Radiation Pressure

Minute pressure exerted by electromagnetic radiation (such as but not limited to light from the sun) on everything it encounters. This can be thought of as the transfer of momentum from photons as they strike the surface of the object. In the environs of stars such pressure can become important given the vast quantities of photons emitted. Under the essentially blackbody conditions that exist inside a star, radiation pressure is proportional to the fourth power of temperature via the equation:

where T = temperature, σ = Stefan-Boltzmann constant, and c = speed of light. Consequently, a small increase in temperature results in a large increase in the radiation pressure.

Most main sequence stars, which have internal temperatures of millions of degrees, are primarily supported against gravity by gas pressure although radiation pressure does contribute a few percent. The internal temperatures of massive stars are hundreds of times higher and, at these extreme conditions, radiation pressure begins to dominate. In the most massive stars, the mass of the star is supported against gravity primarily by radiation pressure, which ultimately sets the upper limit for how massive a star can become.

Other astronomical objects are also influenced by radiation pressure. The pressure from solar photons creates the dust tails of comets within our Solar System. Radiation pressure plays a vital role in the formation of planetary nebulae: as the dying star contracts into a white dwarf, it releases vast amounts of heat. This radiation pressure is so strong that the outer layers of the star are pushed out to form the surrounding gaseous nebula. Similarly, a giant star ejects material and gas into the interstellar medium through radiation pressure.

Image Source: Unknown

Some or all content above used with permission from J. H. Wittke.

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