:::::::BLACK HOLE::::::::::

star is a huge, amazing fusion reactor. Because stars are so massive and made out of gas, there is an intense gravitational field that is always trying to collapse the star. The fusion reactions happening in the core are like a giant fusion bomb that is trying to explode the star. The balance between the gravitational forces and the explosive forces is what defines the size of the star.
As the star dies, the nuclear fusion reactions stop because the fuel for these reactions gets burned up. At the same time, the star's gravity pulls material inward and compresses the core. As the core compresses, it heats up and eventually creates a supernova explosion in which the material and radiation blasts out into space. What remains is the highly compressed, and extremely massive,
core. The core's gravity is so strong that even light cannot escape.

Photo courtesy NASA
Artist concept of a black hole: The arrows show the paths of objects in and around the opening of the black hole.

This object is now a black hole and literally disappears from view. Because the core's gravity is so strong, the core sinks through the fabric of space-time, creating a hole in space-time -- this is why the object is called a black hole.
The core becomes the central part of the black hole called the singularity. The opening of the hole is called the event horizon.
You can think of the event horizon as the mouth of the black hole. Once something passes the event horizon, it is gone for good. Once inside the event horizon, all "events" (points in space-time) stop, and nothing (even light) can escape. The radius of the event horizon is called the Schwarzschild radius, named after astronomer Karl Schwarzschild, whose work led to the theory of black holes. 

 Supermassive black holes in some giant galaxies create such a hostile environment, they shut down the formation of new stars. Strangely, black holes lie at the center of most galaxies, as shown in the next image.

 Dark matter composition is up for debate, with subatomic particles and black holes considered as candidates.

 This is a new composite image of a galaxy cluster located about 2.6 billion light-years away. The three views of the region were taken with NASA's Hubble Space Telescope in February 2006. See a massive radio telescope in the next photo.

 Like most galaxies, NGC 1097, a barred spiral galaxy, has a supermassive black hole at its center.

 In this artist's rendition, the yellow region at the center represents a supermassive black hole. Around are dust grains mixed with heated, outflowing gas. But just how big is "supermassive?" Find out in the next photo!

This is a depiction of a wormhole, or an Einstein-Rosen bridge, bursting open in the vacuum of space. Many believe these curves in spacetime could enable time travel.