Classical Cepheid variable

A young, luminous Population I star that expands and contracts radially in a regular cycle. Its pulsation period is closely related to its intrinsic luminosity, making it a standard candle for measuring distances.

κ-mechanism: A stellar pulsation mechanism in which partial ionization zones trap and release heat, driving periodic expansion and contraction. Helium ionization zones supply the heat-trapping feedback that sustains Cepheid pulsations.
Period–luminosity relation: An empirical relation linking a variable star’s pulsation period to its intrinsic luminosity. A Cepheid’s measured period predicts its luminosity, which can be compared with observed brightness.
Delta Cephei: A bright classical Cepheid in the constellation Cepheus, whose variability gave Cepheids their name. This prototype established the recognizable class of regularly pulsating stars.
Type II Cepheid: An old, low-mass pulsating star whose period–luminosity relation differs from that of classical Cepheids. Its older population and distinct luminosity calibration distinguish it from young classical Cepheids.
Harlow Shapley: An American astronomer who used variable stars to estimate the Milky Way’s size and locate its center. His Cepheid-based work expanded estimates of the Milky Way beyond earlier scales.
Instability strip: A region of the Hertzsprung–Russell diagram where stars become unstable to pulsation. Classical Cepheids cross this region as they evolve, where radial pulsations can grow.
Leavitt's law: The period–luminosity relation discovered by Henrietta Swan Leavitt for Cepheid variables in the Magellanic Clouds. Its discovery made Cepheids usable as distance indicators once their luminosities were calibrated.
John Goodricke: An English astronomer who studied variable stars and proposed a periodic explanation for Algol's brightness changes. His early work on stellar variability preceded the identification of Cepheid periodicity.
RR Lyrae variable: An old, low-mass pulsating star commonly used to measure distances within the Milky Way and nearby galaxies. RR Lyrae stars are typically less luminous and shorter-period than classical Cepheids.
Walter Baade: A German-American astronomer who distinguished stellar populations and revised key extragalactic distance estimates. His recognition of distinct Cepheid populations corrected earlier distance calibrations.

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