Today in History—January 7: This Will Really Pull You In

Black holes sound like science fiction: collapsed stars whose incomprehensibly powerful gravitational pull absorbs everything around them. Scientists had theorized the existence of these mysterious celestial objects for centuries, but how do you discover something in the depths of space that emits no light?

Enter Louise Webster and Paul Murdin. Their findings, published on this day in 1972, were an exploration of the very first black hole identified: Cygnus X-1 (Cyg X-1 for short).

Using the Isaac Newton Telescope in East Sussex, England, the two scientists measured a powerful source of X-ray energy within the Cygnus constellation. In images, Cyg X-1 appears to be just one star. But the scientists discovered that Cyg X-1 is actually a binary star system within the constellation, meaning that it consists of two distinct bodies—one that’s visible, and one that isn’t.

But how did they know that the mysterious second body was a black hole?

Well, the visible star is slowly being pulled into the black hole. In the process of being drawn in, the gas composing the star becomes very hot—leading it to emit an observable X-ray signature. Scientists believe that Cyg X-1 is about 21 times the mass of the Sun, and it’s one of the fastest spinning black holes ever found.

Cyg X-1’s discovery helped scientists identify how black holes fit within the space-time continuum. Although black holes remain mysterious, Webster and Murdin’s discovery helped shed light on these dense pockets of darkness.