Editing Cepheid variable (section)

{{Short description|Type of variable star that pulsates radially}}
{{redirect|Cepheid|}}
[[File:Heic1323a -1243686232.jpg|thumb|upright=1.4|[[RS Puppis]], one of the brightest known Cepheid variable stars in the [[Milky Way]] galaxy<br />(<small>[[Hubble Space Telescope]]</small>)]]

A '''Cepheid variable''' ({{IPAc-en|ˈ|s|ɛ|f|i|.|ɪ|d|,_|ˈ|s|iː|f|i|-}}) is a type of [[variable star]] that [[Instability strip|pulsates radially]], varying in both diameter and temperature. It changes in brightness, with a well-defined stable [[frequency|period]] (typically 1–100 days) and amplitude. Cepheids are important cosmic benchmarks for scaling [[cosmic distance ladder|galactic and extragalactic distances]]; a strong direct [[period-luminosity relation|relationship]] exists between a Cepheid variable's [[luminosity]] and its [[periodic function|pulsation period]].

This characteristic of classical Cepheids was discovered in 1908 by [[Henrietta Swan Leavitt]] after studying thousands of variable stars in the [[Magellanic Clouds]]. The discovery establishes the ''true luminosity'' of a Cepheid by observing its pulsation period. This in turn gives the distance to the star by comparing its known luminosity to its observed brightness, calibrated by directly observing the [[Parallax in astronomy|parallax]] distance to the closest Cepheids such as RS Puppis and [[Polaris]].

Cepheids change brightness due to the [[Kappa–mechanism|κ–mechanism]],<ref name=":0" /><ref name=":1" /> which occurs when opacity in a star increases with temperature rather than decreasing.<ref name="maeder2009">{{cite book |last=Maeder |first=André |url=https://archive.org/details/physicsformation00maed_615 |title=Physics, formation and evolution of rotating stars |date=2009 |publisher=Springer |isbn=978-3-540-76948-4 |series=Astronomy and astrophysics library |page=[https://archive.org/details/physicsformation00maed_615/page/n383 373] |url-access=limited}}</ref> The main gas involved is thought to be [[helium]]. The cycle is driven by the fact [[Double ionization|doubly ionized]] helium, the form adopted at high temperatures, is more opaque than singly ionized helium. As a result, the outer layer of the star cycles between being compressed, which heats the helium until it becomes doubly ionized and (due to opacity) absorbs enough heat to expand; and expanded, which cools the helium until it becomes singly ionized and (due to transparency) cools and collapses again. Cepheid variables become dimmest during the part of the cycle when the helium is doubly ionized.