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Heavier elements like gold, uranium etc. are formed at the end of a star's life. As the star explodes into a supernova, it gives rise to nebula which is the birthplace of new stars. But as the star has already fused lighter elements into heavier elements, where does the new hydrogen required for the formation of new stars come from? Do heavier elements breakdown during supernova or is it some other process?

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2 Answers 2

For a star to become a nova, it needs to have a mass at least 8 times greater than our sun. For a supernova, it needs to be larger.

The first stage is the hydrogen fusion into heavier elements. The energy created pushes the hydrogen outwards. If there was no fusion, the star would be a low smaller. When the hydrogen fusion in the core slows down (due to a lack of hydrogen), the star shrinks and helium fusion takes over as the main process. At this time, it is believed that the core of the star is about 25% hydrogen and 75% helium. The outer layer is over 99% hydrogen. Eventually, the star makes iron as a fusion product. At this stage, our sun will have expanded out to Earth's orbit due to the energy pushing out from the core.

The fusion to make an element heavier than iron consumes more energy than what it produces, so when the iron fusion process slows down significantly, the star collapses. This collapse causes a massive pressure and temperature increase in the core, which triggers the fusion of heavier elements; although it is a net loss of energy. This burst of energy blows the star apart. The difference between a nova and a supernova is the amount of fusion that happens before the core is blasted apart.

Since fusion only takes place in the core, and the core is less 1/100 the volume of the star, when the star goes nova (or supernova), the outer layer of the star (still 99%+ hydrogen) is blown outwards.

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So, actually the star blows up because it tries to fuse iron? –  Yashbhatt May 11 '14 at 6:24
In essence, yes. But because there are lots of other elements (there might still be 20% hydrogen), and the pressure and temperature is intense (especially in a supernova), any fusion is possible. If 2 iron atoms combine, the number of protons is 52 (tellurium); but even heavier elements are formed in a supernova. –  LDC3 May 11 '14 at 13:37
Are elements like uranium too formed in the core by fusion? And you mentioned that fusion of heavier elements takes up more energy than it releases. What kind of energy is that? I am asking this because I think gravity does the work. –  Yashbhatt May 11 '14 at 15:42
@Yashbhatt Yes, gravity does the work needed to bring the atoms together. The energy comes from the conversion of matter into energy as Einstein stated: $E=mc^2$. When a proton and a electron come together to form a neutron, the mass of the proton and electron is more than the neutron. This mass is converted to energy. –  LDC3 May 11 '14 at 16:26
What I meant was that starts don't use the energy derived from mass to start fusion, right? –  Yashbhatt May 11 '14 at 16:28

Only a very small fraction of the elements in the core the supernova get converted into heavier elements. Most of the matter remains unchanged.

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Then why does the star go supernova, when it still has some hydrogen left? –  Yashbhatt May 10 '14 at 12:57
Mostly because of gravity. –  this May 10 '14 at 13:05
I know that. But can't it somehow move the hydrogen in its shell towards the core so as to fuse it? –  Yashbhatt May 10 '14 at 13:30
@Yashbhatt If it could, it would. But as it can't, it shan't. All due to a bitch named gravity. Light things float over heavier things. –  Cheeku May 10 '14 at 15:25
@Cheeku Sounds a little poetic. But what about convectional currents? –  Yashbhatt May 11 '14 at 6:23

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