The most obvious part of the Galaxy is stars. Stars contribute to the dynamics of the Galaxy in a way that is interwoven with the gas and dust. For this course, we don't look at stars in a great deal of detail, but concentrate on a few important points. The models are based on gravity, gas dynamics, nuclear physics, etc. The models make predictions that can then be checked against what we observe on the outside of stars.
Stars are much hotter and denser in their cores than on the outside.
For most of their lives, their energy source is the fusion of hydrogen nuclei to make helium nuclei. Here is what the fuel situation looks like for the Sun. When the hydrogen runs out in the core, the core shrinks and gets hotter. Other processes that provide energy can then occur. Helium nuclei fuse to make carbon.