Data captured by ESA's galaxy-mapping probe, Gaia, revealed for the first time how white dwarfs, the remains of stars like our Sun, turn into solid spheres as the hot gas inside them cools.
This process of solidification or crystallization of the material within white dwarfs was predicted 50 years ago, but it was not until the arrival of Gaia that astronomers were able to observe a sufficient number of these objects with such precision to see the pattern revealing this process . .
"Previously, we had distances of only a few hundred white dwarfs and many of them were in groups, where they are all the same age," says Pier-Emmanuel Tremblay of the University of Warwick, UK, the lead author of the article describing the results, published today in the journal Nature.
"With Gaia we now have the distance, brightness and color of hundreds of thousands of white dwarfs for a considerable sample on the outer disk of the Milky Way, spanning a range of early masses and all types of ages."
It is in accurate estimation of the distance of these stars that Gaia makes a discovery, allowing astronomers to assess their true brightness with unprecedented accuracy.
White dwarfs are the remains of medium-sized stars similar to our sun. Once these stars have burned all the nuclear fuel at its core, they lose their outer layers, leaving behind a hot core that begins to cool.
These ultra-dense remnants still emit thermal radiation as they cool, and are visible to astronomers as rather tenuous objects. It is estimated that as many as 97 percent of the stars in the Milky Way will eventually turn into white dwarfs, while the more massive stars will end up as neutron stars or black holes.
The cooling of white dwarfs lasts billions of years. When they reach a certain temperature, the originally hot matter inside the star's core begins to crystallize, becoming solid. The process is similar to liquid water that turns to ice on Earth at zero degree Celsius, 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 stellar star candidates in 300 light-years from Earth as observed by Gaia and could see these crystallizing white dwarfs as a rather distinct group.
"We saw a pile of white dwarfs of certain colors and luminosities that otherwise were not bound in evolution," says Pier-Emmanuel.
"We realized that this was not a distinct population of white dwarfs, but the effect of cooling and crystallization predicted 50 years ago."
The heat released during this crystallization process, which lasts several billion years, seems to retard the evolution of white dwarfs: the dead stars stop darkening and as a result appear up to two billion years younger than they really are. This, in turn, has an impact on our understanding of the star clusters that these white dwarves are a part of.
"White dwarfs are traditionally used for age dating of star populations, such as star clusters, the outer disk and the halo in our Milky Way," explains Pier-Emmanuel.
"Now we will have to develop better models of crystallization to obtain more accurate estimates of the ages of these systems."
Not all white dwarfs crystallize at the same rate. More massive stars cools faster and will reach the temperature at which crystallization occurs in about a billion years. White dwarfs with lower masses, closer to the expected final stage of the Sun, cool more slowly, requiring up to six billion years to become dead solid spheres.
The Sun is still about five billion years old before becoming a white dwarfand astronomers estimate that it will take another five billion years to cool down in a crystal sphere.
"This result underscores the versatility of Gaia and its many applications," says Timo Prusti, ESA's Gaia project scientist.
"It's exciting how to scan stars across the sky and measuring their properties can lead to evidence of plasma phenomena in such a dense matter that it can not be tested in the laboratory."
"Crystallization and piling-up in the cooling sequence of evolving white dwarfs", Nature volume 565, pages 202-205 (2019)