BDanielMayfield wrote: ↑Sat Oct 27, 2018 3:19 pm
Ann wrote: ↑Sat Oct 27, 2018 4:05 am
BDanielMayfield wrote: ↑Fri Oct 26, 2018 8:01 pm
It could have just in fairly recent times transitioned into being the Red Giant that it is today.
Gamma Cas is not a red giant!
Of course! Please pardon my Sol-centric slip. O and B stars never become red giants.
Well, they do. Only the most massive stars, like Eta Carina, may blow their tops without ever becoming red giants. But stars that still belong to spectral classes O and B are never red giants.
Do A class stars also bypass the red giant phase when they exit the MS?
They do indeed! Take Pollux, a red giant whose luminosity in yellow-green light is only some thirty times Solar, which is very puny for a red giant. I'm just guessing, but I wouldn't be entirely surprised if Pollux started out as an early F-type star, of spectral class F0 or F1. Pollux was definitely never a B-type star! And Arcturus, one of the really easy-to-find bright stars in the sky (because you just follow the handle of the Big Dipper until you find it) has a mass of only about 1.5 times Solar according to Jim Kaler
, and it may possibly have started out its main sequence life as another Vega.
In any case, A-type main sequence stars do indeed become red giants. So do F-type stars, and Procyon is on its way to becoming one. The Sun, too, is expected to become a red giant.
I should perhaps comment on what you said about stars that "bypass the red giant stage when they become red giants". When stars leave the main sequence, they may keep their temperature (more or less) and just become brighter for a while. But astronomers believe that they definitely become red giants after that, unless they are massive enough to become Luminous Blue Variables like Eta Carina. The stars stay on the red giant branch for a moderately long time, although the red giant stage is much shorter than the star's main sequence lifetime. During the red giant stage the stars belong to spectral class K, or so I believe.
I believe that after some time, the stars start fusing helium in their cores. During this stage the stars are a little bit hotter and less red than before. But then they exhaust the helium in their cores, and now they enter the aymptotic giant branch, when they become brighter and redder than they have ever been before. I believe that stars at this stage are spectral class M.