The universe is filled with countless stars that have turned into what are essentially spheres of “crystal” in the sky, according to a study published in the journal Nature.
The paper focuses on a class of stellar objects known as white dwarfs, the (relatively) small, dense remnants of larger stars—such as our Sun—which have shed their outer layers, spent most of their hydrogen and helium fuel, and are gradually cooling as they reach the end of their lives. Ninety-seven percent of stars in our galaxy will meet their end in this way.
White dwarfs—which are some of the oldest stellar objects in the universe—are very useful to astronomers because they can be used to accurately determine the age of stars in their vicinity. This is because they have very predictable lifecycles.
Previously, scientists had predicted that a phase transition—when a substance changes from a solid, liquid or gas to another state—occurs during the white dwarf cooling process, causing carbon and oxygen in the core to turn into metallic crystals. This would release a large amount of latent heat and considerably delay the cooling process, which is significant because it could mean that these stars are billions of years older than we thought they were.
However, no direct observational evidence of this phenomenon has been uncovered. To investigate this issue, researchers from the University of Warwick, U.K., examined data collected by the European Space Agency’s (ESA) Gaia satellite on the luminosity and color of 15,000 white dwarf candidates within around 300 light-years from Earth.
The team, led by physicist Pier-Emmanuel Tremblay, noticed that there was an excess, or “pile-up,” of white dwarfs with specific colors and luminosities which do not correspond to any single mass or age. The researchers say this pile-up is evidence of white dwarfs releasing latent heat as their cores crystallize, indicating that the ageing process in these stars is being delayed, sometimes by as much as two billion years.
"This is the first direct evidence that white dwarfs crystallize, or transition from liquid to solid,” Tremblay said in a statement. “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,” he said. “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."
According to Tremblay, the new findings have significant implications for how we study the universe.
"We've made a large step forward in getting accurate ages for these cooler white dwarfs and therefore old stars of the Milky Way,” he said. “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. 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."
The Gaia mission is an ambitious attempt to create a three-dimensional map of our galaxy, the Milky Way, which will contain approximately 1 billion astronomical objects.