Yet recently, theorists have been asserting that it also shows evolutionary trends in addition to those aforementioned. It is easy to make the link between solar analogue stars (white stars that are very similar to the Sun) and red giants - the solar analogues evolve into red giants when their cores begin to collapse and reach the "core helium burning" phase.
However, the diagram clearly doesn't make a link between the two groups - this can only be inferred from the fact that the stars are in roughly the same area in the diagram and appear to graduate upwards. There is no clear axiom of evolution shown.
This is not a problem with the HRD - this is a theoretical incompatibility. Yet even when we take the core deciding factor of stellar evolution into account, the theory is still incompatible.
This causes a few discrepancies, one of which involves solar analogue stars.
It happens a second time at the RV Tauri stage. During this time, a star of the aforementioned mass exists as a pulsating orange-white supergiant. Its luminosity is high, as well as its temperature. This also appears as a mass gain on the diagram, thus rendering the new attributions false.
There are, however, solutions that involve the modification of the HRD to fit the new theories. I have tried to have a crack at this and came up with a 3D diagram with an added axis of mass.
I found other ideas for a third axis when I got a reply on Stack Exchange (https://astronomy.stackexchange.com/questions/4741/is-there-a-flaw-with-the-newer-purposes-and-correlations-attributed-to-the-hr-di/8312?noredirect=1#comment11499_8312). The whole discussion threw the diagram above into question, though I have no idea about what third axis to use - or even if one should be used at all.
That's all for now. See you next time for more!
-Graviton