Trajectory of supergiants on HR diagrams

Question

I am a secondary school student currently studying cosmology. My A Level textbook supplies the following HR diagram with regards to what trajectories different stars follow:

I found myself unable to explain the trajectory followed by supergiants. In the diagram they cool without significant loss in luminosity; by the Stefan–Boltzmann law, this should mean they are expanding. But I am unsure as to the mechanism by which this occurs, and am aware that often these simple-looking trajectories can be subtly affected by different parameters.

I have done my best to browse different articles online, but none seem to explain why the temperature is decreasing. My questions are therefore:

1. Is the trajectory shown on the diagram correct?
2. If so, what stellar processes cause the temperature to decrease while maintaining relatively constant luminosity?

Thank you very much!

Answer 1

Generally speaking, evolving massive stars follow the pattern that when a "shell-burning" phase begins (i.e. nuclear "burning" outside the core), that provides sufficent energy to lift the envelope and cause it to expand. On the contrary, when shell burning is extinguished, then if the core is is not sufficiently degenerate to provide enough degeneracy pressure to hold up the star (which will be the case for massive stars), then the envelope contracts again (or is maybe lost through radiation driven winds) and the star moves back to the left again.

For both massive and less massive stars, the horizontal parts of any evolutionary track are physically a consequence of radiative diffusion of energy out of the star. Essentially, the luminosity is entirely determined by the opacity to radiation near the core, which determines how fast energy can leak out. The surface temperature simply adjusts to radiate this luminosity given the effective radius of the star.

Incidentally, the trajectory of the 1 solar mass star is incorrect in detail and there is a brief (subgiant) phase where such a star also moves to the right at roughly constant luminosity after leaving the main sequence, to the base of the red giant branch at around 4000K.