A photon bubble is a type of radiation-driven instability

Instability

In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds...

 that can occur in the magnetized, radiation-supported gas surrounding neutron stars, black hole

Black hole

A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

 accretion disks or at the edge of ultra-compact HII regions around young, massive stars

OB star

OB stars are hot, massive stars of spectral types O or B which form in loosely organized groups called OB associations. They are short lived, and thus don't move very far from where they were formed within their life. During their lifetime, they will emit copious amounts of ultraviolet radiation...

. The instability occurs as follows. A compressive magnetohydrodynamical wave propagating at right angles to the direction of propagation of the radiation creates variations in the density of the gas. More radiation is able to pass through the low density regions than through the high density regions, and the imbalance in radiation pressure

Radiation pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

 acts to drive gas out of the low density regions, along the magnetic field lines. This further decreases the density of the low density regions, which in turn allows more radiation to propagate through them, leading to runaway growth of the instability.