efore his death during World War I, the German physicist Karl Schwartzschild carried out the first scientific analysis of the properties of what we now call black holes (2: 45). The radius of a black hole's event horizon is still called the Schwartzschild radius in his honor. It still remained, however, a concept with no particular connection to specific problems in astrophysics. Then, in 1939, physicist J. Robert Oppenheimer (the "father of the atomic bomb") and his student Hartland Snyder developed a new theory which outlined conditions under which a black hole might actually be formed (4: 65). Under certain circumstances a massive star can suffer a catastrophic explosion, called a supernova, which blows the outer layers of the star off into space, while the core of the star collapses to a mass of tightly packed neutrons so dense that a star the mass of the Sun would be only a few miles in diameter. Such a collapsed star was called a neutron star, and its surface escape velocity would be a large fraction of the velocity of light.
Supernovae are rare but well known astronomical events. One in the year 1054, recorded by Chinese astronomers, is believed to have produced the dramatic Crab Nebula in Taurus. In 1987, a supernova in the Milky Way's satellite galaxy, the Large Magellanic Cloud, attained naked eye visibility at a distance of about 150,000 light years. For a generation after they were theorized, neutron stars remained a purely theoretical construct, but neutron stars have now been positively identified in supernova remnants, including the Crab Nebula. Recently, astronomers found evidence of a new born neutron star (or "pulsar") within the debris of the 1987 Magellanic Cloud supernova (3: 480).
A neutron star could only form if the remaining core of a supernova was below a certain mass, equivalent to about two solar masses. Above t
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