A black hole is formed when a star is out of nuclear fuel. Nuclear fuel is what keeps a star going. A star that cannot sustain its own mass, so it collapses under its own weight. Let's look at the process of this.

Regular Star

The regular star emits out bright light from the core. They are the easiest form to see. It has plenty of nuclear fuel to sustain its weight.

Superdense Body

The superdense body (dying star) emits little light from the core due to its gravitational pull. They are not as visible as a regular star because it doesn’t emit as much light. It is starting to get low on nuclear fuel therefore it’s starting to weigh more.

The black hole emits no light due to its EXTREMELY powerful gravitational pull. They are completely invisible because they don’t emit light from the core. It is drained of its nuclear fuel so the superdense body collapsed into this small, but extremely massive form.

In 1783, the existence of black holes was only ideas. Now we have more evidence that they exist. Of course, black holes are invisible because light cannot escape from their core. So we must use indirect evidence to find out about them.

When scientists find a region of space where they think there is a black hole, the first thing they do is measure how much mass there is in the area. They do this by looking at the speed things orbit around that area. If the orbital speeds are fast, there is a strong gravitational force in that area. If the strong gravitational force has a small volume without any light coming out of it, that is evidence of a black hole.

As you may know, black holes tend to suck in everything that crosses their event horizon. An event horizon is the black hole’s gravitational boundary or "the place of no return". Once past the event horizon, you’re past the gravity boundary. Once past the gravity boundary, you’re doomed to be pulled down by the amazing force of gravity and hit something called the singularity in the middle of the black hole.

If you were to fall feet first into a black hole, your feet would weigh more than your head because your feet would go through the boundary first. If your feet weighed more than your head, your feet would fall faster than your head. So your body would be painfully stretched out and eventually ripped apart. Now what would you rather do, warm up stretches in gym, or gravity stretches in black holes?

To begin with, let me say that the Sun won’t and cannot become a black hole. Only stars with much more mass than the Sun can become a black hole. The Sun will probably end by having a short phase of being an enormous "red star," destroying Mercury and Venus in the process. Then, the Sun will die by becoming a "white dwarf star."

But if the Sun did become a black hole, the earth would become very dark and cold. The planets that orbit the Sun would not get sucked into the black hole. The event horizon of this black hole would be so small that the planets would not be able to enter it. The event horizon of the Sun would only be 3 kilometers and it would not be strong enough to pull the planets in.

We have had evidence for black holes for a long time. However, we are learning new and interesting facts about them every day. The Hubble Space Telescope and NASA’s Rossi X-ray Timing Explorer (RXTE) spacecraft are helping us find out more information all the time.