The Sun produces light and heat, as opposed to the planets which shine by reflecting sunlight. The power output from the Sun is enourmous - 4 x 10 26 Watts. The biggest power stations on Earth produce 5 thousand million Watts i.e 5 x 10 9 W. So the Sun produces 2 billion times more energy in one second than the biggest power station produces in a year! This lecture looks at how it does it, and in the process looks at the way science works.
So lets have a guess (fancy scientific word is a HYPOTHESIS). What ways are there of producing power ? Then develop the implications of the idea, and then TEST THEM AGAINST OBSERVATIONS. If the hypothesis does not agree with what is observed then it can be rejected. If not, we should think of some other implications of the idea, and test those against observations.... and so on. When a hypothesis has been shown to explain number of different observations, then it might be called a theory instead of a hypothesis. But it can never be PROVED correct, only disproved if it fails to match observations. More on the scientific method.
So now we have a hypothesis which has passed an observational test - so we have a theory that nuclear fusion of hydrogen powers the Sun. But Hydrogen fusion is the same process as in the hydrogen bomb: so why doesn't the Sun explode, and why aren't we and other forms of life destroyed by the radiation (high energy neutrons and gamma rays) ?
This brings us onto looking at the structure of the Sun. Gravity holds the Sun together, in the same way that gravity holds the Earths atmosphere onto the Earth. our atmosphere is more dense at sea level than at the top of a mountain - the weight of gas above compresses the gas below. The same thing happens in water - the deeper you go the higher the pressure. Much the same thing happens on the Sun, where the pressure upwards from the hot gas balances the downwards pressure from the weight of the layers above - this is called hydrostatic equilibrium. (Review concepts of temperature, pressure and density). This makes the Sun very stable. Suppose the central regions expanded a little bit. Then there would be mare space between the particles, so they would interact less often. So there would be less collisions, so less fusion reactions, so less energy, so the gas temperature drops. This means its pressure would drop, but the gravitational pull inwards on the center from the weight of the outer layers is still the same, so it pushes inwards, contracting the central region. The star is stable - adjusting itself back to equilibrium for any small purturbation away from equilibrium.
Nuclear fusion can only take place at very high temperatures as the protons need to be moving very fast in order to get close enough to each other to interact - otherwise their like charges will repel them and push them apart. Moving fast = high temperature so these reactions only happen in the very center of the Sun. Which is the main reason we don''t get fried by high energy radiation and particles from the nuclear fusion reactions!