Cepheid variable

A pulsating star whose regular brightness changes are linked to its intrinsic luminosity. This period–luminosity relation makes Cepheids important distance indicators.

Instability strip: A region of the Hertzsprung–Russell diagram where stars become radially pulsating variables. Cepheids cross this region, where partial ionization destabilizes their outer layers.
Cosmic distance ladder: A sequence of calibrated methods for measuring distances across increasingly large scales in the universe. Cepheids bridge geometric measurements of nearby stars and distance estimates for other galaxies.
Henrietta Swan Leavitt: An American astronomer who discovered the period–luminosity relation for Cepheid variables in the Small Magellanic Cloud. Her comparison of Cepheid periods and apparent brightness established the key distance relation.
RR Lyrae variable: A pulsating horizontal-branch star with a relatively short period and nearly standard intrinsic luminosity. RR Lyrae stars are older and fainter than classical Cepheids, making them useful for different stellar populations.
Cepheid mass discrepancy: The historical mismatch between Cepheid masses inferred from stellar evolution and from pulsation models. Resolving it improves models that connect a Cepheid’s pulsation period to its physical properties.
Kappa mechanism: A stellar pulsation process in which opacity changes trap and release heat, driving oscillations. It supplies the energy that sustains the repeating expansion and contraction of Cepheids.
Hubble constant: The present-day rate at which the universe’s expansion increases with distance. Cepheid-calibrated distances to galaxies help determine this expansion rate.
Small Magellanic Cloud: A dwarf galaxy orbiting the Milky Way, visible from the Southern Hemisphere. Its Cepheids lie at nearly the same distance from Earth, allowing Leavitt to compare their luminosities.
Type II Cepheid: An old, low-mass pulsating star with a period–luminosity relation distinct from classical Cepheids. Its different luminosity relation prevents treating every Cepheid as the same distance indicator.
Metallicity: The abundance of elements heavier than helium in a star or other astronomical object. Chemical composition can affect Cepheid brightness and complicate period–luminosity calibrations across galaxies.

Linked from 38 pages