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asymptotic giant branch (AGB)  The portion of the H-R diagram occupied by enormous, cool stars with helium-burning shells.
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.
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.
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.
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.
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.
horizontal branch star  A star that is undergoing helium fusion in its core and hydrogen fusion in a shell surrounding the core.
instability strip  A region of the H-R diagram occupied by pulsating stars, including Cepheid variables and RR Lyrae stars.
isochrone  Lines in an H-R diagram occupied by stars of different masses but the same age.
main sequence lifetime  The length of time that a star spends as a main sequence star.
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.
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.
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.
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.
RR Lyrae star  A member of a class of giant pulsating stars, all of which have pulsation periods of about 1 day.
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.
thermal pulse  The rapid consumption of helium in a shell withinan a symptotic giant branch star.
triple _ process  A pair of nuclear reactions through which three helium nuclei (alpha particles) are transformed into a carbon nucleus.
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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