It is my understanding that stars the size of our sun will go through two red giant phases. The first one will occur when fusion of hydrogen to helium begins to happen in a shell surrounding the collapsing helium core. Then, as temperature and pressure increases in the core, the so called helium flash will occur when fusion of helium to carbon begins (the triple-alpha process). This will lessen the "pressure" in the core, and the star will shrink again. Later, as the star begins to run out of helium, and the carbon core begins to collapse, a second red giant phase will occur.

My question is as follows: Stars between roughly 2.2 - 8 times the mass of our sun will, as far as I understand, initiate the helium-carbon fusion without any helium flash. Does this mean that these stars will not shrink in the same manner as our sun will between the two red giant phases? After all, we will not have the same sudden release of "pressure" in the core here, so will these stars just remain red giants? Or will there be a gradual shrinking of these stars too in the stage before the carbon core begins to collapse?

If someone can explain this to me, or correct any misunderstandings I might have, then I would greatly appreciate it!


2 Answers 2


Stars over the full range up to 8 solar masses will go through both the first ascent red giant phase (RGB, burning hydrogen in a shell) and the asymptotic giant branch phase (AGB, burning hydrogen and helium in shells).

The occurrence or not of the helium "flash" has little impact. The progression to helium core burning just happens more smoothly in higher mass stars.

The physical reason that stars become giants is still a debate. There is no doubt it happens, and it happens in evolutionary models too, but handwaving descriptions of why are fraught with difficulties.

What is sure, is that giantism is associated with shell burning.

It can be difficult to find good pictures of evolutionary tracks - the RGB and AGB are close together and authors often cram too many tracks onto the same plot. Here is one from Lagarde et al. (2012). These cover from the pre main sequence to the end of the AGB. The plots on the left and right just use a couple of different prescriptions for various mixing processes - see the paper for details. You can see the separate RGB and AGB phases of the plots.

Evolutionary tracks from Lagarde et al. 2012.

  • $\begingroup$ Thank you so much! Really appreciate your answer! $\endgroup$
    – user12277
    Nov 8, 2023 at 21:25

First from what I understand (which is not much) Helium flash only happens for stars with 0.8 to 2.0 solar masses our sun has a solar mass of 2.2 solar masses or higher go into the red giant phase after becoming a main sequence star after that it will become unstable and eject its outer layer of dust and gas. after that its core will collapse completely because of gravity and become a white dwarf.

I am sorry for my previous unhelpful answer

source https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-lifecycles.html https://www.savemyexams.com/igcse/physics/edexcel/19/revision-notes/8-astrophysics/8-2-stellar-evolution/8-2-2-the-life-cycle-of-solar-mass-stars/

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    $\begingroup$ I understand that stars with mass between 2.2 and 8 solar masses will not have a helium flash. However, my questions is whether or not such stars will have one or two red giant phases. $\endgroup$
    – user12277
    Nov 6, 2023 at 22:44
  • $\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Nov 6, 2023 at 23:05

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