Researchers have discovered why some white dwarfs remain bright for billions of years: a fundamental process in which lighter crystals rise and denser liquids sink, balancing energy and maintaining surface brightness.

Your astronomy textbook may describe white dwarfs as the cool and relatively uninteresting remnants of dead stars. This view is challenged by the previously unexplained existence of slowed-down white dwarfs, which have defied expectations by shining as brightly as some familiar main-sequence stars for billions of years.

The new study, led by Simon Blouin and co-authors from the University of Warwick and the Institute for Advanced Study in Princeton, New Jersey, shows that low-density crystals form and float in the cores of stars exhibiting this strange behavior. Liquids containing heavy impurities sink. This solid-liquid distillation process interrupts cooling for billions of years and explains all the observed features of the unusual population of slowing white dwarfs.

Life cycle of stars and cooling of white dwarfs

The life cycle of a star begins in a nebula of gas, where gravity begins to pull matter together until it accumulates in quantities that enable the new solar core to bring together hydrogen nuclei and emit light into the universe. Eventually most stars exhaust their nuclear fuel, shed their outer layers into a planetary nebula, and become Earth-sized white dwarfs where fusion no longer occurs.

These stars, which did not have a fuel source for fusion, were expected to cool before the end of time. These cooling assumptions inform estimates of the age of the white dwarf and influence our understanding of the formation of the Milky Way.

Gaia satellite observations and research results

The expectation of white dwarf cooling comes from data from the European Space Agency’s Gaia satellite, which showed in 2019 that the white dwarf population may have stopped cooling for more than eight billion years, nearly twice the age of Earth and nearly twice the age of most of Earth. contradicts observations. The age of the universe since the Big Bang. explosion

Blouin and colleagues’ findings explain the long-lasting glow of white dwarfs by a “distillation process” (light crystals form and float to the top, while denser liquids sink to the bottom) that causes the release of gravitational energy. The energy released as a result of this process almost exactly balances the energy emitted by the white dwarf into space, making the surface brightness and temperature almost constant.

“It will be important to take this mechanism into account when using white dwarfs as cosmic clocks in the future to measure the age of stars,” explains Blouin.

Contributed by Simon Blouen

Simon Blouen is a Canadian Institute for Theoretical Astrophysics (CITA) National Research Fellow with Professor Falk Gerwig at the University of Victoria. Blouin earned his PhD in physics from the University of Montreal in 2019, followed by postdoctoral fellowships at Los Alamos National Laboratory and UVic. Their work uses a wide variety of simulation techniques to develop white dwarf models. This improves the ability of physicists and astronomers to use these stars as precise cosmic clocks that help understand the history of star formation in our Milky Way galaxy.

In his latest study published recently Nature Blouin and colleagues explain the second stellar life of white dwarfs by identifying a mechanism that keeps lagging white dwarfs hot for billions of years.