I'm quite new to the world of physics/cosmology and their neighboring subjects. I've just begun learning about black holes, and so far, I've understood everything I've come across except for one term: Fuel. I've read several articles that pertain to black holes. In each of these articles, a certain "fuel" is mentioned. To quote one of these articles: "- when a sufficiently massive star runs out of fuel, it is unable to support itself against its own gravitational pull, and it should collapse into a black hole." The lack of explanation leaves me flummoxed. What is this "feul" the star runs out of? Any information regarding the above would be greatly appreciated. Thanks!