Editing Variable star (section)

==== Cepheids and cepheid-like variables ====
{{Main|Cepheid variable}}
This group consists of several kinds of pulsating stars, all found on the [[instability strip]], that swell and shrink very regularly caused by the star's own mass [[resonance]], generally by the [[fundamental frequency]]. Generally the [[κ mechanism|Eddington valve]] mechanism for pulsating variables is believed to account for cepheid-like pulsations. Each of the subgroups on the instability strip has a fixed [[period-luminosity relation|relationship]] between period and absolute magnitude, as well as a relation between period and mean density of the star. The period-luminosity relationship was first established for Delta Cepheids by [[Henrietta Swan Leavitt|Henrietta Leavitt]], and makes these high luminosity Cepheids very useful for determining distances to galaxies within the [[Local Group]] and beyond. [[Edwin Hubble]] used this method to prove that the so-called spiral nebulae are in fact distant galaxies.

The Cepheids are named only for [[Delta Cephei]], while a completely separate class of variables is named after [[Beta Cephei]].

=====Classical Cepheid variables=====
{{Main|Classical Cepheid variable}}

Classical Cepheids (or Delta Cephei variables) are population I (young, massive, and luminous) yellow supergiants which undergo pulsations with very regular periods on the order of days to months. On September 10, 1784, [[Edward Pigott]] detected the variability of [[Eta Aquilae]], the first known representative of the class of Cepheid variables. However, the namesake for classical Cepheids is the star [[Delta Cephei]], discovered to be variable by [[John Goodricke]] a few months later.

=====Type II Cepheids=====
{{Main|Type II Cepheids}}

Type II Cepheids (historically termed W Virginis stars) have extremely regular light pulsations and a luminosity relation much like the δ Cephei variables, so initially they were confused with the latter category. Type II Cepheids stars belong to older [[Population II]] stars, than do the type I Cepheids. The Type II have somewhat lower [[metallicity]], much lower mass, somewhat lower luminosity, and a slightly offset period versus luminosity relationship, so it is always important to know which type of star is being observed.

=====RR Lyrae variables=====
{{Main|RR Lyrae variable}}

These stars are somewhat similar to Cepheids, but are not as luminous and have shorter periods. They are older than type I Cepheids, belonging to [[Population II]], but of lower mass than type II Cepheids. Due to their common occurrence in [[globular cluster]]s, they are occasionally referred to as ''cluster Cepheids''. They also have a well established period-luminosity relationship, and so are also useful as distance indicators. These A-type stars vary by about 0.2–2 magnitudes (20% to over 500% change in luminosity) over a period of several hours to a day or more.

=====Delta Scuti variables=====
{{Main|Delta Scuti variable}}

Delta Scuti (δ Sct) variables are similar to Cepheids but much fainter and with much shorter periods. They were once known as ''Dwarf Cepheids''. They often show many superimposed periods, which combine to form an extremely complex light curve. The typical δ Scuti star has an amplitude of 0.003–0.9 magnitudes (0.3% to about 130% change in luminosity) and a period of 0.01–0.2 days. Their [[stellar classification|spectral type]] is usually between A0 and F5.

=====SX Phoenicis variables=====
{{Main|SX Phoenicis variable}}
These stars of spectral type A2 to F5, similar to δ Scuti variables, are found mainly in globular clusters. They exhibit fluctuations in their brightness in the order of 0.7 magnitude (about 100% change in luminosity) or so every 1 to 2 hours.

=====Rapidly oscillating Ap variables=====
{{Main|Rapidly oscillating Ap star}}
These stars of spectral type A or occasionally F0, a sub-class of δ Scuti variables found on the main sequence. They have extremely rapid variations with periods of a few minutes and amplitudes of a few thousandths of a magnitude.