  Compact relativistic stars allow us to study the nature of matter under
extreme conditions, probing regions of parameter space that are otherwise
inaccessible. Nuclear theory in this regime is not well constrained: one key
issue is whether neutron stars are in fact composed primarily of strange quark
matter. Distinguishing the two possibilities, however, has been difficult. The
recent detection of seismic vibrations in the aftermath of giant flares from
two magnetars (highly magnetized compact stars) is a major breakthrough. The
oscillations excited seem likely to involve the stellar crust, the properties
of which differ dramatically for strange stars. We show that the resulting mode
frequencies cannot be reconciled with the observations for reasonable magnetar
parameters. Ruling out strange star models would place a strong constraint on
models of dense quark matter.
