A black hole is the result of a reallllllly huge star (8 x bigger than our sun or more) running out of hydrogen fuel. It swells up and tries to compensate by burning other gases but after several million years the other gases run out too. At this point the star EXPLODES! into a massively bright supernova, far brighter than any other star in the sky. The big swollen ball of star shrinks in on itself, creating a very dense ball of rock that has an extremely strong gravitational pull. Once the light fades from the cooling star it becomes what we call a black hole. The huge gravitational pull of the black hole means that anything that gets too close will be absorbed and crushed. If you see a black hole..run!