A Propos Crossing the Hertzsprung Gap

Abstract

The evolution of intermediate-mass and massive stars speeds up considerably after they finish their hydrogen core-burning. Due to this accelerated evolution, the probability to observe stars during this episode is small. In suitable stellar aggregates, in particular star clusters of appropriate ages, the fast evolutionary phase causes a depopulated area~--~referred to as the Hertzsprung gap~--~in color-magnitude diagrams and derivatives therefrom. The explanation of the speed-up usually resorts to the star's Kelvin-Helmholtz timescale and the Sch\"onberg-Chandrasekhar instability is called upon. This exposition challenges this viewpoint with counterexamples and argues that a suitably defined nuclear timescale is enough to explain the fast evolution. A thermal instability, even though it develops in stars evolving through the Hertzsprung gap, is not a necessary condition to trigger the phenomenon.