Stars are the most important objects in the Universe--huge balls of gas that grow dense and hot enough to sustain nuclear fusion in their core. They are the ultimate source of all high-energy radiation such as visible light, and also the means by which the lightweight raw materials of the universe (see page 394) are transformed into heavier and more complex elements--including those that make up our own bodies.
In the context, I think 'raw materials' as 'an element' in physics such as hydrogen and helium. Or, maybe raw materials just mean matter or particles of matter. Could you explain them?
Embarrassingly, but to be frank, I have no basic knowledge of physics and I'm not an English-native speaker.