Animation of a black hole deforming spacetime. Spacetime is represented by a grid, which is distorted by mass. Each star is seen to rest within an associated gravity well, a small dip in spacetime where its gravity exerts influence. Light passing within this dip will curve around it. For an exceedingly compact object like a black hole, the gradient of the dip is such that light cannot escape from the gravity well. The boundary at which this becomes inevitable is called the event horizon.