Long after the melting has gone and its furnaces have cooled, the embers of our Sun will form a giant crystal hanging from the sky – one of the countless incinerated ones in our entire galaxy.
Astronomers have discovered evidence that massive white dwarf stars solidify into metallic crystals early in retirement. Poetic descriptions aside, this could challenge how we calculate the ages of some of the oldest objects in the Universe.
Using data from the European Space Agency's Gaia satellite, researchers in the United Kingdom, Canada and the United States have found support for a 50-year hypothesis describing the stages that many stars pass before ending their lives as crystals.
"This is the first direct evidence that white dwarfs crystallize, or transition from liquid to solid," says physicist Pier-Emmanuel Tremblay of the University of Warwick.
"It was predicted fifty years ago that we should observe an accumulation in the number of white dwarfs in certain luminosities and colors due to crystallization and only now this has been observed."
While massive stars, much larger than ours, come out with a bang, most of the suns in the Universe are of a mediocre mass that sees them age much more quietly.
Once the hydrogen is gone, stars like our sun begin to cool and contract. This provides a brief surge of energy that blows its atmosphere to huge proportions, sending in a lot of heat.
Meanwhile, its core continues to shrink, compressing helium into even heavier elements such as carbon and oxygen.
The end result is a white dwarf – a ball the size of Earth so dense, a tiny 1cm3 part of its core would weigh about 10 tonnes.
The eventual fate of these warm hearts of dying stars will eventually be a frozen corpse called black dwarf.
Given how long the white dwarves are expected to cool, few (if any) must have gotten to that point yet. Finding one would profoundly alter how we think about the age of the Universe.
But how does a white dwarf melt its heat? Internal mechanics make a big difference in how heat seeps into the surface and has long been the subject of debate.
At the bottom of the white dwarf its electrons move freely, sliding through a crowded crowd of carbon and oxygen nuclei and slowly charging the heat toward its more conductive surface.
Theoretically, around 10 million degrees, there is not enough energy to allow the nuclei's positive nuclei to move out of position. They lock in place, forming a vast crystalline structure that releases a significant amount of energy.
The issue is all about timing. In small white dwarfs, the crystallization coincides with a process that connects the core with the outer layers, allowing thermal energy to be easily eliminated. Once connected, the star cools quite efficiently.
Much heavier stars are more of a mystery. Finding evidence of its own sequence has been difficult, both thanks to the tiny size of the white dwarfs in general and to the less obvious cooling signature of the more massive varieties.
The researchers collected data on more than 15,000 objects that were likely to be white dwarfs, all in about 300 light-years from Earth. After comparing their masses and ages, they discovered that there were more stars than there should be some shine and color.
This pattern aligned perfectly with theoretical predictions describing how the white dwarves of a given mass lose their heat, suggesting that crystallization occurs much earlier in white dwarfs with masses exceeding those of our own Sun.
"All white dwarfs will crystallize at some point in their evolution, although more massive white dwarfs go through the process earlier," Tremblay says.
"It means that billions of white dwarfs in our galaxy have completed the process and are essentially crystal spheres in the sky."
Confirming this model has some rather large implications for the aging of some of the most common objects in the Milky Way.
With crystallization occurring before white dwarfs can expel this heat, their cooling process is impeded, slowing their aging process by up to 2 billion years.
Not only did the researchers find sets of crystallization early in heavy white dwarfs, considerably more energy than expected was lost when the heat was released.
"We believe this is due to oxygen crystallizing first and then sinking into the core, a process similar to sedimentation on a river bed on Earth," says Tremblay.
For astronomers, this finding provides observations that may help confirm how stars like our own Sun change over time, giving us a much better idea of the evolution of our galaxy.
The rest of us can look at the cosmos and enjoy among the burning furnaces of stellar dust, there are even more gemstones than we ever imagined.
This research was published in Nature.