Gas and Radiation Pressure

In a hot plasma, there are two sources of pressure: the gas pressure, generated by the gas particles, and radiation pressure, generated by the photons in the gas. At lower temperatures such as those in the Sun, the pressure of the gas almost completely dominates the pressure of the radiation.

However, radiation pressure increases with the fourth power of the temperature while the gas pressure increases only with the first power of the temperature, so as temperature increases eventually radiation pressure becomes more important than gas pressure. Since more massive stars are hotter, radiation pressure is relatively more important for them. Calculations indicate that for stars more massive than about 20-30 solar masses, the radiation pressure becomes more important than the gas pressure.

Upper Mass Limit for the Main Sequence

The upper limit for the mass of a main sequence star is probably set by the ability of a star to hold on to its outer envelope against the radiation pressure fueled by rapid fusion reactions in a masssive core. We now show that this upper limit is thought to be about a hundred solar masses.

Eddington Luminosity
The flux of photons out of the star exerts a force on the particles in its outer layers but that force is opposed by the gravitational force. We may ask, at what luminosity does the force from the photons directed outward exactly balance the inward pull of gravity? This critical luminosity is called the Eddington luminosity. If it is exceeded, we may expect that the star literally uses its own light to blow itself off into space, layer by layer, until the luminosity is lower than the Eddington luminosity.

The Eddington luminosity places an upper limit on the mass of a normal star because the actual luminosity increases with mass and will exceed the Eddington luminosity at some point. Estimates based on this idea suggest that about 100 solar masses is the maximum stable mass for a star, and observations suggest that indeed stars with more mass than that are very rare.

Envelope Loss from Massive Stars
Very massive stars may go through a stage in their lives where they expel large portions of their envelopes into space at velocities as large as 1000 km/s. A few such stars are known in our galaxy; they are called Wolf-Rayet stars. They are thought to be stars that were very massive when they were young and have since blown off their outer layers of gas, exposing the hot helium core. As a result, they are usually very strong UV emitters.

The adjacent image shows the nebula N2359. This is a wind-blown shell of gas that has been expelled from the Wolf-Rayet star HD56925 (marked with the arrow.) The star lies on the edge of a thick molecular cloud. The nebula contains shock waves associated with interaction of the wind and the interstellar medium, and also is glowing from excitation of material expelled previously from the star.