The Red and Yellow Hypergiants

TL;DR

This paper reviews the characteristics and evolutionary significance of red and yellow hypergiants, emphasizing their mass-loss behaviors, spectral changes, and differences from typical red supergiants.

Contribution

It provides a comprehensive review of red and yellow hypergiants, highlighting their unique properties and evolutionary context in massive star development.

Findings

Hypergiants exhibit high, episodic mass-loss rates.

Spectral type changes are linked to pseudo photosphere formation.

Yellow hypergiants often show evidence of past red supergiant activity.

Abstract

The red and yellow hypergiants are a rare and important phase in the evolution of the most massive stars that can reach the cool part of the HR Diagram. The hypergiant phase is commonly characterized by high, often episodic mass-loss rates and significant changes in spectral type, probably due to the formation of a pseudo photopsphere during a high mass-loss episode. Many of the yellow hypergiants are the immediate successors to the most luminous red supergiants, and often show evidence in their dusty, circumstellar envelopes from past red supergiant activity. In this paper we review the yellow and red hypergiants with an emphasis on how they differ from more normal red supergiants.