Monsters in the sky - Black Holes
A Black Hole is formed when a star larger than about 3 times the mass of the Sun runs out of fuel.
A star gets its energy by turning Hydrogen into Helium and heavier elements by a process called nuclear fusion. If you are not familiar with fusion see the short article here >>>
The energy produced by fusion creates light and heat but also creates an outward pressure which holds the star up against it’s gravity which tries to make the star collapse. It is much like the pressure of air in a balloon prevents it from collapsing. When the hydrogen in the centre of the star runs out the fusion reaction ceases as does the outward pressure caused by the reaction. There is now nothing to hold the star up and so it collapses in on itself.
If you are far enough away from the singularity it presents no danger. However as you get closer the gravity starts to draw you in. Up to a point you can simply fire your rockets and move away. But if you get too close the gravity is too strong and even an infinite amount of energy is not enough to enable you to move away. This point of no return is called the event horizon and marks the surface of the black hole. If you cross the event horizon you will be sucked into the singularity and nothing can save you. Even light would be drawn in.