What happens if star can't get rid of enough mass in a supernova explosion to produce a remnant neutron core below three solar masses (below which only can neutrons produce enough pressure to counteract gravity); or if the collapse of the core is so dramatic as to smash through the neutron pressure barrier? When an object of mass M has radial size less than R = 2GM/c2 (the
Schwartzschild radius; 3 kilometers for a mass of 1 solar mass), then the surface gravitation becomes so intense that not even light may escape; the object disappears from view. Although not visible in any form of electromagnetic radiation, the object's gravitational field would still be felt in the surrounding space. Such a
black hole could be detected by its gravitational influence on other objects.
Evidence for such collapsed objects appears to exist in the form of
binary x-ray systems. Here a compact object may accrete material from its companion that is swelling to become a red giant star. As this material falls in toward the compact star, angular momentum conservation produces a rapidly rotating accretion disk near the compact star. The energy released from infall of additional matter and its collision with this accretion disk appears in the form of X-rays, gamma rays, and other energetic photons. Application of Kepler's Third Law to the observed orbital motion of the visible companion in several X ray sources (for example, Cygnus X-1) suggests that the masses of the unseen companions are too large to be any kind of known star; thus presumably the unseen stars are black holes.
In summary, objects termed stars may represent a wide variety of physical conditions, as shown in Table
1 and Figure
1 :
A Brief History of Astronomy
Final End States of Stars
