What happens when a white dwarf star crystallizes?

The answer

White dwarfs - the dense remnants of Sun-like stars - undergo crystallization as they cool over billions of years, with their carbon and oxygen interiors solidifying into a crystalline lattice similar to a diamond. In 2019, astronomers using the ESA's Gaia spacecraft analyzed over 15,000 white dwarfs and found direct observational evidence of this phase transition: a pile-up of white dwarfs at certain luminosities and colors where the release of latent heat from crystallization temporarily slows their cooling. Approximately 97% of all stars will eventually become white dwarfs. The crystallization process releases gravitational energy as heavier elements settle toward the core, adding up to two billion years to a white dwarf's cooling time. The oldest crystallized white dwarfs in our galaxy are essentially Earth-sized diamond spheres with masses comparable to the Sun, making them among the densest visible objects in the cosmos.