All across the universe, dying stars are turning into crystals. This is not a plot from a Doctor Who episode, but the reality for white dwarfs – small, dense stellar core remnants. White dwarfs are the last stage of evolution for many small stars, including the Sun. Now, new research has discovered that as they age, they slowly turn into a crystal structure.
The idea that white dwarfs solidify was first proposed 50 years ago, but astrophysicists failed to find hard evidence in support of this idea. New observations have now delivered the much-needed proof. Using the European Space Agency’s Gaia satellite, the researchers were able to identify 15,000 white dwarf candidates within 300 light-years from Earth. Studying the distribution of their luminosities and colors, the team realized there was an excess of these stars at very specific values.
As these stars age, they cool down. As the temperature drops below 10 million degrees, the oxygen and carbon in their cores crystalize. Oxygen begins this process sooner, pushing the carbon upwards and releasing energy. This latent heat is the reason why white dwarfs of different ages cluster around specific colors and luminosities. The findings are reported in Nature.
"This is the first direct evidence that white dwarfs crystallize, or transition from liquid to solid. It was predicted 50 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," lead author Dr Pier-Emmanuel Tremblay, from the University of Warwick, said in a statement.
"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 team estimated that some of these white dwarfs are much older than previously thought.
"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," added Dr Tremblay. "Before Gaia we had 100-200 white dwarfs with precise distances and luminosities – and now we have 200,000. This experiment on ultra-dense matter is something that simply cannot be performed in any laboratory on Earth."