The size of the broad-line region can be calculated from the brightness of the quasar radiation that is lighting it up.[7] From the size of this region and the speed it is orbiting, the law of gravity reveals that the mass of the black hole in TON 618 is 66 billion solar masses.[2] With a mass this high, TON 618 falls into the new classification of ultramassive black holes.[8][9] A black hole of this mass has a Schwarzschild radius of 1,300 AU.