Instability strip

A region of the Hertzsprung–Russell diagram where stars undergo self-excited pulsations. It contains variable stars such as Cepheids and RR Lyrae.

Eddington valve: A stellar pulsation mechanism in which increased opacity traps heat and drives expansion. The helium ionization layers act as this valve, feeding energy into pulsations.
Hertzsprung–Russell diagram: A plot of stellar luminosity against surface temperature or spectral type. The instability strip is located by temperature and luminosity on this diagram.
Classical Cepheid variable: A young, massive pulsating star whose period is related to its intrinsic luminosity. Classical Cepheids are prominent strip occupants and key distance indicators.
Period–luminosity relation: An empirical relation linking a pulsating variable star’s period to its intrinsic luminosity. Cepheids in the strip use this relation to measure distances beyond the Milky Way.
Classical instability strip: The diagonal region of the Hertzsprung–Russell diagram containing classical pulsators such as Cepheids and RR Lyrae. This conventional name often denotes the same strip, distinguishing it from other stellar instability regions.
κ-mechanism: A pulsation-driving process in which opacity changes during compression trap radiation and add energy to a stellar oscillation. It explains how stars inside the strip sustain pulsation against damping.
Stellar luminosity: The total energy a star emits per unit time across all wavelengths. Luminosity supplies one coordinate used to locate the strip and its stars.
RR Lyrae variable: An old, low-mass pulsating star commonly found in globular clusters. RR Lyrae stars occupy the strip at lower luminosities than classical Cepheids.
Leavitt's law: The relation between pulsation period and luminosity established for Cepheid variable stars. It makes classical Cepheids from the strip standard candles.
Solar-like oscillations: Stellar oscillations excited stochastically by turbulent convection rather than maintained by a heat-engine instability. Unlike strip pulsations, these oscillations are continually excited and damped by convection.

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