Black Holes

When a star has three or more solar masses left after it exhausts its nuclear fuels, it can become a black hole.

Like the white dwarf and neutron star, this star's density and gravity increase with contraction.

Consequently, the star's gravitational escape velocity (speed needed to escape from the star) increases. When the star has shrunk to the Schwarzschild radius, named for the man who first calculated it, its gravitational escape velocity would be nearly 300,000 kilometers per second, which is equal to the speed of light. Therefore, light could theoretically never leave the star.

Reduction of a giant star to the Schwarzschild radius represents an incredible compression of mass and decrease in size. As an example, mathematicians calculate that for a star of 10 solar masses (ten times the mass of our Sun) after exhaustion of its nuclear fuels, the Schwarzschild radius is about 30 kilometers.

According to the Law of General Relativity, space and time are warped, or curved, by gravity. Time is theorized TO POINT INTO THE BLACK HOLE FROM ALL DIRECTIONS. To leave a black hole, an object, even light would have to go backward in time. Thus, anything falling into a black hole would disappear from our Universe.

The Schwarzschild radius becomes the black hole's "event horizon", the hole's boundary of no return. Anything crossing the event horizon can never leave the black hole. Within the event horizon, the star continues to contract until it reaches a space-time singularity, which modern science cannot easily define. It may be considered a state of infinite density in which matter loses all of its familiar properties.

Theoretically, it may take less than a second for a star to collapse into black hole. However, because of relativistic effects, we could never see such an event. This is because, as demonstrated by comparison of clocks on spacecraft with clocks on Earth, gravity can slow, perhaps even stop, time. The gravity of the collapsing star would slow time so much that we would see the star collapsing for as long as we watched.

Once a black hole has been formed, it crushes into a singularity anything crossing its event horizon. As the black hole devours matter, its event horizon expands. This expansion is limited only by the availability of matter. Incredibly vast black holes that harbor the crushed remains of billions of solar masses are theoretically possible.

Evidence that such super dense stars as white dwarfs and neutron stars do exist has supported the idea that black holes, representing what may be the ultimate in density, must also exist. Potential black holes, stars with three or more times the mass of our Sun, pepper the sky. But how can astronomers detect a black hole?