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 gotta exams at the corner.. Please help!!