Our sun may look like an eternal ball of burning gas, but one day it will die. This may sound like a bummer, especially for anything that's living on Earth in a few billion years, but there is a bright side to the solar doom. According to research published in the journal Nature in January 2019, our dead star will leave behind a shimmering legacy and turn into a massive crystal. This isn't the type of crystal that you'd find in an average chandelier, however.
Before we start talking about supersized stellar crystals, we first need to understand how stars like our sun live and die.
From Giant to Dwarf
The sun is fueled by nuclear fusion. Its massive gravity crushes hydrogen atoms together in its core to create helium, and the vast quantities of energy released by these fusion processes push outward, maintaining a happy equilibrium. So long as there's plenty of hydrogen fuel feeding this process, the core remains about the same size and temperature (around 15 million Kelvin), producing energy that radiates throughout the solar system, ultimately nurturing the evolution of life on a certain habitable planet called Earth. This hydrogen-burning phase lasts for 90 percent of the lifetime of our sun, a period of stellar life known as the "main sequence." We are currently about 4.5 billion years into our sun's main sequence days — or approximately halfway through its life.
What happens when that hydrogen is all used up? Things start to get a little crazy, to put it mildly. Without the outward pressure of the energy created by fusing hydrogen, the sun's gravity overwhelms the core, crushing it into a smaller space and boosting its temperature tenfold. That's OK though; the heavier helium nuclei will begin to fuse together, creating the outward pressure once again to maintain equilibrium. It's predicted that this will start happening in about 5 billion years, marked with a sudden outrush of energy known as a "helium flash." As the helium fuses, carbon and oxygen are formed and the temperature of the core rises yet again.
Soon after, even heavier elements also begin to fuse, and the sun on the whole will look a bit worse for the wear. It will begin to swell, blasting interplanetary space with savage solar winds that begin to strip away its upper layers. Though our sun isn't massive enough to explode as a supernova, it will turn into a red giant star, possibly expanding beyond the orbit of Earth. And that's not good. Our planet will be toast.
After the death of our star, it'll leave behind the wispy remains of solar plasma — creating a beautiful planetary nebula enriched with newly formed heavy elements that will go on to create the next generation of stars and planets — and in its core will be a hot stellar remnant known as a white dwarf, a tiny, dense star shimmering brightly, a testament to the sun that used to be in its place.
White dwarfs can sustain themselves for billions of years before fizzing out and dimming forever, but this isn't the end of the story. Using observations by the European Gaia mission, which is currently making precision measurements of stars throughout our galaxy, researchers at the University of Warwick in the U.K. have stumbled on a white dwarf secret that has remained hidden.
The Crystal Dwarf
Soon after forming, white dwarfs are extremely hot, radiating the intense energy that was once held in the core of the main sequence star that came before them. Over the billions of years after forming, white dwarfs slowly cool and, at a certain point, the oxygen and carbon they contain will go through a phase transition — akin to liquid water freezing and turning into solid ice, only at much more extreme temperatures and pressures — solidifying to form a huge crystal.
"All white dwarfs will crystallize at some point in their evolution, although more massive white dwarfs go through the process sooner," Pier-Emmanuel Tremblay, from the University of Warwick's Department of Physics and leader of the study, said in a press release. "This means that billions of white dwarfs in our galaxy have already completed the process and are essentially crystal spheres in the sky. The sun itself will become a crystal white dwarf in about 10 billion years."
Tremblay's team analyzed the Gaia observations to measure the luminosities and colors of 15,000 white dwarfs within 300 light-years of Earth. What they found was an excess (or a "pile-up") in the population of stars of specific colors and brightness. They realized that this group of stars represented a similar phase in stellar evolution where the conditions are right for this phase transition to occur, causing a delay in cooling, thus slowing down the aging process. The researchers found that some of these stars had extended their lifespan by up to 2 billion years.
"This is the first direct evidence that white dwarfs crystallize, or transition from liquid to solid," added Tremblay, also in the 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."
Crystallized white dwarfs aren't just a stellar curiosity; their quantum makeup is unlike anything we can recreate in the laboratory. As the white star material crystalizes, its material becomes ordered on a quantum level, nuclei aligning themselves as a 3D lattice creating a metallic oxygen core and an outer layer enriched with carbon.
So, there we have it, after stars like our sun die, their stories aren't over. All white dwarfs will go through this crystallization phase, littering the galaxy with massive diamond-like stellar remnants.