The Balance between Radiation Pressure and Gravity

The Balance between Radiation Pressure and Gravity

• Radiation pressure and gravity are two forces acting in opposite directions within a star.
• Gravity, always acting inward, pulls the star material towards its core.
• Radiation pressure, always acting outward, results from the energy produced in the star’s core, pushing the star’s matter outwards.
• The balance between these two forces is referred to as hydrostatic equilibrium.

Establishment of Hydrostatic Equilibrium

• Hydrostatic equilibrium is reached when the inward force of gravity is exactly balanced by the outward force exerted by radiation pressure.
• When inside a star, if radiation pressure wins, the star expands, which in turn decreases the force of gravity due to increased distance between particles.
• If gravity wins, the star contracts, which increases temperature and pressure, producing more radiation pressure.
• Over time, an equilibrium is established where the star neither expands nor contracts significantly - it has reached its state of hydrostatic equilibrium.

Role in Stellar Evolution

• The hydrostatic equilibrium is crucial in governing the various stages of stellar evolution.
• A star spends most of its life in a state of hydrostatic equilibrium, which occurs during the main sequence stage.
• The equilibrium may be upset if there is a change in energy output from the core, such as when a star exhausts the hydrogen fuel in its core and leaves the main sequence.
• The resulting struggle between gravity and pressure forces leads to the star entering new phases of life, such as a red giant or supernova stage, through stellar evolution.

Hydrostatic Equilibrium and Earth

• Hydrostatic equilibrium is constantly at work not only in stars but also in the Earth’s atmosphere.
• It helps in determining the distribution of atmospheric pressure and is crucial for weather prediction models. It also gives us an understanding of the stability of gas planets and stars.