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...