Expansion of a star to become a red giant

[lys04]

TL;DR Summary

Process of star becoming a red giant

How exactly does a star become a red giant, i.e how does its radius expand so much?

When a star runs out of hydrogen to fuse into helium the star's core is essentially all helium, but the core isn't hot enough to start the triple alpha process to start fusing helium yet. So gravity takes over and starts shrinking down the star until to a point where a shell of hydrogen around the core is hot enough to start fusing into helium. Now I get to this point and I'm stuck as to how this leads to the expansion of the star?

 

[Bandersnatch]

Think of a star as composed of two separate parts: the core, and the outer layers. The core (and only the core) collapses, increasing its temperature as it does, which makes it radiate more. The increased radiation pressure from the core raises the outer layers higher.

 

[lys04]

Think of a star as composed of two separate parts: the core, and the outer layers. The core (and only the core) collapses, increasing its temperature as it does, which makes it radiate more. The increased radiation pressure from the core raises the outer layers higher.

But doesn't the radiation pressure need to overcome gravity first in order to raise the outer layers higher?

 

[phinds]

But doesn't the radiation pressure need to overcome gravity first in order to raise the outer layers higher?

And what makes you think the radiation has any problem overcoming gravity? We are not talking about a black hole here.

 

[berkeman]

But doesn't the radiation pressure need to overcome gravity first in order to raise the outer layers higher?

Sure.

Can you please post links to the reading you have been doing about your question, so that we can help you figure out what is confusing you in that reading? Thanks.

 

[Drakkith]

So gravity takes over and starts shrinking down the star until to a point where a shell of hydrogen around the core is hot enough to start fusing into helium. Now I get to this point and I'm stuck as to how this leads to the expansion of the star?

I believe @Ken G addressed this before and said that the shell of fusing hydrogen makes up more volume than the hydrogen core previously did, which means that much more energy is being released during hydrogen-shell fusion vs hydrogen-core fusion, which causes the outer layers to puff outwards under the increased radiation pressure. Correct me if I'm wrong, Ken.

Edit: Hmm. This reference appears to contradicts me, so I am not confident in what I just said. Hopefully someone will clarify.

 

[Astronuc]

A red giant forms after a star has run out of hydrogen fuel for nuclear fusion, and has begun the process of dying.

A star maintains its stability through a fine balance between its own gravity, which holds it together, and the outwards pressure from ongoing thermonuclear fusion processes taking place at its core. Once a star’s core runs out of hydrogen, however, that state of equilibrium is lost and the core begins to collapse. As the core collapses, the shell of plasma surrounding the core becomes hot enough to begin fusing hydrogen itself. As fusion in this shell begins, the extra heat causes the outer layers of the star to expand dramatically, and the surface extends up to several hundred times beyond the former size of the star. The energy at the star’s surface becomes far more dissipated, causing the star's bloated surface to cool, turning from white or yellow to red. A red giant is formed. This process can take hundreds of millions of years and applies to intermediate mass stars (with a mass greater than 80% and less than 800% of the Sun’s mass), which then go on to form planetary nebulae. When a more massive star runs out of hydrogen at its core, it forms a red supergiant instead, prior to exploding as a supernova.

Reference: https://esahubble.org/wordbank/red-giant/

The further out the hydrogen gets, the lower the gravity field, so the less radiation pressure needed to support it. There is both radiation pressure and the thermal (kinetic) energy of the plasma.

See also the discussion here. Halfway down the page is a nice diagram, and further on is a comparison of the sun (Sol) and Betlegeuse
https://pressbooks.online.ucf.edu/a...olution-from-the-main-sequence-to-red-giants/