In this unit, we discuss the interior of the Sun. This is a tricky proposition because we cannot see the interior of the Sun in visible light. As a consequence, most of what we know about the interior of the Sun (and hence stars) comes from well-crafted theory that has been tweaked and improved through comparisons to observations.
Fortunately, however, the basic notions about the structure of the Sun can be understood from very simple observations. The first is that the Sun is held together by gravity, the mutual attraction between the 10**57 particles which make up the Sun. The second observation is that the Sun is able to maintain its current size because its interior is very hot, As a result, the Sun generates a large internal gas pressure which counteracts the tendency of gravity to contract the Sun. The maintenance of this mechanical equilibrium is referred to as hydrostatic equilibrium.
Another note is that we can see the Sun!!! We see the Sun because it emits photons. Since photons are a form of energy, this means that the Sun is continuously losing energy. This implies that the Sun should be getting cooler so, as a consequence, its internal gas pressure should be getting smaller and thus the Sun should be shrinking. The Sun has been around for over 4.5 billion years which means that the Sun must continuously replace the energy it loses to radiation. The Sun needs an efficient way in which to generate energy. It finds such a process in the fusion of hydrogen nuclei into helium nuclei; the Sun is a huge nuclear fusion reactor.
Now let us examine these points in more detail.