Type II Cepheid

A low-mass pulsating variable star, usually found in an old stellar population, whose brightness changes periodically as it expands and contracts.

Stellar pulsation: Periodic expansion and contraction of a star caused by oscillations in its interior. Type II Cepheids change brightness because their outer layers repeatedly expand and contract.
Cepheid variable: A pulsating variable star whose period is related to its luminosity. Type II Cepheids are a distinct low-mass branch of the Cepheid family.
W Virginis variable: A Type II Cepheid subclass with periods generally between about 4 and 20 days. W Virginis stars are the intermediate-period members of the Type II Cepheid family.
Distance ladder: A sequence of methods for measuring astronomical distances across increasingly large scales. Period–luminosity measurements let Type II Cepheids contribute to distance estimates.
Anomalous Cepheid: A pulsating variable star more luminous than an RR Lyrae star but generally less massive than a classical Cepheid. Its periods and luminosities can overlap those of Type II Cepheids, but its origin differs.
Instability strip: A region on the Hertzsprung–Russell diagram where stars develop radial pulsations. Type II Cepheids cross this region during evolutionary phases that make them pulsate.
Classical Cepheid: A young, massive pulsating star that follows a period–luminosity relation. Its higher mass and younger population distinguish it from Type II Cepheids.
BL Herculis variable: A Type II Cepheid subclass with periods generally shorter than about 4 days. BL Herculis stars occupy the short-period end of the class.
Distance modulus: The difference between an object's apparent and absolute magnitudes, related to its distance. Comparing observed brightness with inferred luminosity yields a Type II Cepheid's distance modulus.
Population II star: An old star, usually metal-poor, formed early in a galaxy's history. Type II Cepheids belong to this broad stellar population, though the labels describe different things.

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Classical Cepheid variableCompared with: Its older population and distinct luminosity calibration distinguish it from young classical Cepheids.