  We show that the optically-thick dusty envelopes surrounding young high-mass
stars are subject to the photon bubble instability. The infrared radiation
passing through the envelope amplifies magnetosonic disturbances, with growth
rates in our local numerical radiation MHD calculations that are consistent
with a linear analysis. Modes with wavelengths comparable to the gas pressure
scale height grow by more than two orders of magnitude in a thousand years,
reaching non-linear amplitudes within the envelope lifetime. If the magnetic
pressure in the envelope exceeds the gas pressure, the instability develops
into trains of propagating shocks. Radiation escapes readily through the
low-density material between the shocks, enabling accretion to continue despite
the Eddington limit imposed by the dust opacity. The supersonic motions arising
from the photon bubble instability can help explain the large velocity
dispersions of hot molecular cores, while conditions in the shocked gas are
suitable for maser emission. We conclude that the photon bubble instability may
play a key role in the formation of massive stars.
