Read this excerpt from "Why Are They Called Black Holes?" After many millions or billions of years, the star runs out of hydrogen and energy. Then the star "dies" and collapses into a dense little ball with tons of gravity. Albert Einstein’s theory of relativity predicted that black holes would have to exist if a dead star had enough gravity. Using Einstein's theory, astronomers calculated that if a star more than three times as heavy as our sun died, the force of gravity pulling matter in could overwhelm any other force pushing matter out. In this excerpt, the author makes a connection between a scientific theory and the calculations that scientists make. the beginning of a star's existence. the idea that all things live and die. the question of why gravity exists.