GCSE Physics P1 "Life Cycle of Stars" help

Hi all,

I'm consolidating my notes for Physics P1 (Edexcel) and I've realised I'm not too clued up about one aspect of the life cycle of stars :/

As I understand it in my notes, when a main sequence star becomes a red (super)giant, it stops the nuclear fusion of hydrogen because the hydrogen supply has been exhausted, and it instead begins fusing helium protons together. Base knowledge - tick

My notes explain that the core of the star becomes denser but that the star expands. My confusion comes from this: is this true and if so, why does the core of the star become denser and why does the star expand?

I suppose my guess is that when the hydrogen supply is exhausted, the star releases less gas because of less nuclear fusion (and so it exerts a lowered force of gas pressure), so the resultant force between the gas pressure and the gravitational force of the star is changed from zero (which is what defined it as a main sequence star) and so the star has a higher gravitational force - which leads the core of the star to collapse. Yet surely if the core of the star collapses because it has a higher gravitational force, the star should not expand as well?

This probably goes well beyond the syllabus, but I'm cautious because statement 3.12 asks us to "describe the role of gravity in the life cycle of stars". I see from some research that there's such thing as variable stars between MSSs and red (super)giants, with stars finding hydrogen from pockets outside of the core before the supply completely burns out (and that carries a sort of understandable way to answer my questions, albeit a very long answer), so I have a feeling the question is even unanswerable thanks to P1 oversimplification :P

All help would be very much appreciated. Thank you ever so much in advance. And all the very best of luck in your exams

I'm y11 IGCSE so I'm not an expert. You seem smart enough to understand that the gcse question probably just wants you to say something general about gravity causing nuclear fusion of the star. Tick A* gcse thanks very much. So I guess it's curiosity?

http://abyss.uoregon.edu/~js/ast122/lectures/lec16.html

it's explained pretty well here. The core exhausts it's supply of hydrogen and the helium rich core begins to contract and heat up. This heats hydrogen around the core and hydrogen fusion can begin again. The volume of hydrogen around the core is huge and the pressure produced overcomes the force of gravity and causes the star to expand. As this is turned into helium, the process repeats until the star exhausts all it's hydrogen and it can briefly switch to using hydrogen as a fuel.

As to the core being denser... it is. As fusion slows in the core (when hydrogen supply is low) the pressure created is not enough to balance the force of gravity and the core contracts, heating it up.

Hey there,

Thanks so much for your reply! This is really, really helpful; I really appreciate it yeah, I suppose this all does go well beyond the "something general about gravity", as you so accurately described it, which they're looking for in the markschemes, but it might be useful background knowledge to make sense of things

Now just to hope that there'll be a six-mark question on it!

Best of luck to you in your exams too Thanks so much once again.