  It was predicted more than 40 years ago that the cores of the coolest white
dwarf stars should eventually crystallize. This effect is one of the largest
sources of uncertainty in white dwarf cooling models, which are now routinely
used to estimate the ages of stellar populations in both the Galactic disk and
the halo. We are attempting to minimize this source of uncertainty by
calibrating the models, using observations of pulsating white dwarfs. In a
typical mass white dwarf model, crystallization does not begin until the
surface temperature reaches 6000-8000 K. In more massive white dwarf models the
effect begins at higher surface temperatures, where pulsations are observed in
the ZZ Ceti (DAV) stars. We use the observed pulsation periods of BPM 37093,
the most massive DAV white dwarf presently known, to probe the interior and
determine the size of the crystallized core empirically. Our initial
exploration of the models strongly suggests the presence of a solid core
containing about 90% of the stellar mass, which is consistent with our
theoretical expectations.
