Once a star has formed, it spends most of its active lifetime burning its initial nuclear fuel, hydrogen, by a process of nuclear fusion that combines four hydrogen atoms together to make a helium atom and energy. Eventually, the star's hydrogen supply is spent, and, if the star is massive enough, the star's helium supply begins to fuse together to make carbon atoms, which then go on to make heavier and heavier atoms until all the nuclear fuel is converted to the element iron, which is incapable of further energy-releasing reactions. Once there exist no further resources of heat and pressure to counterbalance the inward pull of gravity, the star must collapse.
Our own sun has already lived about 5 billion years and will live another 5 billion years, quietly burning its hydrogen, before it swells into a red giant star. Then, in the relatively brief period of about 100 million years, it will exhaust the rest of its nuclear fuel and collapse. More massive stars spend themselves more quickly and less massive stars more slowly. For example, a star of 10 times the mass of our sun burns up its core of hydrogen gas and becomes a red giant in only about 30 million years.
A burned-out star may end its life in several ways. If the mass remaining after the red giant phase does not exceed several times that of our sun, it becomes a dense, dim white dwarf star or, after a violent stellar explosion called a supernova, an even denser cold star called a neutron star. A white dwarf is about 100 times smaller than a younger star of the same mass. A neutron star is yet another 1,000 times smaller than a white dwarf and is composed almost entirely of neutrons, uncharged subatomic particles, packed together side by side. A typical neutron star has an incredible density: the mass of Manhattan Island squeezed into a cherry. Furthermore, that star can spin very rapidly, between 1 and 1,000 revolutions per second; it can anchor magnetic fields that are trillions of times stronger than the earth's, and it can produce periodic pulses of intense radio waves. White dwarfs and neutron stars support themselves against further collapse by the resistance of their subatomic particles to being squeezed more closely together. They can remain in such balance almost forever. Yet these massive spheres that were once shining stars have no source of energy, other than their energy of rotation, and so eventually grow dim and cold.
The first white dwarf was identified in 1914 and the first neutron star in 1967. Astonishing as it may seem, astrophysicists had predicted the characteristics of neutron stars before their discovery. Swiss-born astronomer Fritz Zwicky and German-born astronomer Walter Baade, working together in California, correctly forecasted the existence and properties of neutron stars as early as 1933, only two years after the discovery of the neutron itself in terrestrial laboratories. Such accurate predictions testify not only to the power of theoreti-