Collapsed Stellar Cores | White Dwarfs | Neutron Stars | Black Holes
...4.... Black Holes
The next obvious question to ask is what happens when there is so much mass in the stellar core that even neutrons can't hold it up. Scientists know of no force in nature that can stop the collapse of such a core. The collapse continues until the density is so high that even light cannot escape from the object. The radius at which this occurs is called the Schwarzschild Radius. Any light originating from below the Schwarzschild Radius cannot break free of the object's gravity. Since light cannot get out to us the object becomes invisible, hence the name Black Hole. For a black hole having three times the mass as the Sun the Schwarzschild radius is about 9 km.  The scaling is easy: if you double the mass of the black hole, the Schwarzschild radius also doubles.

It is a result of Einstein's Theory of General Relativity that gravity causes light to bend, just like a lens does.  For gravity as weak as the Earth's, the effect is not noticeable - hence the concept does not seem intuitive to us.  But it certainly would be close to a black hole, leading to some unusual effects. To see what these are we'll use an applet to view space from near or even inside a black hole.  It shows you how space appears at any distance from the black hole and for any direction.
Black Hole Applet

Instructions: open the applet. Click anywhere within the frame and then click anywhere on the bottom left control panel to activate the controls.   Along the bottom of the frame is a scale with a blue circle showing how far you are from the center of the black hole.  The three line segments extending from the circle show the current field of view.  Your left and right keyboard arrows should change the distance, and your up and down arrows should change the viewing angle.  If not, the blue circle can be dragged to a new distance, and the middle line segment can be dragged to change your viewing angle.

The location "3 M" is the Schwarzschild Radius, inside of which light (and you) cannot escape.   Experiment with different distances and viewing angles to see how space appears to be distorted by the bending of light around the black hole.