*Review* **Red Supergiants, Yellow Hypergiants, and Post-RSG Evolution**

### **Michael S. Gordon 1,\*,† and Roberta M. Humphreys 2**


Received: 14 October 2019; Accepted: 26 November 2019; Published: 3 December 2019

**Abstract:** How massive stars end their lives remains an open question in the field of star evolution. While the majority of stars above -9 M will become red supergiants (RSGs), the terminal state of these massive stars can be heavily influenced by their mass-loss histories. Periods of enhanced circumstellar wind activity can drive stars off the RSG branch of the HR Diagram. This phase, known as post-RSG evolution, may well be tied to high mass-loss events or eruptions as seen in the Luminous Blue Variables (LBVs) and other massive stars. This article highlights some of the recent observational and modeling studies that seek to characterize this unique class of stars, the post-RSGs and link them to other massive objects on the HR Diagram such as LBVs, Yellow Hypergiants and dusty RSGs.

**Keywords:** evolved stars; Yellow Hypergiants; Red Supergiants; stellar mass loss
