An international team of astronomers has uncovered the first direct evidence for the theory that the dying husks of ancient stars solidify into enormous cosmic crystals as they age. The study relied on data collected by ESA's Gaia spacecraft, which is currently engaged on a mission to create an enormous three-dimensional map of the Milky Way.

When a medium mass star like our Sun dies, it doesn't mark its passing with the glorious spectacle of a supernova. Instead, having depleted its stockpile of hydrogen, it will swell into a mighty red giant, and eventually, having run out of nuclear fuel altogether, it will blow away its outer layers, leaving only its core behind. This core is referred to as a white dwarf.

However, the death of a red dwarf and its subsequent transformation into a white dwarf does not mark the end of its evolutionary journey. Over the course of billions of years, the stellar husk continues to bleed heat, until the core finally cools to around 10 million °C. At this point, the heart of the star begins to crystallize.

Prior to crystallization, atoms are thought to be packed so tightly in the core of a white dwarf that their electrons become unbound, leaving a conducting electron gas and positively charged nuclei in a fluid form.

When the transition occurs, the atoms in the super dense interior of the star form into an ordered structure and solidify, in a similar way to how liquid water transitions into ice. The core is then formed of a crystallized metallic oxygen interior, with a carbon enhanced mantle.

The crystallization process would dramatically slow the cooling process, theoretically prolonging the life of a star by as much as 2 billion years. Fifty years ago, it was predicted that the onset of this transition would be accompanied by the release of a huge amount of latent heat that had previously been stored within the stellar body.

Because the life cycles of white dwarfs are so well understood, astronomers are able to use them as a tool to estimate the age of nearby populations of stars. Therefore, gaining a greater understanding of how crystallization can stave off the cooling process, essentially making the stars appear younger than they really are, would help astronomers improve the accuracy of the white dwarf dating technique.

Using photometry and parallax data harvested by ESA's Gaia satellite, a team of astronomers led by Dr. Pier-Emmanuel Tremblay from the University of Warwick's Department of Physics, set out to find direct evidence of white dwarf crystallization.

The team selected 15,000 white dwarf candidates that were located within 300 light-years of Earth, and analyzed their brightness and colors.

Dr. Tremblay and his team discovered an excess population of stars with specific colors and luminosities that, when compared to stellar evolution models, coincided with the point at which latent heat is released during the onset of crystallization.

The study, published in the journal Nature, represents the first direct evidence of white dwarfs transitioning from a liquid to a solid state.

According to the authors of the new paper, the release of heat energy alone would not be enough to account for their observations. They believe that the missing energy could be released in the form of gravitational energy, created as carbon is pushed to the surface of the star by falling oxygen that had crystallized earlier in the process.

"All white dwarfs will crystallize at some point in their evolution, although more massive white dwarfs go through the process sooner," said Dr. Tremblay. "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."

Source: University of Warwick