The region on the Hertzsprung–Russell diagram occupied by several types of pulsating variable star, including Cepheid variables, classical Cepheids, Delta Scuti stars, RR Lyrae stars, and W Virginis stars. It runs diagonally upwards and to the right from the main sequence at about absolute magnitude 2, and includes extremely luminous supergiants at the very top; here it tends to merge with the broader region occupied by RV Tauri, red semiregular, and long-period (Mira) variables. The RR Lyrae variables are located where the instability strip intersects the horizontal branch in globular clusters, while classical Cepheids and W Virginis stars lie at the intersection of the strip with the supergiant branch in metal-rich and metal-poor stars, respectively. Hot, massive stars enter the strip as they evolve off the main sequence. During this phase of the stars' lives, their expanding outer layers become unstable and they pulsate regularly. Stars can move across the instability strip and back again several times as they continue to evolve.
Subjects: Astronomy and Astrophysics.