About fifty years ago, astronomers predicted what the ultimate fate of our sun will be. According to the theory, the sun will exhaust its hydrogen fuel billions of years from now and expand to become a Red Giant, followed by it shedding its outer layers and becoming a white dwarf. After a few more billion years of cooling, the interior will crystallize and become solid.
Until recently, astronomers had little evidence to back up this theory. But thanks to the ESA’s Gaia Observatory, astronomers are now able to observe hundreds of thousands of white dwarf stars with immense precision — gauging their distance, brightness, and color. This, in turn, has 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 which describes these findings recently appeared in the journal Nature under the title “Core crystallization and pile-up in the cooling sequence of evolving white dwarfs.” The study was led by Pier-Emmanuel Tremblay, an assistant professor at the University of Warwick, and included multiple researchers from Warwick’s Astronomy and Astrophysics group, the Université de Montréal, and the 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. Whereas larger stars (like blue super-giants) eventually go supernova and become neutron stars or black holes, smaller stars like our sun will shed their outer layers to become planetary nebulae, and eventually conclude their life cycle as a white dwarf.
These ultra-dense stars continue to emit radiation as they cool, a process which lasts billions of years. Eventually, their interiors will be cool enough — about 10 million degrees C (50 million degrees F) — that the extreme pressure being exerted on their cores will cause the material there to crystallize and turn solid. It is estimated that this will be the fate of up to 97 percent of stars in the Milky Way, while the rest will become neutron stars or black holes.
Since white dwarfs are among the oldest stars in the Universe, they are incredibly useful to astronomers. Since their lifecycle is predictable, they are used as “cosmic clocks” to estimate the age of groups of neighboring stars with a high degree of accuracy. But determining what happens to white dwarfs towards the end of their life cycle has been challenging.
Previously, astronomers were limited when it came to the number of white dwarfs they could study. All of that changed with the deployment of Gaia, a space observatory that has spent the past few years precisely measuring the positions, distances and motions of stars for the sake of creating the most detailed 3D space catalog ever made.
“Previously, we had distances for only a few hundreds of white dwarfs and many of them were in clusters, where they all have the same age. With Gaia we now have the distance, brightness and color of hundreds of thousands of white dwarfs for a sizeable sample in the outer disc of the Milky Way, spanning a range of initial masses and all kinds of ages.”
For their study, the astronomers used Gaia data to analyze more than 15,000 stellar remnant candidates within 300 light years of Earth. From this sample, they were able to identify an excess in the number of stars (aka a pileup) that had specific colors and luminosities that didn’t correspond to any single mass or age.
This pile-up, once compared to evolutionary models of stars, appeared to coincide with the developmental stage where stars lose heat in large quantities. This process slows down the natural cooling process and causes the dead stars to stop dimming, which makes them appear up to 2 billion years younger than they actually are.
“This is the first direct evidence that white dwarfs crystallize, or transition from liquid to solid,” explained Tremblay in a Warwick press 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.”
This pattern, where luminosity is unrelated to age, was one of the key predictions made about crystallizing white dwarfs 50 years ago. Now that astronomers have direct evidence of this process at work, it is likely to impact our understanding of what stellar groupings white dwarfs should be included in.
“White dwarfs are traditionally used for age-dating of stellar populations such as clusters of stars, the outer disc, and the halo in our Milky Way,” said Tremblay. “We will now have to develop better crystallization models to get more accurate estimates of the ages of these systems.”
For example, while all white dwarfs will crystallize at some point in their evolution, the time it takes varies based on the star. More massive white dwarfs cool down more rapidly and reach the temperature at which crystallization occurs sooner (in about one billion years). Smaller white dwarfs, which is what our sun will become, may require as much as 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. Meanwhile, our sun can be expected to undergo this transition in about another ten billion years. At that point, our sun will have exited its Red Giant Branch phase, become a white dwarf, and begun the process of crystallization.
This is just the latest revelation to come from the Gaia mission, which has spent the past five years cataloging celestial objects in the Milky Way and neighboring galaxies. Before the mission ends (expected to happen by 2022), two more data releases are scheduled, with the DR3 release scheduled for 2021 and the final release still to-be-determined.