The instability strip is a narrow, almost vertical region in the Hertzsprung-Russell diagram which contains many different types of variable stars (RR Lyrae, Cepheid variable, W Virginis and ZZ Ceti stars all reside in the instability strip).

Most stars more massive than the Sun enter the instability strip and become variable at least once after they have left the main sequence. It is within this region that they suffer instabilities that cause them to pulsate in size and vary in luminosity.