Instability strip

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Instability strip is a region in HR diagram which is occupied by pulsating stars (RR Lyrae variable, Cepheid variable, W Virginis variable, ZZ Ceti variable, RV Tauri variable, Delta Scuti variable, SX Phoenix variable).

Mathematically, the instability strip is a narrow rectangular region in HR diagram, starting from the main sequence in the region of A and F stars (1-2 solar mass) and extending almost vertically up (slightly inclined to the right) to the highest luminosities. The lower part of instability strip appears as the Hertzsprung gap on the HR diagram.

[edit] Radial pulsations

All stars in the instability strip pulsate due to He III (doubly ionised helium). In normal A-F-G stars He is neutral in the stellar photosphere. Deeper below the photoshere at about 25000-30000K begins the He II layer (first He ionization). Second ionization (He III) starts at about 35000-50000K.

When the star contracts, the density and temperature of the He II layer increase. He II starts to transform to He III (second ionization). Opacity increases, energy flux from the interior of the star is effectively absorbed. Temperature of the layer increases and it starts to expand. After expansion, density and temperature decreases, He III begins to recombinate into He II. Outer layers contract and the cycle starts from the beginning.

The phase shift between a star's radial velocity pulsations and brightness variability depends on the distance of He II zone from the stellar surface in the stellar atmosphere.