Discovery tests theory on cooling of white dwarf stars | Science & Technology

Open any astronomy textbook to the section on white dwarf stars and you’ll likely learn that they are “dead stars” that continuously cool down over time. New research published in Nature is challenging this theory, with the University of Victoria (UVic) and its partners using data from the European Space Agency’s Gaia satellite to reveal why a population of white dwarf stars stopped cooling for more than eight billion years.

“We discovered the classical picture of all white dwarfs being dead stars is incomplete,” says Simon Blouin, co-principal investigator and Canadian Institute of Theoretical Astrophysics National Fellow at UVic. “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.”

Understanding the age and other aspects of white dwarf stars helps scientists reconstruct the formation of the Milky Way Galaxy. Using 2019 Gaia data, Blouin collaborated with Antoine Bédard of the University of Warwick and Institute for Advanced Study researcher Sihao Cheng to make the discovery.

Over 97 per cent of stars in the Milky Way will eventually become white dwarfs. Scientists have long considered these stars to be at the end of their lives. Having depleted their nuclear energy source, they stop producing heat and cool down until the dense plasma in their interiors freezes into a solid state, and the star solidifies from the inside out. This cooling process can take billions of years.

According to the new paper, 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 centre of the star releases gravitational energy, and this energy is enough to interrupt the star’s…

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