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NGC 6791 is a well studied open cluster1 that it is so close to us that canbe imaged down to very faint luminosities. The main sequence turn-off age (~8Gyr) and the age derived from the termination of the white dwarf coolingsequence (~6 Gyr) are significantly different. One possible explanation is thatas white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in thedeep interior of these stars. At lower temperatures, white dwarfs are expectedto crystallise and phase separation of the main constituents of the core of atypical white dwarf, 12C and 16O, is expected to occur. This sequence of eventsis expected to introduce significant delays in the cooling times, but has nothitherto been proven. Here we report that, as theoretically anticipated,physical separation processes occur in the cores of white dwarfs, solving theage discrepancy for NGC 6791.

A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes