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.

 

[pmacias]

Helium fusion occurs in the evolution of a star after the hydrogen fuel at its core has been exhausted. This is because the core of the star needs to reach a certain temperature and pressure for helium fusion to occur. As the hydrogen fuel is used up, the core becomes more compact and the temperature and pressure increases, eventually reaching a point where the fusion of helium can begin.

The reason why the star does not immediately use the hydrogen fuel on its surface is due to the balance between gravity and the energy produced by fusion. As the core becomes more compact, the gravity pulling the star inward also increases, making it difficult for the hydrogen fuel at the surface to reach the necessary temperature and pressure for fusion to occur. It is only when the core is compact enough that the hydrogen at the surface can also reach the necessary conditions for fusion.

The expansion of a star is caused by the energy produced by fusion at its core. As the core fuses hydrogen into helium, it releases a tremendous amount of energy, which pushes against the outer layers of the star, causing it to expand. This expansion is what leads to the star becoming a red giant.

Helium fusion typically occurs after the star has become a red giant. This is because the core has become hot and compact enough for helium fusion to occur, causing the outer layers to expand and the star to become a red giant. However, some smaller stars may skip the red giant phase and go straight from being a main sequence star to a white dwarf without ever fusing helium.

In summary, helium fusion occurs in the evolution of a star after the hydrogen fuel at its core has been exhausted. It is triggered by the increase in temperature and pressure as the core becomes more compact, and it is this fusion process that leads to the expansion of the star and its eventual transformation into a red giant.