Why does the sun shine? The Sun’s radiance results from the process of nuclear fusion, which transforms hydrogen into other light elements, and ultimately iron (H..Fe).

What is actually happening is that 4 atoms of Hydrogen come together in a series of steps that we are going to leave out for now, they form a helium atom. That is made from 2 protons and 2 neutrons. Therefore, some conversion of 4 protons into 2 neutrons into 1 Helium. This works because there is quantum mechanical tunneling going on. The 4 positively charged protons would actually repel themselves.

Because this tunneling effect it is hot enough so that these protons can be eventually combined into 1 helium nucleus.

If we weigh 1 He atom and (2n+2p) separately we would find out that 1He < (2n+2p) around ~0.73% (mass defect) and the energy released as the popular equation showed E=mc^2. For atoms that are heavier than iron, the fusion process happen requires external energy to happen.

In the end, the star ends up with an iron core. These fusion processes have been going on in the center and growing larger and larger as more and more elements are being made. Eventually, there is a big, fat iron core sitting there. Since it can’t get energy out anymore, the star has a big problem because it needs to have an energy source. Without that, it explodes as a supernova.

It is really nice how nuclear physics and astrophysics can come together because nuclear physics governs what is happening inside the core and then the astrophysics provides what we can actually observe. By putting these two together we can understand stellar evolution. That is actually governed by the nuclear physics processes, specifically fusion, in the core.

By measuring the luminosity of a star as well as its temperature we can place it on this diagram and then learn which evolutionary state the star is in, which tells us what is going on in its core.