(catalog number Sco X-1), brightest X-ray source in the sky, the first such object discovered in the direction of the constellation Scorpius. Detected in 1962, its X-radiation is not only strong but, like other X-ray sources, quite variable as well. Its variability exhibits two states, one at higher output with great variability on a time scale of minutes and another at lower output with the variability correspondingly lessened.

Scorpius X-1 was observed in visible light for the first time in 1966. Optically it is much less impressive, bluish in colour and appearing only faintly. The complete nature of Scorpius X-1 is not yet known, but it appears to be a close double star, one component of which is optically invisible—either a white dwarf star or a neutron star. Such is the case with most galactic X-ray sources. The X rays are generated when matter from the optically visible, bluish hot star falls onto the invisible collapsed companion. This matter is tremendously accelerated and crushed by the enormous gravity of the collapsed star. Unlike the majority of binary X-ray sources, the visible member does not appear to be very massive.