Your astronomy textbook might describe white dwarfs as the cold and relatively uninteresting remnants of dead stars. The previously unexplained existence of delayed white dwarfs defies expectations and shines as brightly as some familiar main sequence stars for billions of years.
Researchers have discovered the reason some white dwarfs have kept glowing for billions of years: It's a core process in which lighter crystals rise and denser liquids sink, balancing energy and maintaining surface brightness.
New research by Simon Blouin and co-authors from the University of Warwick and the Institute for Advanced Study in Princeton, New Jersey, reveals that in the cores of these strangely behaving stars, less dense crystals form and float, while denser liquids containing heavy impurities sink. This solid-liquid distillation process interrupts billions of years of cooling and explains all the observed properties of the anomalous population of delayed white dwarfs.
Stellar Life Cycle and White Dwarf Cooling
A star's life cycle begins in a gas nebula, where gravity begins to pull matter together until enough matter accumulates that the core of a new sun begins to fuse hydrogen nuclei together and emit light into the universe. Eventually, most stars will run out of nuclear fuel, shed their outer layers and enter planetary nebulae, eventually becoming Earth-sized white dwarfs that will no longer undergo nuclear fusion.
Without a fuel source for nuclear fusion, these stars are expected to simply cool down for the rest of their time. These assumptions about cooling provide the basis for estimates of the age of white dwarfs, which in turn affects our understanding of the formation of the Milky Way.
Gaia satellite observations and research results
The expectation of white dwarf cooling conflicts with observational data from the European Space Agency's Gaia satellite, which showed in 2019 that the white dwarf population was apparently able to stop cooling in more than 8 billion years, which is almost twice the age of the Earth and more than half the age of the universe since the Big Bang.
The discovery by Blouin and his collaborators explains the reason for the white dwarf's persistent glow -- a "distillation process" in which light crystals form and float while denser liquids sink, resulting in the release of gravitational energy. The energy output from this process almost completely balances the energy radiated by the white dwarf into space, keeping its surface luminosity and temperature essentially unchanged.
"Going forward," Blouin explains, "it will be important to take this mechanism into account when using white dwarfs as cosmic clocks to measure stellar ages."
Simon Blouin's contribution
Simon Blouin is a national researcher at the Canadian Institute for Theoretical Astrophysics (CITA) and studied under Professor Falk Herwig at the University of Victoria. Blouin received his PhD in physics from the University of Montreal in 2019, and then completed postdoctoral research at Los Alamos National Laboratory and the University of Victoria in the United States. His work uses a variety of simulation techniques to improve white dwarf models. This improves the ability of physicists and astronomers to use these stars as precise cosmic clocks, helping to infer the history of star formation in the Milky Way.
In their latest work, just published in the journal Nature, Blouin and his collaborators identify the mechanism that keeps delayed white dwarfs hot for billions of years, explaining the white dwarf's second stellar life.
Compiled from:ScitechDaily