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
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.
Carroll & Ostlie