White dwarfs are stellar embers depleted of nuclear energy sources that cool over billions of years1. These stars, which are supported by electron degeneracy pressure, reach densities of 107 grams per cubic centimetre in their cores2. It has been predicted that a first-order phase transition occurs during white-dwarf cooling, leading to the crystallization of the non-degenerate carbon and oxygen ions in the core, which releases a considerable amount of latent heat and delays the cooling process by about one billion years3. However, no direct observational evidence of this effect has been reported so far. Here we report the presence of a pile-up in the cooling sequence of evolving white dwarfs within 100 parsecs of the Sun, determined using photometry and parallax data from the Gaia satellite4. Using modelling, we infer that this pile-up arises from the release of latent heat as the cores of the white dwarfs crystallize. In addition to the release of latent heat, we find strong evidence that cooling is further slowed by the liberation of gravitational energy from element sedimentation in the crystallizing cores5,6,7. Our results describe the energy released by crystallization in strongly coupled Coulomb plasmas8,9, and the measured cooling delays could help to improve the accuracy of methods used to determine the age of stellar populations from white dwarfs10.
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The Gaia DR2 catalogue of white dwarfs used in this study is available from the University of Warwick astronomy catalogues repository, https://warwick.ac.uk/fac/sci/physics/research/astro/research/catalogues/gaia_dr2_white_dwarf_candidates_v2.csv. All modelling was performed with our extensive white-dwarf evolution code. We have opted not to make this multi-purpose code available, but the cooling sequences calculated for this work are available on request.
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This research received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme number 677706 (WD3D) and under the European Union’s Seventh Framework Programme (FP/2007- 2013)/ERC Grant Agreement number 320964 (WDTracer). This work made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC was provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Support for J.J.H. was provided by NASA through Hubble Fellowship grant #HST-HF2-51357.001-A, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.