Hydrogen can be fused into helium at a temperature of about 10 million K. Fusion of helium and heavier elements requires still higher temperatures. When a star lacks nuclear energy sources, energy can be produced by gravitational contraction or collapse.
The opacity of a gas describes its ability to block the flow of radiation. Where opacity is low, radiation flows nearly unimpeded, so it easily carries energy outward through a star. Where opacity is high, the flow of radiant energy is inefficient. Convection currents develop and carry most of the energy outward.
When a gas is degenerate, its particles are compressed so much that they strongly resist further compression. As a result, the temperature of a degenerate gas can decrease without causing a similar decrease in pressure.
The Vogt-Russell theorem states that the initial mass and chemical composition of a star determine its entire evolution. Stars with close binary companions are exceptions to the Vogt-Russell theorem.
As a star evolves, its luminosity and surface temperature change. Thus, the evolution of the star can be described by its changing position in an H-R diagram. An H-R diagram of a star cluster is useful because it represents a snapshot of the evolution of a group of stars that have different masses but the same age and chemical composition.
Main sequence stars are those that consume hydrogen in their cores. Despite this common property, main sequence stars of different masses show a wide range of sizes, luminosities, temperatures, and internal structures.
Massive stars spend much less time on the main sequence than less massive main sequence stars because their greater luminosities result in more rapid consumption of their hydrogen. While on the main sequence, stars change little in temperature and luminosity.
After the hydrogen in the core of a star is used up, fusion begins in a thin shell surrounding its helium core. The star swells and cools to become a red giant. Rising temperature in the core of the star eventually initiates the fusion of helium. In stars like the Sun, this happens in an explosive helium flash.
While burning helium in its core, a star becomes hotter, moving across the H-R diagram at constant luminosity. During part of this horizontal track, the star develops pulsations that cause its size, temperature, and brightness to vary periodically. The period-luminosity relationship for Cepheid variables can be used to determine their distances.
When the supply of helium in the core of a star is exhausted, the shell of helium surrounding the core begins to be consumed. A star with a helium-burning shell once again swells and cools, becoming an asymptotic giant branch (AGB) star. AGB stars shed their outer layers in cool winds in which dust particles form. The dust shields the star and converts its light to infrared radiation. Eventually, for all but the most massive AGB stars, the wind strips away nearly all the outer layers of the star.
At the end of its AGB phase, a star quickly becomes hotter. When its surface temperature reaches 30,000 K, it ionizes the gas that it had earlier shed as a cool wind. The ionized matter glows as a planetary nebula.
Elements as massive as iron can be formed in energy-generating nuclear reactions in stars. More massive elements are made by the capture of neutrons by nuclei. The mix of elements and isotopes that results from neutron capture depends on whether the rate of neutron captures is rapid (the r-process) or slow (the s-process).
