  Bubbles in a first-order electroweak phase transition are nucleated with
radii $R_0$ and expand with velocity $v$. If $v$ is subsonic, a bubble becomes
unstable to non-spherical perturbations when its radius is roughly $10^4\,
R_0$. These perturbations accelerate the transition, and the effective velocity
of bubble growth rapidly becomes supersonic. The transition should subsequently
proceed spherically via detonation. If for some reason the onset of detonation
is postponed, the surface area of the bubbles may be enhanced by $10^5$. We
discuss consequences for electroweak baryogenesis.
