To begin with, Ive seen some lectures of Gyula Dvid whos a professor at the ELTE. The ELTE is the biggest university in Hungary and Gyula Dvid is one of the best teachers at the deparment of theoretical physics. I hope he doesnt mind that I stole his presentation.

The topics are the termodynamics of a star, the lifetime and the life-events of a star.

Id like to start by looking at this nice question which was asked by the daughter of the professor: she asked that why we couldnt heat with ash.

1) Why cant we heat with ash?

Firstly, the stars were born in huge nebulas. In these huge particle dusts the gravity starts to pull together everything. During this process the nebula is heating up and radiating heat across the universe. At one point the contraction stops. It happens when the gravity has pulled the particles close enough to enable them to emit sufficent energy. This energy comes mostly in heat and particle drizzle.

The closeness is enough when the center of a star reaches some million degree and this is the term of the nuclear fusion. In the nuclear fusion the particles of the nebula ,which are mostly hidrogenes, form a new paticle, a lighter one, emitting the difference in mass in form of heat and kinetic energy.

Obviously, youve realised that I havent been talking about heating with ash. So, here it comes. Whats ash in our world? Its basicly the soft grey powder remaine after something had been burnt. Altough we surely know what ash is in our world, we dont all know what it is for the stars, for our Sun. You may recall that I mentioned the nuclear fusion, and a lighter particle which was formed. Firstly, its helium. Helium is the gas inside of balooning baloons. This is the ash(1) of a star.

When the stars run out of fuel, the hidrogene, they can possibly burn the helium. So, all we need is a gigantic Sun in our garage to burn ash. The miracle of the Sun is the complexity of its form. We are not able to provide the neccesary circumstances on Earth for nuclear fusion because we need a never-melting container, we need to heat up the system to 10 million degrees to enable it to start the fusion. However, for instance, the Sun itself is a container for the matter, and gravity provides the heating. Amazing, how the nature works.

For helium we have to proivde enough energy to fusion. The exterior of the star starts to inflate while the gravity shrinks the center to heat it up. This form of a star is called RED GIANT. Actually, we have to possess one of these to burn ash which is unfortunate as it would be at least more than a hundred 10 times bigger than our Sun.

From this point theres to possible ways to progress. In our case, accordin to the Sun. Its goint to run out of helium in the next 5 billion years, become a red giant,shrink and fade out slowly because it wont be big enough to continue nuclear fusion. This form is called the WHITE DWARF. I forgot to mention, that the enormous collision during the transformation into a red giant is going to burn the Earth to the ground, and make a real hell out of it.

The other path is becoming a SUPERNOVA. The supernovae could become more than a 100 times bigger than our Sun, and could shine a tenbillionfold lighter than our Sun now. It means, that if a red giant ,which is able to become a supernova due to it mass, had happen to collide 150 billion kilometres from the Earth, it would shine lighter than the Sun next to us. They shrink as the energy of the nuclear fusion is not enough anymore to them, to fight against gravity and to maintain their size. Owing to the the fact that they are extremely huge, and extremely light, it wouldnt suggest that they live long. The red giant collapsed because it has run out of helium as well as he did run out of hydrogene before. So, the ash(1) was burnt, which formed coal, and now the coal has burnt as well so we have oxygen. And so on...

From this point the supernovae can die in any minute, it depedns on its beginner mass. If it was heavy enough, than it would continue the fusion until the ash is going to be iron. Iron is the last phase since it has the lowest energy per nucleon.

When the nuclear fusion is over, the gravity shrinks the center of the star. If the star wasnt heavy enough, it would become a neutron star. Theres no denser stuff in the whole universe than a neutron star. Everyone who talks about neutron stars mentions that a spoon of its matter weighs more than the Himalaya. They rotate extremely fast and emit electromagnetic waves towards everething.

And at last but not least. The monstrous creature of the galaxy are the blackholes. If a supernova was really heavy that it would become a blackhole. Blackholes have immesurable gravity, nothing can escape them, even light. Light bends and falls back to the center of a blaackhole. The center of a blackhole is a singularity in physics. It means that theres no law which describes it and it has infinite mass. Now scientists think that every galaxy has at least one big black hole in the middle, and these monsters keep the galaxies together.