Floating Crystals Halt Cooling of Some High-Mass White Dwarfs, Astronomers Say

Astronomers have proposed a new theory that explains why a puzzling population of white dwarf stars stopped cooling for at least eight billion years.

White dwarfs are stellar remnants devoid of a nuclear energy source, gradually cooling over billions of years and eventually freezing into a solid state from the inside out.

Recently, it was discovered that a population of freezing white dwarfs maintains a constant luminosity for duration comparable with the age of the Universe, signaling the presence of a powerful, yet unknown, energy source that inhibits the cooling.

“We discovered the classical picture of all white dwarfs being dead stars is incomplete,” said Dr. Simon Blouin, an astronomer at the University of Victoria.

“For these white dwarfs to stop cooling, they must have some way of generating extra energy.”

“We weren’t sure how this was happening, but now we have an explanation for the phenomenon.”

According to the team, in some white dwarfs, the dense plasma in the interior does not simply freeze from the inside out.

Instead, the solid crystals that are formed upon freezing are less dense than the liquid, and therefore want to float.

As the crystals float upwards, they displace the heavier liquid downward.

The transport of heavier material toward the center of the star releases gravitational energy, and this energy is enough to interrupt the star’s cooling process for billions of years.

“This is the first time this transport mechanism has been observed in any type of star, which is exciting, as it is not every day we uncover a whole new astrophysical phenomenon,” said Dr. Antoine Bédard, an astronomer at the University of Warwick.

“Why this happens in some stars and not others is uncertain, but we think it is likely due to the composition of the star.”

“Some white dwarf stars are formed by the merger of two different stars,” Dr. Blouin said.

“When these stars collide to form the white dwarf, it changes the composition of the star in a way that can allow the formation of floating crystals.”

White dwarfs are routinely used as age indicators: the cooler a white dwarf is, the older it is assumed to be.

However, due to the extra delay in cooling found in some white dwarfs, some stars of a given temperature may be billions of years older than previously thought.

“This new discovery will not only require that astronomy textbooks be revised but will also require that astronomers revisit the process they use to determine the age of stellar populations,” Dr. Blouin said.

The team’s paper appears today in the journal Nature.

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A. Bédard et al. Buoyant crystals halt the cooling of white dwarf stars. Nature, published online March 6, 2024; doi: 10.1038/s41586-024-07102-y