Temperature, Pressure, and Gravity

The formation of stars is the story of how temperature, pressure, and gravity act on the cold dark clouds of hydrogen and dust that incubate stars. This is a large and complex subject, but the main principles that govern the formation of stars can be stated simply:

  • Gravity, acting mutually on all particles in a cloud, attempts to contract the cloud to the smallest possible radius.
  • Energy is conserved. Thus, when a cloud contracts its gravitational potential energy is converted to other forms, primarily kinetic energy of the gas (internal heat). This raises the gas temperature.
  • At fixed volume, increasing the temperature of a gas raises the pressure that it exerts by increasing the average velocity of the particles in the gas. The increased pressure resists the attempt by gravity to contract the cloud.
  • For the contraction to proceed further, the cloud must radiate some of its internal heat energy into space. The rate at which it can do this is governed by the rate at which it can transport energy to its surface. Thus, the contraction rate is ultimately set by the energy transport rate.
  • Internal energy transport in normal stars is dominated by radiative transport (energy transport by photons), unless the rate of temperature decrease with radius becomes too high. In that case, the star switches to convection to transport its energy.
  • A star in equilibrium must radiate its internal energy into space at the same rate that it is producing it. If this is not so, the star is not in equilibrium and will be unstable to changes that will try to bring it into equilibrium.

    • These simple concepts are sufficient to understand much of the evolutionary history of a star, from its formation to its death.