White Dwarf Star Enters Its Crystallization Era, Turning Into A 'Cosmic Diamond'

The night sky may appear velvety dark to us, with stars flashing coldly against it like cut gems. And it could be somewhat true for some of them.

A certain kind of dead star eventually hardens and crystallizes as it cools. A white dwarf that is 104 light-years away and primarily made of carbon and metallic oxygen has been discovered by astronomers to be doing just that; its temperature-mass profile indicates that the star's core is changing into a dense, hard, "cosmic diamond" made of crystallized carbon and oxygen.

A manuscript on the finding that was approved for publication in the Monthly Notices of the Royal Astronomical Society is accessible on the preprint server arXiv.

The previously known triple HD 190412 has a crystallizing white dwarf companion, according to a team of international astronomers led by Alexander Venner of the University of Southern Queensland in Australia. "In this work we present the discovery of a new Sirius-like quadruple system at 32 parsecs distance, composed of a crystallizing white dwarf companion to the previously known triple HD 190412," the team writes.

This is the first crystallizing white dwarf whose overall age may be externally restricted because of its relationship with these main sequence partners. We take use of this feature by attempting to objectively estimate a cooling delay caused by core crystallization in the white dwarf.

The universe as a whole must undergo transformation. Every star in the heavens that shines brilliantly with the light produced by atomic fusion eventually runs out of fuel for their flames and transforms into something else.

That object is a white dwarf star for the great majority of stars, which are those with masses less than or equal to eight times that of the Sun and include the Sun.

When the star's fuel runs out, its outer layers are ejected into space, leaving the remaining core to collapse when the star's fusion-driven pressure is no longer sufficient to support it. This ultradense object will be about the size of Earth (or the Moon! ), but it will contain as much mass as 1.4 Suns.

Although the mass of white dwarf stars is very compacted, a phenomenon known as electron degeneracy pressure prevents it from collapsing much farther. This prevents the white dwarf from being even denser, as observed in a neutron star or black hole, because no two electrons can occupy the same states.

Despite being faint, white dwarf stars are still visible due to lingering heat. They gradually cool and are predicted to transform into stars known as black dwarfs when they completely lose their heat and turn into a frigid mass of crystalline carbon.

Since the universe is just around 13.8 billion years old and this process is thought to take a quadrillion years (or a million billion years), we don't expect to see one any time soon.

What we can do is spot the telltale traces of crystallization beginning in the centers of the white dwarfs we can see in our neighborhood.

The carbon and oxygen atoms inside the white dwarf cease moving about freely and begin to establish bonds, organizing themselves into a crystal lattice as the white dwarf crystallizes. This process results in the release of energy, which is then converted to heat.

White dwarf stars experience a kind of plateau or slowdown in their cooling as a result, which may be seen in the star's color and brightness and causes it to look younger than it actually is.

It is necessary to know a star's precise distance in order to evaluate its brightness precisely; in recent years, this has become much more feasible thanks to the Gaia mission's high-precision stellar mapping work.

As a result, scientists can now recognize crystallizing white dwarfs with far more assurance.

Using the Gaia data, Venner and his colleagues were looking for numerous star systems by finding stars whose relationships to other stars may have been hazy.

And they discovered that HD 190412, which had previously been believed to be a system of three stars, was gravitationally tied up with a recently discovered white dwarf star (remember, these things are really faint).

The triplet became a quadruplet once the white dwarf, now known as HD 190412 C, was found, but there was more going on. It appears to be crystallizing based on its characteristics.

White dwarfs have a density of about one million kilograms per cubic meter, but diamond has a density of roughly 3,500 kilograms per cubic meter, hence it is uncertain whether or not the crystal in that white dwarf is made of diamond. There are denser allotropes of carbon, but there is also a lot of diamond floating around in space.

The team was able to externally restrict the white dwarf's age, which has never been done previously for a known crystallizing white dwarf, thanks to the other three stars in the system.

The age of the system is around 7.3 billion years. It seems that the white dwarf is 4.2 billion years old. According to the researchers, the 3.1 billion year difference indicates that the white dwarf's cooling pace has slowed by around 1 billion years due to crystallization.

Although the date alone is insufficient to change our theories about white dwarf crystallization, the finding and the system's closeness to Earth raise the possibility that there are many more systems like it that we may use as benchmarks for this intriguing process.

We think that the fact that this system was found at a distance of only 32 parsecs indicates that there are probably many other Sirius-like systems with crystallizing white dwarfs. The researchers argue that future discoveries could therefore enable more robust evaluations of white dwarf crystallization models.

We draw the conclusion that the HD 190412 system's discovery has provided a fresh perspective on crystallizing white dwarfs.