Abstract

Contributed Talk - Splinter StellarEndpoints

Friday, 11 September 2026, 17:06   (MW-2235)

Why Do Stars Turn Red?

Po-Sheng Ou, Ke-Jung Chen
Academia Sinica Institute of Astronomy and Astrophysics (ASIAA)

We revisit a long-standing question in stellar astrophysics: Why do post-main-sequence stars evolve into red giants (RGs) or red supergiants (RSGs)? By comparing stellar evolution models that enter the RG/RSG phase with those that remain blue, we show that envelope expansion is governed by an anti-correlated response: the envelope expands as its inner boundary, defined by the edge of the hydrogen-burning shell, moves inward, rather than as a direct consequence of energy absorption. Building on this insight, we construct steady-state envelope models to determine the physically realizable configurations of stellar envelopes. These solutions reproduce the anti-correlation and show that the RG/RSG phase represents a limiting configuration of envelope expansion associated with the Hayashi limit. They also reveal that configurations in the yellow-supergiant regime are structurally unstable, providing a physical explanation for the Hertzsprung gap.