It is a very typical star in all its properties. Because of this, we can learn much about other stars by simply studying our own. The amount of energy reaching the earth each second. If we then find the surface area of this sphere and apply the solar constant to it, we come up eith the quantity known as luminosity. The interior of the sun is physically unmeasureable.
Astronomers have to infer what the interior is like.
To this end, they build mathematical models and use exterior observations to test them. What they have come up with is the Standard Model of the Sun. The study of how the sun vibrates. These pressure are essentially sound waves. The study of these sound waves, since they pass through the entire sun, provide astronomers with a way to study the interior.