compressed into a single point, estimated to have a density of 109 kg m 3 and a temperature of 10 K(Kelvin). With the Big Bang, this point of mass exploded, ejecting material in all directions. Aggregates of ejected material collapsed gravitationally to form the earliest stars. When temperatures in the cores of early stars reached 10 million K, nuclear fusion of hydrogen(H) into helium(He) and higher elements began, releasing energy that powered the stars. As early stars aged, they ultimately exploded, ejecting stellar material back into space. Table 2.1 gives the abundance of hydrogen in the universe today relative to the abundances of other interstellar elements.