Instability strip

The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars:Delta Scuti variables, SX Phoenicis variables, and rapidly oscillating Ap (roAp) stars near the main sequence; RR Lyrae variables where it intersects the horizontal branch; and the Cepheid variables where it crosses the supergiants.

RV Tauri variables are also often considered to lie on the instability strip, occupying the area to the right of the brighter Cepheids (at lower temperatures), since their pulsations are attributed to the same mechanism.

Position on the HR diagram

The instability strip intersects the main sequence in the region of A and F stars (1–2 solar mass (M_☉)) and extends to G and early K bright supergiants (early M if RV Tauri stars at minimum are included). The lower part of instability strip appears as the Hertzsprung gap on the Hertzsprung–Russell diagram. Above the main sequence, the vast majority of stars in the instability strip are variable. Where the instability strip intersects the main sequence, the vast majority of stars are stable, but there are some variables, including the roAp stars.