A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.

Because no light can get out, people can't see black holes. They...

An even better illustration for center of mass is the binary star system . When two stars of comparable mass cohabit the same corner of space, they orbit about a point between each other.

Space and time are the grand stage where the play of the universe unfolds. But space isn't a fixed stage and doesn't tick the same for everyone everywhere. In short, they are relative. Matter bends space and bent space tells matter how to move.

• Put some stars and ...