The unqualified terminstability strip usually refers to a region of theHertzsprung–Russell diagram largely occupied by several related classes of pulsatingvariable stars:^[1]Delta Scuti variables,SX Phoenicis variables, andrapidly oscillating Ap stars (roAps) near themain sequence;RR Lyrae variables where it intersects thehorizontal branch; and theCepheid variables where it crosses the supergiants.

RV Tauri variables are also often considered to lie on the instability strip, occupying the area to the right of the brighter Cepheids (at lower temperatures), since theirstellar pulsations are attributed to the same mechanism.

TheHertzsprung–Russell diagram plots the realluminosity of stars against theireffective temperature (theircolor, given by the temperature of theirphotosphere). The instability strip intersects themain sequence, (the prominent diagonal band that runs from the upper left to the lower right) in the region of A and F stars (1–2solar mass (M_☉)) and extends to G and early K bright supergiants (early M if RV Tauri stars at minimum are included). Above the main sequence, the vast majority of stars in the instability strip are variable. Where the instability strip intersects the main sequence, the vast majority of stars are stable, but there are some variables, including theroAp stars and theDelta Scuti variables.^[2]

Stars in the instability strip pulsate due toHe III (doublyionized helium),^[1] in a process based on theKappa–mechanism. In normal A-F-G class stars, He in the stellarphotosphere is neutral. Deeper below the photosphere, where the temperature reaches 25,000–30,000 K, begins the He II layer (first He ionization). Second ionization of helium (He III) starts at depths where the temperature is 35,000–50,000 K.

When the star contracts, thedensity andtemperature of the He II layer increases. The increased energy is sufficient to remove the lone remaining electron in the He II, transforming it into He III (secondionization). This causes theopacity of the He layer to increase and theenergy flux from the interior of the star is effectively absorbed. The temperature of the star's core increases, which causes it to expand. After expansion, the He III cools and begins to recombine with free electrons to form He II and the opacity of the star decreases. This allows the trapped heat to propagate to the surface of the star. When sufficient energy has been radiated away, overlying stellar material once again causes the He II layer to contract, and the cycle starts from the beginning. This results in the observed increase and decrease in the surface temperature of the star.^[3] In some stars, the pulsations are caused by the opacity peak of metal ions at about200,000 K.^[4]

The phase shift between a star'sradial pulsations andbrightness variations depends on the distance of He II zone from the stellar surface in thestellar atmosphere. For most Cepheids, this creates a distinctly asymmetrical observed light curve, increasing rapidly to maximum and slowly decreasing back down to minimum.^[5]

There are several types of pulsating star not found on the instability strip and with pulsations driven by different mechanisms. At cooler temperatures are thelong period variableAGB stars. At hotter temperatures are theBeta Cephei andPV Telescopii variables. Right at the edge of the instability strip near the main sequence areGamma Doradus variables. The band ofWhite dwarfs has three separate regions and types of variable: DOV, DBV, and DAV (=ZZ Ceti variables) white dwarfs. Each of these types of pulsating variable has an associated instability strip^[6][7][8] created by variable opacity partial ionisation regions other than helium.^[1]

Most high luminosity supergiants are somewhat variable, including theAlpha Cygni variables. In the specific region of more luminous stars above the instability strip are found theyellow hypergiants which have irregular pulsations and eruptions. The hotterluminous blue variables may be related and show similar short- and long-termspectral and brightness variations with irregular eruptions.^[9]

• Hertzsprung–Russell classifications
• Stellar evolution

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