The Crab Nebula is an interstellar gas cloud, located approximately 6500 light years from Earth.
In an attempt at anecdote, in the prescence of Captain Picard and Lwaxana Troi, Lieutenant Commander Data explained how he was able to precisely calculate the distance between the Crab Nebula and the Omicron Galaxy. He accomplished this by multiplying the square root of pi with nine to the third power. (TNG: "Manhunt")