You can’t see a black hole, of course, but astronomers first deduced the presence of one of these exotic objects, known as Cygnus X-1, revolving around a giant blue star in the constellation Cygnus. You can easily see this blue star, if not its unseen companion, with binoculars or a small telescope.
Cygnus X-1, so-named because it’s the brightest X-ray source in Cygnus, was first detected in 1964 using Geiger counters placed in Aerobee rockets blasted briefly into suborbital flight. In 1971, the Uhuru X-ray satellite confirmed the object was a strong X-ray source and noticed it varied strongly over just milliseconds. This suggested a violent acceleration of matter, which is needed to produce X-rays, happened over a very tiny region, which is why the variability was so quick.
In time, astronomers found the X-rays appeared to emanate from the 9th-magnitude blue star HDE 226868 near the bright star eta Cygni along the long axis of the Northern Cross. Blue stars don’t emit X-rays unless there’s another star in a close orbit to suck in the star’s gas at high. High-speed gas swirling around the black hole creates the X-rays (see image at top). The companion of the blue star had to be massive, at least 10 solar masses and maybe more, far too massive to be a neutron star. And the mass had to be packed into a region less than 100 miles across, so the object had to be amazingly dense!
The only candidate for such an object was a black hole. Until the 1970’s, a black hole was just a theoretical concept predicted by Einstein’s theory of general relativity and studied mathematically in the 1960’s by eminent scientists like John Wheeler and Stephen Hawking. Black holes are produced, the theory goes, when the core of a massive star collapses under its own weight and shrinks down to a single point. No one knew if black holes existed or whether they could be detected, since the gravity of a black hole was so powerful even light cannot escape.
So was Cygnus X-1 really the first-known example of a black hole?
While observational astronomers collected more evidence, Stephen Hawking and the great American physicist Kip Thorne placed a good-natured bet. Thorne was convinced the object was a black hole, and if he was proved correct, Hawking promised to buy Thorne a year’s subscription to Penthouse magazine. If the object proved not to be a black hole, then Thorne would buy Hawking a four-year subscription of the satirical magazine Private Eye.
Hawking, who famously studied the physics of black holes without knowing if they existed, placed the bet as an “insurance policy”. If black holes did not exist, he reasoned, then his life’s work was worth little. But at least he’d get a magazine subscription. Thorne’s motivation was more straightforward.
By 1990, the evidence was overwhelming that Cygnus X-1 was indeed a black hole. It was first of many to be discovered over the next two decades. Hawking happily conceded the bet and paid up, much to the dismay of Thorne’s wife.
To find the star HDE 226868, the 9th-magnitude companion star of Cygnus X-1, look to the Northern Cross asterism of Cygnus. Find the 3rd-magnitude star eta Cygni. Use binoculars or a small scope at low power to look just half a degree to the northeast for a tight pair of faint stars (see maps above). The black hole known as Cygnus X-1 revolves around the southern star of this tight pair.