BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass ( \sim 1.1 M _ { \odot } ) to theoretically crystallize while still inside the ZZ Ceti instability strip ( T _ { eff } \sim 12,000 K ) .
As a consequence , this star represents our first opportunity to test crystallization theory directly .
If the core is substantially crystallized , then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star , affecting mainly the average period spacing .
This is distinct from the “ mode trapping ” caused by the stratified surface layers , which modifies the pulsation periods more selectively .
In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999 .
Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized .