White dwarf star in the process of solidifying. (University of Warwick/Mark Garlick)The first direct evidence of crystallized white dwarf stars has been discovered by an international team of researchers that includes two former astronomers from the University of North Carolina at Chapel Hill. Predicted half a century ago, the discovery of these stars will be published in the January 10 edition of the journal Nature.
Observations have revealed that these stars have a core of solid carbon and oxygen due to a phase transition during their lifecycle, similar to water turning into ice. This phase transition slows their cooling in multiple ways, making them potentially billions of years older than previously thought.
The discovery, led by Pier-Emmanuel Tremblay of the U.K.’s University of Warwick, is largely based on observations taken with the European Space Agency’s Gaia satellite.
Bart Dunlap, a postdoctoral fellow with UT Austin’s Wootton Center for Astrophysical Plasma Properties, along with JJ Hermes, made the discovery independently of the Warwick team while working together at UNC-Chapel Hill and later joined forces with Tremblay. Hermes is now an assistant professor at Boston University.
Almost all stars end up as white dwarfs, and some of them are among the oldest stars in the universe. They are useful to astronomers because their predictable cooling rate allows them to be used as cosmic clocks to estimate the ages of groups of stars. They are the leftover cores of red giant stars, after these huge stars have died and shed their outer layers. They are then constantly cooling as they release their stored-up heat over billions of years.
The Gaia satellite has enabled the selection of a sample of white dwarfs with precise luminosities and colors that is significantly larger and more complete than any previous survey. For the study, the team selected 15,000 white dwarfs within about 300 light-years …