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.