Grouping nearby particles into one larger particle allows fewer computations to occur as fewer particles are being dealt with. This works because gravitational force is inversely proportional to the square of the distance between the particles, leading to a rapid drop-off in force as distance increases.
Lotusword
If the center of mass of the cell is far enough away from the body, the entire subtree under that cell is approximated by a single body at the center of mass of the cell, and the force this center of mass exerts on the body computed. If, however, the center of mass is not far enough away, the cell must be "opened" and each of its subcells visited.
Grouping nearby particles into one larger particle allows fewer computations to occur as fewer particles are being dealt with. This works because gravitational force is inversely proportional to the square of the distance between the particles, leading to a rapid drop-off in force as distance increases.
If the center of mass of the cell is far enough away from the body, the entire subtree under that cell is approximated by a single body at the center of mass of the cell, and the force this center of mass exerts on the body computed. If, however, the center of mass is not far enough away, the cell must be "opened" and each of its subcells visited.