Stellar Radiation Pressure

The idea of pressure can be expressed as force per unit area or energy per unit volume. In the case of a thermal radiator, the energy density of the blackbody radiation may be calculated. If you consider the hot interior of a star, the radiation energy density can be related to the radiation pressure which can act to prevent further gravitational collapse of the star.

For blackbody radiation, the energy density integrated over all wavelengths is

where for astronomical applications the physical constants are combined in the parameter a which is called the radiation constant.
In practice, pressure is a directed quantity against some surface. In a star we visualize the outward pressure of the radiation against some surface that is opaque to the radiation and scatters it backward. When the geometry is analyzed for all incident photon angles upon the surface it yields just a factor of 1/3. The radiation pressure is then
or 1/3 of the integrated photon energy density. For comparison the pressure of an ideal monoatomic gas is two thirds of its energy density.

For temperature T = x 10^ K

the radiation pressure is
Pradiation = x 10^ Pa
Pradiation = x10^Atmospheres

Any of the above numbers may be changed. The standard SI unit for pressure is N/m2 or Pascals. Pressures are sometimes stated in terms of the standard Earth atmospheric pressure.

Radiation pressure and the transparency point

Star Concepts

Star Classification

Carroll & Ostlie
Ch 9
HyperPhysics***** Astrophysics R Nave
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