Data from ESA's Gaia spacecraft showed for the first time how white dwarfs, the dead remnants of stars like our Sun, turn into solid spheres after the hot gas inside them cools.
This process of solidification or crystallization of material inside white dwarfs was predicted 50 years ago, but astronomers have not been able to observe enough of these objects to see a sample that reveals this process.
“We used to have only a few hundred white dwarfs and many of them were in clusters of roughly the same age,” says Pierre-Emmanuel Tremblay of the University of Warwick, UK, lead author of the paper describing the results.
With the help of Gaia, we got more information. A lot of data was received from different objects with different distances, brightness and color. These are hundreds of thousands of white dwarfs in the outer disk of the Milky Way, with different masses and different ages.
It is in accurately estimating the distance to these stars that Gaia is making a breakthrough, allowing astronomers to measure their true brightness with unprecedented precision.
(The main photo shows a white dwarf, a dead remnant of a star like our Sun, with a crystallized solid core.)
White dwarfs are the remnants of medium-sized stars similar to our Sun. After these stars burned off all the nuclear fuel in their core, they released their outer layers, leaving behind a hot core that begins to cool.
These superdense remnants still emit thermal radiation as they cool down and are visible to astronomers as rather faint objects. It is estimated that up to 97% of stars in the Milky Way will eventually turn into white dwarfs, while the most massive ones will turn out to be neutron stars or black holes.
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Cooling white dwarfs takes billions of years. Once they reach a certain temperature, the initially hot matter inside the star's core begins to crystallize, becoming solid. The process is similar to the transformation of liquid water into ice on Earth at zero temperatures, except that the temperature at which this solidification occurs in white dwarfs is extremely high -
about 10 million degrees Celsius.
In this study, astronomers analyzed more than 15,000 candidate stellar remnants 300 light-years from Earth. Gaia was able to see these crystallizing white dwarfs as a distinct group.
“We saw a large number of white dwarfs of different colors and shades that were not related to each other in terms of their evolution,” says Pierre-Emmanuel.
“We realized that this is not a separate population of white dwarfs, but the cooling and crystallization effect predicted 50 years ago."
The heat generated during this crystallization process, which lasts several billion years, appears to slow down the evolution of white dwarfs.
“White dwarfs have traditionally been used for age dating of stellar populations such as star clusters,” explains Pierre-Emmanuel.
“We now need to develop better crystallization models to get more accurate estimates of the age of these systems."
Not all white dwarfs crystallize at the same rate. More massive stars cool faster and reach temperatures at which crystallization occurs in about a billion years. Lower-mass white dwarfs, closer to the expected final stage of the Sun, cool more slowly, taking up to six billion years to turn into dead solid spheres.
The sun still has about five billion years before it becomes a white dwarf, and astronomers estimate it will take another five billion years after that to eventually cool down to a crystalline sphere.
“This result underlines the versatility of Gaia and its many applications,” says Timo Prusti, Project Gaia Scientist at ESA.