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).

Mathematically instability strip is a narrow rectangular region in HR diagram, starting from main sequence at approximately A-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 Hertzsprung gap on the HR diagram.

Astronomy | Stellar evolution