The Crystal Method : Asteroseismology of BPM 37093

More than 40 years have passed since Ed Salpeter and others predicted that the carbon/oxygen cores of the coolest white dwarf stars in our Galaxy will theoretically crystallize. This effect has a dramatic impact on the calculated ages of cool white dwarfs, but until recently we have had no way of testing the theory. In 1992, pulsations were discovered in the massive potentially crystallized white dwarf BPM 37093, and in 1999 the theoretical effects of crystallization on the pulsation modes were determined. Observations from two Whole Earth Telescope campaigns in 1998 and 1999, combined with a new model-fitting method using a genetic algorithm, are now giving us the first glimpse inside of a crystallized star. In 1994, Ed Nather was reading the newspaper and came across a headline which read, " Crisis in the Cosmos-stars older than the universe ". Reading through the accompanying article, he learned that a group of astronomers had fit cosmological models to new observations from the refurbished Hubble Space Telescope and had concluded that the Universe was between 8 and 12 billion years old. Meanwhile, another group of astronomers had used stellar models to fit main-sequence isochrones to observations of a globular cluster and had derived an age of about 18 billion years. When presented with these details, Ed's reaction was: " Where's the crisis? Either the cosmological models are wrong, the stellar models are wrong, or both! " As we now know, the age estimates from these two methods later met somewhere in between. The lesson here is that it is always useful to have independent methods of measuring a quantity, because it gives us a chance to improve our models. In the context of asteroseismology, if we restrict the range of our search for pulsation models to be consistent with the spectroscopic determinations of the surface gravity and effective temperature, we will never find a disagreement between the models. A global search allows the pulsation periods to speak for themselves, and lends more credibility to the final results. 2. Theory There is a very simple explanation for how crystallization affects the derived ages of cool white dwarfs: the process releases latent heat. This is familiar to anyone 1