Data captured by ESA’s galaxy-mapping spacecraft Gaia has revealed for the first time how white dwarfs, the dead remnants of stars like our Sun, turn into solid spheres as the hot gas inside them cools down. This process of solidification, or crystallisation, of the material inside white dwarfs was predicted 50 years ago but it wasn’t until the arrival of Gaia that astronomers were able to observe enough of these objects with such a precision to see the pattern revealing this process. It is in the precise estimate of the distance to these stars that Gaia makes a breakthrough, allowing astronomers to gauge their true brightness with unprecedented accuracy. In this study, the Warwick-led astronomers analysed more than 15 000 stellar remnant candidates within 300 light years of Earth and were able to see these crystallising white dwarfs as a rather distinct group in colour and intrinsic brightness. The cooling of white dwarfs lasts billions of years. Once they reach a certain temperature, the originally hot matter inside the star’s core starts crystallising, becoming solid. The process is similar to liquid water turning into ice on Earth at zero degrees Celsius, except that the temperature at which this solidification happens in white dwarfs is extremely high – about 10 million degrees Celsius. The heat released during this crystallisation process, which lasts several billion years, seemingly slows down the evolution of the white dwarfs: the dead stars stop dimming and, as a result, appear up to two billion years younger than they actually are. That, in turn, has an impact on our understanding of the stellar groupings these white dwarfs are a part of.

• Caption to figure: White dwarf star in the process of solidifying. Credit: University of Warwick/Mark Garlick
• Publication: https://www.nature.com/articles/s41586-018-0791-x
• DOI: 10.1038/s41586-018-0791-x