The star collapses in upon itself in a matter of seconds (a far cry from the million- and billion-year time scales used so far), and a tremendous amount of gravitational energy is released. (There is nothing mysterious about this gravitational energy. When lead shot is dropped from a height onto a steel plate, and the metal is warmed from the impact, this is the conversion of gravitational potential energy into heat. On a much larger scale, it is gravitational energy that heats the interior of an infant star and touches off the hydrogen-fusion process.) The enormous heat generated in this final collapse detonates more nuclear reactions in the outer layers of the star, and literally blows it apart in a supernova (page 25). The material of the former star is spread far and wide across interstellar space, with heavy elements arising both from the interior of the star and from the supernova explosion itself.

First-generation stars therefore turn hydrogen into heavier elements. After a supernova explosion, these elements are mixed with the original hydrogen of interstellar space, ready to serve as the raw material for a new generation of stars like our sun. As these second generation stars (page 26) coalesce, they may form planets around them, and if so, these planets are enriched in the heavier elements. The core of Earth, for example, is believed to be made up of metallic iron.

One of the most striking objects in the sky is the Crab Nebula in the constellation of Taurus, shown opposite...