Why Do Stars Turn Red? I. Post-Main-Sequence Expansion Mechanism

TL;DR

This paper proposes a new physical mechanism, the refined mirror principle, explaining why stars expand into red giants and supergiants, based on stellar envelope response to the hydrogen-burning shell rather than energy absorption.

Contribution

It introduces the refined mirror principle as the key driver of stellar envelope expansion, supported by numerical experiments with MESA, providing a unified framework for post-main-sequence evolution.

Findings

Envelope expansion is governed by the refined mirror principle, not energy absorption.

Two evolutionary pathways to RG/RSG phase are identified: during helium-core contraction and after contraction ceases.

Structural transition involves mass redistribution and extended convection in the envelope.

Abstract

In this series of papers, we address the long-standing question of why post-main-sequence stars expand into red giants (RGs) or red supergiants (RSGs). This paper aims to identify the key physical mechanism that drives stellar evolution toward the RG/RSG phase. Using the Modules for Experiments in Stellar Astrophysics (MESA), we perform controlled numerical experiments by systematically varying stellar parameters in evolutionary models, and compare those that successfully evolve into RG/RSGs and those that do not. We show that envelope expansion toward the RG/RSG phase cannot be explained by energy absorption. Instead, it is governed by a refined form of the "mirror principle," in which the stellar envelope responds oppositely to its inner boundary, defined by the outer edge of the hydrogen-burning shell, rather than directly to the helium core. This behavior arises naturally from hydrostatic equilibrium, as the burning shell establishes a moving, nearly constant-pressure inner boundary for the envelope. We identify two evolutionary pathways toward the RG/RSG phase that both follow this refined mirror principle: (1) direct envelope expansion during helium-core contraction, and (2) continued expansion after contraction ceases, driven by a decline in nuclear energy generation rate. The final approach to the RG/RSG phase is marked by a structural transition in the envelope, characterized by mass redistribution and the development of an extended convective region. We present a unified physical framework for envelope expansion toward the RG/RSG phase, based on the refined mirror principle and the final structural transition, and outline an evolutionary roadmap leading to the RG/RSG phase.