Someday—probably billions of years from now—our sun will die. What exactly happens to the sun at the end of its long life, however, has long been up for debate. New observations reported in a study in the journal Nature reveal that most stars, including the sun, will likely turn into giant space crystals about the size of the Earth that will mark the spot where our solar system once was.
The finding comes from the European Space Agency’s Gaia Space Telescope, which took a close look at the color and brightness of 15,000 star remnants known as white dwarfs within about 300 light years of Earth. Fifty years ago astronomers first predicted that, at the end of a white dwarf’s life, it would cool enough to transition from a liquid to a solid and crystallize, but they had no proof. This new study gives the first observational evidence that the star remnants do indeed cool down into cosmic disco balls.
In fact, lead author and astronomer Pier-Emmanuel Tremblay of Warwick University tells Deborah Netburn at The Los Angeles Times that the majority of stars in the known universe will eventually crystallize.
“In tens of billions of years from now, the universe will be made largely of dense crystal spheres,” he says. “In the future, these objects will be completely dominant.”
So, what is a white dwarf? Basically, it’s one of the final stages of a star’s life. Medium-sized stars fuel their existence by fusing hydrogen into helium in their super-heated cores. The energy and pressure released from those nuclear reactions generate heat and outward pressure to keep the star stable. Eventually, however, small- to medium-sized stars—defined as anything with a mass less than about 8 times the mass of our sun—will convert most of their hydrogen to helium. The pressure from those reactions will not be able to overcome the force of gravity from the star's core. The star will then begin to collapse on itself, then start to heat up again and begin fusing its last remaining hydrogen outside the core in a burning shell that causes the star to massively expand into a red giant. That will become hot enough to fuse its helium core into the heavier elements oxygen and carbon. After that, it will blow off its outer layers, and what remains is a white dwarf, or the spent core of the star that will slow cool over several billion years.
According to Netburn, if the white dwarfs simply cooled off over time and did not turn into crystals, the stars would change color and lost brightness in a smooth, predictable path, turning from blue to orange to red as they cooled.
But the Gaia telescope data showed that many white dwarfs stopped cooling off for millions and sometimes billions of years instead of following that predictable path and instead, released energy. The most reasonable explanation is that during that time period the white dwarf is crystallizing, a process that gives off energy.
“We saw a pile-up of white dwarfs of certain colors and luminosities that were otherwise not linked together in terms of their evolution,” Pier-Emmanuel says in a press release. “We realized that this was not a distinct population of white dwarfs, but the effect of the cooling and crystallization predicted 50 years ago.”
It was believed by some researchers that if white dwarfs did crystallize, the energy given off by the process would be too small for astronomers to detect. But that’s not the case, and the energy given off during the process is at the upper end of predictions. In another press release, Tremblay says that likely has to do with the composition of the dwarfs.
“Not only do we have evidence of heat release upon solidification, but considerably more energy release is needed to explain the observations. We believe this is due to the oxygen crystallizing first and then sinking to the core, a process similar to sedimentation on a river bed on Earth,” he says. “This will push the carbon upwards, and that separation will release gravitational energy.”
While knowing that these stars become crystal spheres is pretty interesting, it has practical ramifications for astronomers. Because white dwarfs were known to cool at a steady rate, they are often used to date star clusters. But the rate at which a white dwarf crystallizes depends on its mass, with larger stars going through the crystallization process after one billion years while smaller stars could take billions of years longer to begin crystallization. The researchers say that they need to create better models of how these star crystallize in order to use them to better date star clusters.
Earth still has some time to go before until the Sun turns itself into a massive astro-chandelier. It’s estimated it will take about 5 billion years before it burns through its fuel and becomes a white dwarf, and it will take another 5 billion years to cool off and crystallize.