The Instability strip is a nearly vertical region in the Hertzsprung–Russell diagram which is occupied by pulsating variable stars (including RR Lyrae variable, Cepheid variable, W Virginis variable, ZZ Ceti variable, RV Tauri variable, Delta Scuti variable, SX Phoenicis variable and rapidly oscillating Ap stars).
The instability strip intersects the main sequence in the region of A and F stars (1–2 solar mass) and extends upwards almost vertically (slightly inclined to the right) to the highest luminosities. The lower part of instability strip appears as the Hertzsprung gap on the Hertzsprung–Russell diagram.
Read more about Instability Strip: Pulsations
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