Q: What fusion processes occur in the Stars and how?

At the beginning of the universe pretty much nothing existed. The entire universe which was extremely small at the time was pretty much pure energy. With the expansion of the universe it cooled resembling the behavior of gases, which are much more familiar to us. As the universe cooled down actual matter came into existence from pure energy, obeying the famous Einstein law of conversion from mass to energy and vice-versa.

As the universe became increasingly cool it allowed the existence of atoms. But only the basic atoms were formed at first. The most simplistic atom of course is hydrogen consisting of only an electron orbiting a proton. But a question arises with this explanation. How did heavier elements form with the universe being so cold all of a sudden? Well the answer is all elements up to iron were formed in stars through various fusion processes, while elements heavier than iron were formed in super-massive explosions called supernovae.

So, how exactly do fusion processes occur in the Stars and what makes them possible?

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