About fifty years ago, astronomers predicted what the ultimate fate of our Sun will be. According to the theory, the Sun will deplete its hydrogen fuel billions of years from now and will expand to become a Red Giant, followed by it it loses its outer layers and becomes a white dwarf. After a few billion years of cooling, the interior will crystallize and solidify.
Until recently, astronomers had little evidence to support this theory. But thanks to the observatory Gaia of the ESA, astronomers are now able to observe hundreds of thousands of white dwarf stars with immense precision, measuring distance, brightness and color. This in turn allowed them to study what the future holds for our Sun when it is no longer the warm yellow star that we know and love today.
The study describing these discoveries recently appeared in the journal Nature with the title "Core crystallization and accumulation in the cooling sequence of the evolving white dwarfs." The study was conducted by Pier-Emmanuel Tremblay, an assistant professor at the University of Warwick, and included more researchers from Warwick's Astronomy and Astrophysics Group, the Université de Montréal and the 39; University of North Carolina.
When it comes to stellar evolution, decades of observations combined with theoretical models have allowed astronomers to conclude what will happen to a star based on its classification. While the larger stars (like the blue super-giants) eventually become supernovae and become neutron stars or black holes, smaller stars like our Sun will spread to become planetary nebulae and eventually end their life cycle like a dwarf. white.
These ultra-dense stars continue to emit radiation as they cool, a process that lasts billions of years. In the end, their interiors will be quite cold – around 10 million ° C (50 million F) – that the extreme pressure exerted on their cores will cause the material to crystallize and become solid. It is estimated that this will be the fate of up to 97% of the stars in the Milky Way, while the rest will become neutron stars or black holes.
Because white dwarfs are among the oldest stars in the Universe, they are incredibly useful for astronomers. Since their life cycle is predictable, they are used as "cosmic clocks" to estimate the age of neighboring groups of stars with a high degree of accuracy. But determining what happens to white dwarfs near the end of their life cycle has been difficult.
Previously, astronomers were limited when it came to the number of white dwarfs they could study. All this changed with the deployment of Gaia a space observatory that has spent the last few years accurately measuring the positions, distances and stars of the stars to create the most detailed 3D spatial catalog ever made.
As Pier-Emmanuel Tremblay, an ERC * Starting Grant Fellow, indicated in a recent ESA press release:
"Previously, we had distances only for a few hundred white dwarfs and many of them were in cluster, where they all have the same age.With Gaia we now have the distance, the brightness and the color of hundreds of thousands of white dwarfs for a considerable sample in the outer disk of the Milky Way, covering a range of initial masses and all age types. "
For their study, astronomers used Gaia data to analyze more than 15 000 stellar star candidates within 300 light years from Earth. From this sample, they were able to identify an excess in the number of stars (ie a pile-up) that had specific colors and brightness that did not correspond to any single mass or age.
This pile-up, once compared to the evolutionary models of the stars, seemed to coincide with the stage of development in which the stars lose heat in large quantities. This process slows down the natural cooling process and causes dead stars to stop darkening, which makes them appear up to 2 billion years younger than they actually are.0
"This is the first direct test that white dwarfs crystallize, or transition from liquid to solid, "explained Tremblay in a Warwick press release. "It was predicted fifty years ago that we should observe an accumulation in the number of white dwarfs at certain luminances and colors due to crystallization and only now has this been observed."
This model, in which brightness is not related to age, was one of the main predictions made on the crystallization of white dwarfs 50 years ago. Now that astronomers have direct evidence of this process in the workplace, it is likely that they will influence our understanding of which star groups should be included in the white dwarfs.
"White dwarfs are traditionally used for the age of stellar populations as groups of stars, the outer disk and the halo in our Milky Way," said Tremblay. "Now we will have to develop better crystallization models to get more accurate estimates of the age of these systems."
For example, while all white dwarfs will crystallize at some point in their evolution, the time required varies by star. More massive white dwarfs cool more quickly and reach the temperature at which crystallization occurs first (in about a billion years). The smallest white dwarfs, which will become our Sun, could take up to six billion years to make the same transition.
"This means that billions of white dwarfs in our galaxy have already completed the process and are essentially crystal spheres in the sky," said Tremblay. In the meantime, our Sun can be expected to undergo this transition in about another ten billion years. At that point, our Sun will come out of its Red Giant Branch phase, it will become a white dwarf and the process of crystallization began.
This is only the last revelation of the mission of Gaia who has spent the last five years cataloging celestial objects in the Milky Way and in nearby galaxies. Before the end of the mission (scheduled for 2022), two more data releases are planned, with the DR3 version scheduled for 2021 and the final version still to be determined.
* The research was made possible thanks to funding from the European Research Council (ERC).
Further reading: University of Warwick, ESA Nature