Our sun and billions of stars just like it are headed for a strange, cold destiny.
New research suggests that long after our roiling, boiling life-giving star runs out of fuel, it will slowly transform into a cold, dead, super-dense crystal sphere about the size of Earth that will linger like a translucent tombstone for close to eternity.
“In tens of billions of years from now, the universe will be made largely of dense crystal spheres,” said astrophysicist Pier-Emmanuel Tremblay, who led the work published this week in the journal Nature. “In the future, these objects will be completely dominant.”
To come to this conclusion, Tremblay and his collaborators used data collected by the European Space Agency’s Gaia telescope to analyze the color and brightness of 15,000 white dwarf stars within 300 light-years of Earth.
White dwarf stars are among the oldest objects in the universe and represent one of the final phases of life for stars like the sun.
Our sun is about halfway through the main sequence phase, which means it creates energy by fusing hydrogen into helium in its core.
In about 5 billion to 6 billion years, it will run out of hydrogen. Then its core will shrink, and the rest of the star will puff up into a relatively short-lived red giant phase, which will last about 500 million to 1 billion years before it contracts once again.
After this contraction, the star can still create energy by fusing helium to create carbon and oxygen, said Tremblay, who works at the University of Warwick in Coventry, England.
This form of energy generation, however, occurs quickly and will last for only a few million years.
When that process comes to an end, the sun will enter the white dwarf stage. Essentially, it will be a retired star made up primarily of oxygen and carbon gas.
White dwarf stars start off extremely hot, but although they initially radiate enough heat that we can see them in our telescopes, they slowly lose their energy over billions of years.
“It’s like taking a hot coal out of a fire and letting it cool off into the night,” said J.J. Hermes, an astronomer at Boston University who worked on the study.
It is not possible to observe crystal structures in white dwarf stars directly, but it is possible to see evidence of the crystallization process, the authors said.
If the stars did not crystallize, they would cool at a steady rate, going from blue to orange to red and losing brightness along a smooth, predictable slope. But that’s not what the Gaia data show.
Instead, the researchers found an unexpectedly high number of white dwarf stars that seemed stuck in a certain color and brightness region.
This pileup, or traffic jam, in the data suggests that at around the same point in the cooling process, the stars simply stop getting colder.
“We see them sitting there for hundreds of millions and even billions of years,” Hermes said, “when they should be cooling on a much shorter timescale.”
The only explanation is that these stars have an extra energy source, Tremblay said.
And it could be coming from crystallization.
As matter crystallizes from a liquid into a solid, it releases energy. You can see this when water goes from a liquid to a solid in the freezer, Hermes explained.
If you were keeping track with a thermometer, you would find that the temperature of water stalls at zero degrees Celsius for a bit — the exact time that the H2O molecules are rearranging themselves into the crystal structure of ice. Once the crystal arrangement is in place, the ice will continue to cool at a more or less steady rate until it reaches the same temperature as the environment in the freezer.
The same thing is happening in the cores of these white dwarf stars, except over a much longer time period, the study authors said. As the oxygen and carbon in the star crystallize, they release heat, causing the star to stall its cooling for roughly 2 billion years.
Experts said the finding itself was not surprising — for more than 50 years, astronomers have thought that white dwarf stars must crystallize as they cool. However, they agreed that the work is impressive.
“What’s remarkable is the ability of the observational community to make such an excellent measurement of the release of heat as the crystallization occurs in the core of the star,” said Lars Bildsten, a theoretical physicist at UC Santa Barbara who was not involved in the paper.
Although many scientists thought it was likely that white dwarf stars would form crystals as they cooled, there was disagreement about whether the energy released from the process would be detectable, Tremblay said.
The new finding suggests that the energy is not only detectable, but also at the upper end of predicted estimates by theoreticians, he said.
“The data used from the Gaia mission allows us to make a great step forward,” said Martin Barstow, a professor of astrophysics at the University of Leicester in England.
But just as the water in your freezer continues to cool after it releases all its latent energy, white dwarfs eventually resume their cooling as well.
And when the process is complete, they become what are known as black dwarfs — cold crystal spheres that are not detectable with our telescopes because they don’t emit energy.
One day in the far-distant future, Tremblay said, 97% of stars in the universe will meet this fate.
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