Stellar evolution is different according to the mass of the star. If you have a small star, let's say one-tenth of the solar mass, it lives forever. If you have a star of about the solar mass, it finally uses up its hydrogen first in the center and then it undergoes the transformation that I mentioned. Namely the center collapses, the outside expands, and the reason why the outside expands is that it is important to let the radiation out, to make it possible for... for all the radiation which is made in the center to escape to the outside, and that makes it necessary for the density near the center to decrease and the star gets blown up to giant size. Now if you take the sun, it will do precisely what I said and in the center the Salpeter process will take place and you will make carbon and oxygen and as the star ages, there is a shell in which energy is produced from the hydrogen reaction with the carbon cycle, and this shell gradually moves outside as the hydrogen in the... near the center gets used up, and finally all the hydrogen is gone. And then follows another shell in which helium is processed, but the energy which you get from that is only about 10% of the energy which you get from the hydrogen reaction. Well so finally the energy producing qualities of the star are pretty well exhausted and so gravitation takes hold, and the sun - maybe five billion years in the future - will finally succumb to gravitation and contract into a white dwarf. The white dwarfs are the final product of a star of medium size, let's say, the sun up to... several times the sun.