When does helium fusion occur in an evolution of star?

[ehabmozart]

When does helium fusion occur in an evolution of star?

Well, i just need help in this confusing part of a star's life. First of all, when the sun finishes the H-fuel at core, why doesn't it immediatly use the H-fuel on the surface.. And then another doubt is, what causes the expansion of the Star and when does the helium fusion occur.. Before or after becoming a red giant. My point of view is that after the H-fuel at core ends, the helium inside is at high pressure, causing the particles around the star exert a pressure on h at surface causing fusion at higher rates which leads to expansion ... But the other question is that if energy is so much , the g is also so much .. isn't it?? I just need good responses and not just links to wikipedias... Thanks in advance to whoever dominates.. I got to tough exams within days!

 

[Drakkith]

I don't understand. Half of your questions are readily answered in an easy to understand way in the wikipedia article I linked in your other post. Did you bother to actually read it? It says right in the article when helium fusion begins and why the star expands.

First of all, when the sun finishes the H-fuel at core, why doesn't it immediatly use the H-fuel on the surface..

There is no way for new fuel to get to the core, as the core is not convective. In low mass stars like red dwarfs the entire star is convective and will cycle new fuel into the core. Thus the lifetime of a red dwarf is phenomenally long compared to solar mass or larger stars. (10 billion years for the sun compared to over 1 trillion for red dwarfs) I know you don't want wikipedia articles, but the whole process is explained in the stellar structure article here: http://en.wikipedia.org/wiki/Stellar_structure

 

[phyzguy]

This web site has movie of a sun-like star as it progresses through it's evolution. It's called "Evolution of a 1 MSun Star, by Josiah Schwab", and you can watch the radius, luminosity, temperature, composition, etc. as it evolves. Maybe it will help answer your questions.

 

[iced199]

As Drakkith stated, there is no way for the helium in the core to escape to the rest of the star and hydrogen from the rest of the star to replace it. The reason is because the radiative zone (the one right above the core) is non convective and does not allow hydrogen to flow into the core and helium to flow out. Thus, only the hydrogen in the core of stars like the Sun ever get used up. This is a huge loss for the stars, as this means they only get to use 10-15% of their entire hydrogen supply, making them die early. Red dwarfs, since they don't have radiative zones, cycle through all their hydrogen, and thus, live far longer. The enemy of stars is probably the radiative zone.