Upwards of a billion stars in our galaxy have burnt up their internal
energy sources, and so can no longer produce the heat a star needs to oppose
the inward force of gravity. These stars, of more than a few solar masses,
evolve, in general, much more rapidly than does a star like the Sun. Moreover,
it is just these more massive stars whose collapse does not halt at
intermediate stages (that is, as white dwarfs or neutron stars). Instead, the
collapse continues until a singularity (an infinitely dense concentration of
matter) is reached.
It would be wonderful to observe a singularity and obtain direct evidence
of the undoubtedly bizarre phenomena that occur near one. Unfortunately in most
cases a distant observer cannot see the singularity; outgoing light rays are
dragged back by gravity so forcefully that even if they could start out within a few kilometers of the singularity, they
would end up in the singularity itself.
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