Model 1: The Sun and the Proton-proton Chain

Studying the Sun provides scientists with a plethora of data as well as an idea of how all stars are born, live, and die. Despite all of the information gained from observation, there are parts of the Sun (most of its interior) that scientists only postulate about based on data from models of the Sun. Combining what we do know (surface temp, chemical composition because of spectra, mass because of Newton, and the temperature required in order to fuse hydrogen protons into helium protons) we have created a model for the insides of the Sun. 

The layers of the sun from inside to outside: core, radiative, convective, photosphere, chromosphere, corona. 

Core and radiative zone are where fusion takes place. The pressure and temperature are right here. 

Convective zone is mostly plasma made up of hydrogen. 

Photosphere and chromosphere are the two upper most layers of the Sun. The chromosphere is hotter than the photosphere.

The corona is the wispy highest layer. It is made up of very hot, ionized gas. However, if you were to be inside of it (somehow without dying from the lack of oxygen, etc etc) you wouldn't be immediately burned. Imagine, for example, the way the hot air feels as you open the oven after you've been cooking something. You feel the heat and you could identify that it is very hot, but you are not burned the same way you would be if you touched the metal inside the oven. 

Inside the core of the Sun, it is so hot that the electrons associated with the protons of the hydrogen atoms are no longer bound to them. The electrons are in a free state. Two hydrogen protons bond together, and then those two bond with another to form a helium atom. The interesting part (okay, one of the interesting parts) comes when you look into the mass of the atoms before and after. The total sum of the protons after added up is less than before when they were single hydrogen protons. A tiny bit of mass is gone! What happened to it? It left in the form of a neutrino, a positron, and a gamma ray. By the time the gamma ray makes it from the Sun's core to the Sun's surface, it has lost energy and is viewed as visible light. 

Neat!