Massive stars may undergo multiple fusion processes as they near the end of their lifespans. Our sun will eventually start fusing helium in its inner core so that carbon is formed. As this occurs, the radiation energy will be large, and the sun will expand into a red giant phase. After a while, as the star begins to run out of helium "fuel", the star will contract again. As the fusion shells then move out towards the surface, the outer layers will eventually be discarded from the star, and we are left with a white dwarf.

At least that is my understanding (I'm a newbie when it comes to this stuff, so go easy!)

Stars with much larger mass than our sun, will, however, have the potential to also start fusion of carbon into even heavier elements. And these heavier elements my fuse further still, all the way up to, in the most extreme cases, iron.

My question is as follows:

Will these stars undergo a phase of expansion and contraction every time a new fusion process begins? That is, it will have an expansion phase as the helium fuses into carbon, just like our sun, but when the star contracts again, and the carbon begins to fuse heavier elements, will the star then experience a second expansion process into a second red giant phase? And when the next fusion process begins, will the star undergo a third expansion phase, etc. Or is it so that these stars, like our sun, only have one red giant phase, but in this phase all the heavier fusion processes being without an expansion phase for each process? Or may both scenarios occur depending upon the properties of the star?

If someone can clarify this for me, then I would truly appreciate it. Searching on Google did not give me a clear answer to this question, and sometimes I even got conflicting viewpoints.

Any input will really be appreciated!

  • 1
    $\begingroup$ As I mention in astronomy.stackexchange.com/a/41415/16685 it can take a while for the energy released in the core (or in a shell surrounding the core) to affect the outer parts of the star. $\endgroup$
    – PM 2Ring
    Apr 20, 2023 at 12:49

2 Answers 2


The answer is that most massive stars should become red supergiants at least once, but whether there is much subsequent back and forth between being red supergiants and rather smaller, hotter (though still luminous stars) is greatly model-dependent and uncertain.

There are some problems with your understanding of the processes.

A low mass star like the Sun becomes a red giant before it begins helium burning. In fact "giantism" is caused by nuclear burning in a shell around the core. In the case of stars of a few solar masses or less, the ascent up the red giant branch is caused by hydrogen shell burning.

When helium ignites in the core, the star shrinks. The star will then become a giant again (an asymptotic giant branch star) when helium core burning ends and it is burning hydrogen and helium in shells around the core.

For more massive stars, the situation is more messy. Their interiors are not degenerate, nuclear burning phases merge more smoothly one into the other, there is thought to be considerable (rotation-driven) mixing between layers and extensive mass loss can complicate the picture.

Generally speaking, high mass stars are expected to be become red supergiants at least once and the expansion is associated with the generation of a large fraction of their energy from a shell around the core. Then, depending on the mass, initial composition and mass loss prescription, some models have them oscillate back and forth between being red supergiants and somewhat warmer giants, at almost constant luminosity. They may even shed their outer layers to become hot, blue giants (Wolf-Rayet stars) again.

Other models I have seen have the star completing its internal fusion processes while in the initial red supergiant phase, particularly in the 10-20 solar mass range. The variety and model-dependence can perhaps be judged from theoretical evolutionary tracks in the picture below (taken from Martins & Palacios 2013).

Tracks for high mass stars in the HR diagram

  • $\begingroup$ Thank you so much! This is a really detailed and helpful explanation! $\endgroup$
    – user12277
    Apr 23, 2023 at 17:39

The version I was led to believe (undergrad is just not enough time to state everything precisely) is that there are specific phases. There is a "not yet fusing at all, just gravitational heating", and then there is "H -> He", and the Sun will stop here. There are two types of "H -> He", the pp chain and the CNO cycle. Our Sun is doing the former. The transition from pp chain to CNO is smooth, and there will not be an expansion/contraction.

Those that can do CNO, IIRC, would have a sudden transition when the triple-alpha process kicks in. I do not remember if there would be a flash when this happens. After the triple-alpha process kicks in, the alpha process will also happen, and that generates all the elements up to iron.

Anything after that has to wait for the fusion to suddenly stop, leading to sudden gravitational collapse and explosive rebound, i.e. the supernova explosion.

Wikipedia is rather helpful and detailed and readable here. https://en.wikipedia.org/wiki/Stellar_nucleosynthesis

  • $\begingroup$ Thanks a lot for your comment. This is interesting, and the link provides a lot of detailed information about the various fusion processes, but it still does not, as far as I can see, answer whether or not the stars will expand and contract each time a new fusion process begins once a star reaches the end of its lifespan. $\endgroup$
    – user12277
    Apr 20, 2023 at 8:48
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    $\begingroup$ Yes, I am aware of that, but that was not the aim of my post. I was trying to tell you that it will NOT make a fast expansion and contraction at every single new beginning. Only a special few transition points would have such a thing. $\endgroup$ Apr 24, 2023 at 2:18
  • $\begingroup$ Ah, Ok. I see. Thanks a lot for your clarification! $\endgroup$
    – user12277
    Apr 24, 2023 at 18:06

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