How unpredictable are stellar evolution paths on the HR diagram?

[Simfish]

stellar evolution pathways...?

I find this especially interesting since the trajectories of stars on the HR diagram are more or less based on this assumption.

But obviously, small perturbations can lead to major changes in the long-run. But it seems that most of those changes involve only a small fraction of mass of the star (for example, star spots don't seem to change the star significantly). But perhaps trajectories are more unpredictable for extremely high mass stars?

 

[D H]

Will two stars of the exact same mass and chemical composition undergo identical stellar evolution pathways...?

I find this especially interesting since the trajectories of stars on the HR diagram are more or less based on this assumption.

No. They are based on the assumption that stars of similar mass and composition will follow similar stellar evolution pathways, and moreover, that a averaging things out over lots and lots of stars will minimize variations due to unmodeled effects such as rotation rate, magnetic field, companion stars, etc.