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Hydrostatic Equilibrium
Hydrostatic equilibrium in a star represents a balance between inward and outward forces, producing a stable form. The inward acting force of gravity causes a star to collapse. At the same time, the outward acting force of gas pressure and radiation causes the star to expand. If one of these forces exceeds the other, the star will be dynamically unstable, either collapsing or exploding. However, with main-sequence stars, such as the sun, the forces are balanced---creating hydrostatic equilibrium. The outward forces are produced by fusion reactions in the star's core.
Stellar Thermostat
Hydrostatic equilibrium serves as the thermostat for main-sequence stars. If a star's core begins to cool down, the outward force will be reduced, causing the star to contract. The contraction will compress the core, increasing temperatures and the rate of fusion. This increases the outward force, causing the star to expand. The expansion reduces the star's density, lowering temperatures at the core and the rate of fusion. The overall result is a positive feedback system that maintains the hydrostatic equilibrium by controlling the rate of fusion reaction rate, as long as the star has fuel to burn.
Stellar Mass
Hydrostatic equilibrium is directly tied to a star's mass. The mass determines the required internal pressure to achieve hydrostatic equilibrium. An increase in mass results in a corresponding increase in the force of gravity, or inward pressure. This determines what amount of outward force is required to balance the forces. The star's mass also determines the density of the star at this point of hydrostatic equilibrium. This relationship limits the size of stars. Too little mass and there will not be enough gravity to trigger fusion. Too much mass and the outward force of radiation will cause excess mass to be blown off.
Limits On Stellar Mass
The natural range of star mass is between 0.08 and 100 solar masses, where one solar mass equals the mass of the sun. The lower limit represents a mass that is about 80 times the mass of Jupiter. The upper limit represents the mass of the largest stars astronomers have discovered, such as Eta Carinae at 100 solar masses. Theoretically, the upper limit could extend as high as 200 solar masses. Astronomers hypothesize that stars this size were common shortly after the birth of the universe. These massive stars had very short lives, but created the heavy elements found in the universe.
Stellar Luminosity and Lifespan
Luminosity is also related to mass and hydrostatic equilibrium. Massive stars require high fusion rates to generate more outward forces. This results in a brighter star, but also causes the star to burn through its fuel at a much faster rate. Small changes in mass result in large changes in a star's luminosity and lifespan.