Most of us are familiar with Jane Taylor’s nursery rhyme, “Twinkle, Twinkle, Little Star,” which compares the dense balls of burning hydrogen that litter the universe to “diamonds in the sky.” According to new research, Taylor’s charming metaphor can be taken quite literally.

As detailed in a paper published on Wednesday in Nature, astronomers at the University of Warwick in the UK obtained the first direct evidence of a star’s core turning into a crystal of solid oxygen and carbon. What’s more, the research suggests that the sky is full of stars that are undergoing or have completed crystallization and that our own Sun will one day transform into a crystal as well.

Indeed, as University of Warwick physicist Pier-Emmanuel Tremblay put it in a statement, ”billions of white dwarfs in our galaxy have already completed the process and are essentially crystal spheres in the sky.”

White dwarfs are the ultra-hot and dense cores of medium-sized stars that remain after they have exhausted their nuclear fuel and shed their outer layers as clouds of gas called nebulae. After shedding their outer layers, the white dwarfs—by this point not much larger than the Earth—begin a cooling process that can last for billions of years.

The presumably predictable cooling process of white dwarfs meant that astronomers could measure their temperature and use them as a sort of cosmic clock to determine the age of other nearby celestial objects. Yet according to the Warwick astronomers, if the center of white dwarfs are solidifying as crystals then this would dramatically slow the cooling process. This means that many white dwarfs may be billions of years older than previously calculated.

The theory that white dwarf cores turn into a solid crystal was first proposed over 50 years ago, but it wasn’t until recently that astronomers had the data they needed to verify this prediction. This data was provided by the European Space Agency’s Gaia satellite, which was launched in 2013 for a decade-long mission to measure the motion of stars with unprecedented accuracy.

“Before Gaia, we had 100 to 200 white dwarfs with precise distances and luminosities,” Tremblay said in a statement. “Now we have 200,000.”

Tremblay and his colleagues used data from Gaia to select 15,000 potential white dwarf stars within 300 light years of Earth. After analyzing the brightness and color spectrum of these stars, they found that there was an excessive number of white dwarfs with characteristics matching those predicted of white dwarfs undergoing crystallization half a century ago.

“This is the first direct evidence that white dwarfs crystallize, or transition from liquid to solid,” Tremblay said in a statement.

“Not only do we have evidence of heat release upon solidification, but considerably more energy release is needed to explain the observations,” Tremblay added. “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 on Earth. This will push the carbon upwards and that separation will release gravitational energy.”