| asymptotic giant branch (AGB) | The portion of the H-R diagram occupied by enormous, cool stars with helium-burning shells.
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| Cepheid variable | A member of a class of yellow pulsating stars that vary in brightness as they expand and contract. The period of a Cepheid is related to its luminosity.
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| degenerate gas | A gas in which a type of particle (electrons or neutrons) are as tightly packed as permitted by the Pauli exclusion principle. In a degenerate gas, temperature has essentially no influence on pressure.
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| equation of state | The relationship among pressure, density, and temperature for a gas or fluid. The ideal gas law, for which pressure is proportional to the product of temperature and density, is an example of an equation of state.
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| evolutionary track | The path in an H-R diagram followed by the point representing the changing luminosity and temperature of a star as it evolves.
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| helium flash | The explosive consumption of helium in the core of a star when helium fusion begins in a degenerate gas in which pressure doesn’t rise as energy is produced and temperature increases.
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| horizontal branch star | A star that is undergoing helium fusion in its core and hydrogen fusion in a shell surrounding the core.
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| instability strip | A region of the H-R diagram occupied by pulsating stars, including Cepheid variables and RR Lyrae stars.
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| isochrone | Lines in an H-R diagram occupied by stars of different masses but the same age.
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| main sequence lifetime | The length of time that a star spends as a main sequence star.
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| Pauli exclusion principle | A physical law that limits the number of particles of a particular kind that can be placed in a given volume. A gas in which that limit is reached is degenerate.
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| period-luminosity relationship | The relationship between the period of brightness variation and the luminosity of a Cepheid variable star. The longer the period of a Cepheid is, the more luminous the Cepheid.
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| planetary nebula | A luminous shell surrounding a hot star. The gas in a planetary nebula was ejected from the star while it was a red giant.
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| r-process | The process of building up massive nuclei in which neutrons are captured at a rate faster than the newly produced nuclei canundergo radioactive decay.
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| RR Lyrae star | A member of a class of giant pulsating stars, all of which have pulsation periods of about 1 day.
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| s-process | The process of building up massive nuclei in which neutrons are captured at a rate slower than the newly produced nucleican undergo radioactive decay.
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| thermal pulse | The rapid consumption of helium in a shell withinan a symptotic giant branch star.
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| triple _ process | A pair of nuclear reactions through which three helium nuclei (alpha particles) are transformed into a carbon nucleus.
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| Vogt-Russell theorem | The concept that the original mass and chemical composition of an isolated star completely determine the course of its evolution.
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