A photon bubble is a type of radiation-driven
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. Not all systems that are not stable are unstable; systems can also be mar ...
that can occur in the magnetized, radiation-supported gas surrounding
neutron stars
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
,
black hole accretion disks
An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...
or at the edge of ultra-compact HII regions around young,
massive stars.
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 is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is a ...
acts to drive gas out of the low density regions, along the
magnetic field lines
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
. 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.
References
Fluid dynamics
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