Tech & Science

The Universe Is Filled With Stars Made of Crystal

The universe is filled with countless stars that have turned into spheres of “crystal” in the sky, according to a study published in the journal Nature.

The paper focuses on a class of stellar objects known as white dwarfs, the (relatively) small, dense remnants of stars—such as our sun—which have shed their outer layers, spent most of their hydrogen and helium fuel, and are gradually cooling as they reach the end of their lives. Ninety-seven percent of stars in our galaxy will meet their end in this way.

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White dwarfs—which are some of the oldest stellar objects in the universe—are very useful to astronomers because they can be used to accurately determine the age of stars in their vicinity. This is because they have very predictable life cycles.

Previously, scientists had predicted that a phase transition—when a substance changes from a solid, liquid or gas to another state—occurs during the white dwarf cooling process, causing carbon and oxygen in the extremely dense core to turn into metallic crystals. This would release a large amount of heat and considerably delay the cooling process, which is significant because it could mean that these stars are billions of years older than we thought they were.

However, no direct observational evidence of this phenomenon has been uncovered. To investigate this issue, researchers from the University of Warwick, U.K., examined data collected by the European Space Agency’s (ESA) Gaia satellite on the luminosity and color of 15,000 white dwarfs located within a distance of 300 light-years from Earth.

The team, led by physicist Pier-Emmanuel Tremblay, noticed that there was an excess, or “pileup,” of white dwarfs which had very similar luminosities and colors, regardless of their mass or age, indicating that they had reached a phase where they were releasing large amounts of heat from their cores.

According to Tremblay, this is the "first direct evidence" that white dwarfs crystallize, or transition from liquid to solid, and it shows that the ageing process in these stars is being delayed, sometimes by as much as 2 billion years.

"Ninety-seven percent of all stars will go through [the white dwarf phase] in their evolution," he told Newsweek. "The huge amount of extra energy released by solidification gives the universe an extra waiting time of about 2 billion years, and we need to account for that when looking at the chronology of the universe. Not all stars will crystallize at the same time, but white dwarfs that have undergone crystallization are particularly good chronometers. They can be used to precisely trace back the evolution of the Milky Way."

"All white dwarfs will crystallize at some point in their evolution, although more massive white dwarfs go through the process sooner,” he said in a statement. “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."

Over time, the whole of the white dwarf will crystallize, Tremblay continued.

"Our observations show the transition from fully liquid to 99 percent solid," he said. "The hydrogen-rich surface is much less dense, and it will take at least 20 billion years before it also becomes fully solid. After that, the star will be a fully solid black dwarf, emitting no meaningful light and remaining stable for more than 10 to the power of 21 times the current age of the universe!"

Tremblay noted that the new findings have significant implications for how we study the universe, while pointing out the important contribution of the Gaia satellite to the latest study.

"We've made a large step forward in getting accurate ages for these cooler white dwarfs and therefore old stars of the Milky Way,” he said. “Much of the credit for this discovery is down to the Gaia observations. Thanks to the precise measurements that it is capable of, we have understood the interior of white dwarfs in a way that we never expected. Before Gaia we had 100-200 white dwarfs with precise distances and luminosities—and now we have 200,000. This experiment on ultra-dense matter is something that simply cannot be performed in any laboratory on Earth."

Data collected by the Gaia mission has led to the creation of a three-dimensional map of our galaxy—containing more than 1 billion astronomical objects—which shows the Milky Way in unprecedented detail.

This article has been updated to include additional comments from Pier-Emmanuel Tremblay.

189743 A white dwarf star is in the process of solidifying in this artist’s rendering. Scientists have found the first direct evidence that white dwarf stars crystallize as they cool. University of Warwick/Mark Garlick

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