|Matter | Pages 104-105|
Too Much Matter
A neutron star is one possible end point of the demise of a massive star. If the star's core is very massive--more than three times the mass of our Sun--even neutrons can't survive the crushing power of gravity. As the core shrinks, its gravitational pull becomes unstoppable. The core collapses in on itself and vanishes as the rest of the star explodes in a titanic supernova. The core becomes a black hole that emits no visible light. Indeed, all of the familiar characteristics of stars, including color, luminosity, and chemical composition, no longer exist. Black holes retain only three properties: mass, spin, and electric charge.
Incredible as all this sounds, the laws of gravity as we currently understand them predict just this. Theoretical physicists have developed several models that describe different varieties of black holes. One type, known as the Schwarzschild black hole (near right), is characterized by its mass alone. All of the mass concentrates at an infinitely dense point known as a singularity, located at the center of the hole's "surface"--the event horizon. However, another type of black hole, the Kerr black hole, is probably much more common in the universe. It is simply a Schwarzschild black hole that spins.
Schematically illustrated on the opposite page, the Kerr black hole's rotation forces the singularity to take the shape of a ring within the event horizon rather than a point. The rotation also creates a donut-shaped region where space warps so severely that the path of a light beam will close back upon itself and trace a stable orbit around the hole. If light approaches closer than this zone, it spirals inward helplessly toward the event horizon. Such is the inescapable fate of light and anything else that wanders too close to the black hole's deep gravitational well.
Space near a rotating black hole is so severely warped that a beam of photons, which tracks the fabric of space itself, can encircle the hole in a stable orbit.
If a light beam crosses the zone for stable orbits, it relentlessly spirals inward and disappears from view at the event horizon. Along the way, the light gains energy and its color shifts to the violet end of the spectrum. These effects are invisible, however, to an observer outside the event horizon.