They’re mostly made out of Hydrogen. However, because the way that they shine is to do nuclear fusion, which turns Hydrogen into Helium and eventually heavier elements, they’re not entirely Hydrogen.
n fact we know that all stars are mostly made of Hydrogen by comparing the light from the Sun with that from the stars.
That’s an image of the light from the Sun on the left and you see a load of dark lines, these are the colours of light that Hydrogen and Helium like to absorb (what the Sun in made of). Looking at distant stars on the right we see the similar dark bands telling us that all stars are made of the same stuff. The reason for the shift in colour is because the stars are moving away from us because of the Big Bang.
Hydrogen and Helium make up 99.9% of the star! Everything else comes in tiny amounts. It’s this nuclear fusion of Hydrogen into Helium that makes the star ‘shine’, as it generates a lot of heat and light.