How Do Low Mass Stars Evolve After Exhausting Hydrogen Fuel?

[ehabmozart]

In my book, it is written that in an evolution of a star, specifically the sun, IT will run low on hydrogen fuel but will have produced so much energy that it will expand slightly... Once H- fuel is used, the star will start fusing helium nuclei. This complex process can cause an explosion that throws some material from the star into the space forming a planetary nebula.. My questions are:
1) Why would the star expand after the hydrogen fuel is over. At that time, the gravity dominates and it should collapse.. Am i right?
2) Y helium fusion is a complex process. I mean what is a planetary nebula.. In my book it seems like a coolapsed star with some dust around it...
3) If possible, i would like to know the stages of this low mass star in an easy detailed way if possible.
I got to exams at the corner.. Please help!

 

[SHISHKABOB]

when the hydrogen in the core has all fused into helium, the core begins to contract. This causes the "shell" of hydrogen surrounding the core to heat up, and begin burning. This happens because the shell of hydrogen follows the core of helium as it collapses inwards. The shell of burning hydrogen then increases the energy output of the star, and thus its luminosity and size.

At some point, the core will stop collapsing due to electron degeneracy pressure. The core will continue to heat up after that. Once it reaches a temperature of roughly 10^8 K, the helium will begin to burn and form Carbon. Because the whole entire core reaches this temperature nearly simultaneously, all of the helium starts burning at once and this is called the "Helium flash". After the helium starts burning, the star contracts again.

It will start this process over again once all the Helium has burned into Carbon, and there will be a helium shell, etc.

During this process, as the star becomes really really big, the outer layers of the star will sort of fall off, because they are so far out and dispersed. This is what causes the planetary nebula.

Once the star has burned through everything it can burn, it becomes a white dwarf, and then sits there for a very long time, doing nothing but cooling off.

Try wikipedia http://en.wikipedia.org/wiki/Red_giant or http://en.wikipedia.org/wiki/Stellar_evolution#Mid-sized_stars

for more information