Scorpius X-1 (α 16h 19m 55.1s, δ -15° 38' 25'') was the first X-ray source detected outside of our solar system, in June 1962, by astronomers at MIT. It was detected by mounting a Geiger counter on a sounding rocket launched from White Sands Missile Range in southern New Mexico. The rocket flight was undertaken to study X-rays from the moon, but the detector also detected Sco X-1. It was detected first because it is far brighter than any other extrasolar X-ray source; Sco X-1 is intrinsically bright in X-rays, but it is relatively close by -- only a few thousand light years away.

Scorpius X-1 is a low-mass X-ray binary, which means that it consists of a massive, compact object like a neutron star or a black hole, and a low-mass stellar companion. The X-rays come from accretion, where material from the companion overflows its Roche lobe and spirals down onto the compact object. The luminosity comes from the transformation of the falling material's gravitational potential energy to heat by viscosity in the accretion disk.

The system is also a Z-source, a sub-class of X-ray binaries which follow a Z-shaped pattern in a plot of the high-energy versus low-energy X-ray luminosity. The position in the Z is related to the how fast the star is accreting. They are at the top of the Z when they are near their Eddington limit -- when the pressure of the light is nearly strong enough to overcome gravity.

Besides being bright in X-rays Scorpius X-1 is also an optically variable star. However, it is very faint with an apparent magnitude of only 12.5. Therefore it is only visible with a telescope and a good star chart, in the constellation Scorpius. It is best observed (if you can find it!) between May and July.