Astronomers from the University of Warwick have found direct evidence suggesting that the white dwarf stars are solidifying into crystals.

A white dwarf star in a dense stellar core remnant, which is mostly made up of electron-degenerate matter. All the luminosity of these stars come from the stored thermal energy as there is absolutely no fusion taking place on them. Willem Luyten proposed the name ‘White Dwarf’ in 1922 following the discovery of their unusual faintness in 1910. According to a theory, they are the final evolutionary state of the stars who don’t have enough mass to turn into a neutron star. The nearest known white dwarf star is Sirius B, which is at a distance of 8.6 light years from us.

Once a main-sequence star puts an end to its Hydrogen-fusing period, it enters into a state called the red giant where it combines Helium to Oxygen and Carbon in its core by making use of the ‘Triple Alpha’ process. In case a red giant doesn’t have sufficient mass to produce the required temperatures for this infusion procedure, an inert mass of Carbon and Oxygen tends to gather at the center of the star. When such a star forms a planetary nebula after getting rid of its outer layers, the core that is left behind is called the white dwarf. All these observations indicate that these dead remnants of stars are potentially billions of years older than the previous estimates.

Most of the white dwarfs are made up of Oxygen and Carbon but their chemical composition can vary on the basis of their mass. For instance, stars having very low mass might fail to fuse Helium with other elements which give birth to a Helium White Dwarf. Similarly, a progenitor who has a mass between 8 and 10.5 solar masses might succeed in fusing Carbon but it will not be able to blend Neon. Consequently, an Oxygen-Neon-Magnesium White Dwarf is formed.

The latest discovery is largely based on observations taken with the Gaia Satellite of the European Space Agency (ESA). Dr. Pier-Emmanuel Tremblay led this research at the Physics Department of the University of Warwick, which was published in the journal ‘Nature’. The fact that these stars incredibly handy as cosmic clocks due to their predictable lifecycles urged the researchers to extract more information about these celestial objects. The researching team selected a pool of 15,000 dwarf white stars within 300 light years of Earth for their study.

They examined the data on the color and luminosities of the stars and discovered that an excessive number of stars, having specific colors and luminosities, do not correspond to a single age or mass. This pile-up strongly matches to a phase (mentioned in the evolutionary models of stars) in their development during which large amounts of latent heat is released to slow down their cooling process. Researchers estimate that some of these stars have slowed down their aging by nearly 2 billion years (15% of our galaxy’s age). Tremblay referred to that by saying,

“This is the first direct evidence that white dwarfs crystallize, or transition from liquid to solid. It was predicted fifty years ago that we should observe a pile-up in the number of white dwarfs at certain luminosities and colors due to crystallization and only now this has been observed. All white dwarfs will crystallize 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.”

The term ‘Crystallization’ refers to a process where the atoms of a material arrange themselves in an orderly manner to attain a solid state. Extreme pressures in white dwarf stars unbound electrons from their atoms, which results in positively charged nuclei in a fluid form and a conducting electron gas. As the core cools down to a temperature of around 10 million degrees, this positively charged fluid starts to solidify into a metallic core at the heart of the star. Tremblay was quite fascinated by this discovery and he expressed his feelings in the following words:

“Not only do we have evidence of heat release upon solidification, but considerably more energy release is needed to explain the observations. We believe this is due to the oxygen crystallizing first and then sinking to the core, a process similar to sedimentation on a river bed on Earth. We’ve made a large step forward in getting accurate ages for these cooler white dwarfs and therefore old stars of the Milky Way. Much of the credit for this discovery is down to the Gaia observations. 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.”