Period–luminosity relation

The period–luminosity relation connects a pulsating star’s cycle period to its intrinsic luminosity. It lets astronomers infer a star’s luminosity, and hence distance, from its observed pulsation.

Cepheid variable: A pulsating star whose regular brightness changes are linked to its intrinsic luminosity. Classical Cepheids provide the relation’s best-known calibration for measuring extragalactic distances.
Stellar pulsation: Periodic expansion and contraction of a star that changes its radius, temperature, and brightness. The relation applies to stars whose changing structure produces a measurable cycle.
Henrietta Swan Leavitt: An American astronomer who discovered the period–luminosity pattern among Cepheid variables in the Magellanic Clouds. Her analysis established the empirical foundation of the relation.
Type II Cepheid: An old, low-mass pulsating star whose period–luminosity relation differs from that of classical Cepheids. Confusing the two Cepheid populations can bias inferred distances.
Leavitt law: The relation between pulsation period and luminosity discovered in Cepheid variables. This name commonly denotes the Cepheid form used to estimate distances.
Cepheid instability strip: A region of the Hertzsprung–Russell diagram where stars become unstable to radial pulsation. Cepheids occupy this region, where opacity-driven oscillations sustain their cycles.
Harvard College Observatory: An astronomical research institution founded in Cambridge, Massachusetts, in 1839. Leavitt made the photographic observations and comparisons there.
RR Lyrae variable: An old, low-mass pulsating star used as a standard candle in the Milky Way and nearby galaxies. Its shorter periods and different luminosity calibration distinguish it from classical Cepheids.
Distance ladder: A sequence of astronomical methods that calibrates distances from nearby objects to the distant universe. Cepheid luminosities anchor steps leading to measurements of the Hubble constant.
Eddington valve mechanism: An opacity-driven process that converts radiative energy into pulsations in certain stars. It explains how Cepheid envelopes maintain the oscillations whose periods are measured.

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