Introduction

From the moment of its birth, a star is engaged in an internal, unrelenting struggle. Gravitational forces oppose the outward pressure generated by nuclear fusion deep in its core. For most of a star’s lifetime, this gravitational collapse and outward pressure are equally balanced. In the later stages of a star’s life, this gravitational collapse and outward pressure become imbalanced, causing profound changes in the star.

A star’s physical properties—its size, temperature, color, and brightness—are the product of the star’s initial mass and this clash of forces, and these physical properties change during a star’s lifetime. The many types of stars we observe are like snapshots showing different parts of the stellar evolution story.

More than a century ago, astronomers Ejnar Hertzsprung and Henry Norris Russell independently noticed that there was a relationship between the intrinsic brightness of stars and their colors. Eventually, their work evolved into what is now known as the Hetrzsprung-Russell Diagram, or H-R Diagram.

Rubin Observatory will have the capability to image an incredible number of distant and faint stars, leading to a deeper understanding of stellar evolution. In particular, Rubin data will be able to characterize the properties of dim main sequence stars, and bring new insights to the relationship between a star’s color, magnitude and its initial mass.