The brightest persistent X-ray source in the sky, and the first known celestial X-ray source apart from the Sun. It was discovered in 1962 during a sounding-rocket flight. Scorpius X-1 is a low-mass X-ray binary with an orbital period of 18.9 hours. The optical counterpart is a 13th-magnitude blue star known as V818 Sco, 9000 l.y. away. The X-rays arise from the transfer of material from this star on to a companion neutron star via an accretion disk. Scorpius X-1 is over three times brighter at X-ray wavelengths than the second-brightest constant X-ray source, the Crab Pulsar.