the night sky. However, as stars cool, some of them gradually harden and crystallize. In recent years, astronomers have discovered a white dwarf composed primarily of carbon and metallic oxygen just 104 light-years away, whose temperature-mass profile suggests that the center of the star is transforming into a dense, hard, ‘cosmic diamond’ made up of crystallized carbon and oxygen.

The Discovery

The discovery of the crystallizing white dwarf star is detailed in a paper accepted into the Monthly Notices of the Royal Astronomical Society and available on preprint website arXiv. An international team of astronomers led by Alexander Venner of the University of Southern Queensland in Australia discovered a new Sirius-like quadruple system composed of a crystallizing white dwarf companion to the previously known triple HD 190412 at 32 parsecs distance. By virtue of its association with these main sequence companions, this is the first crystallizing white dwarf whose total age can be externally constrained.

All things in the Universe must change, and every star that hangs in the firmament will one day run out of fuel for their fires and evolve into something new. For the vast majority of stars, that something is a white dwarf star. When the fuel runs out, the star’s outer material is shucked into the surrounding space, and the remaining core, no longer supported by the outward pressure supplied by fusion, will collapse down into an ultradense object, around the size of Earth, but packing in as much mass as 1.4 Suns.

The matter in white dwarf stars is highly compressed, but it’s prevented from collapsing further by something called electron degeneracy pressure. No two electrons can occupy identical states, and this keeps the white dwarf from becoming even denser, as seen in a neutron star or black hole. White dwarf stars are dim, but they still shine with residual heat. Over time, they cool, and are expected to evolve into something called a black dwarf star when they lose all their heat and become a cold lump of crystallized carbon.

Crystallization in White Dwarf Stars

During crystallization, the carbon and oxygen atoms inside the white dwarf stop moving about freely and form bonds, arranging themselves into a crystal lattice. Energy is released during this process, which dissipates in the form of heat. This produces a sort of plateau or slowing in the cooling of white dwarf stars, which can be observed in the color and brightness of the star, making it appear younger than it actually is.

In order to accurately gauge the brightness of a star, it is essential to know with precision how far away it is, something that has been made much more possible in recent years due to the high-precision stellar mapping conducted by the Gaia mission. This means that we can now identify crystallizing white dwarfs with a lot more confidence.

The discovery of the white dwarf, now named HD 190412 C, made the triplet a quadruplet, but its properties suggest that it’s undergoing the crystallization process. Whether or not that white dwarf crystal is diamond is unknown, and it is still uncertain if the white dwarf resembles a diamond with its density. The density of white dwarfs is over around 1 million kilograms per cubic meter, while the density of diamond is about 3,500 kilograms per cubic meter. Denser allotropes of carbon do exist; on the other hand, there’s plenty of diamond floating around out there in space.

The system’s age is around 7.3 billion years, and the white dwarf’s age appears to be around 4.2 billion years. The discrepancy is 3.1 billion years, suggesting that the crystallization rate has slowed the cooling rate of the white dwarf by approximately 1 billion years, the researchers say. The discovery and its proximity to Earth suggest that there might be many more such systems out there that we can exploit for benchmarking this fascinating process.

The discovery of the HD 190412 system has opened up a new avenue for understanding crystallizing white dwarfs. The researchers propose that the discovery of this system at only 32 parsecs suggests that similar Sirius-like systems containing crystallizing white dwarfs are likely to be numerous. Future discoveries may, therefore, allow for stronger tests of white dwarf crystallization models.


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