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 tightlypacked 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, eq
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