Piecing Together a Black Hole

1971: Uhuru

Many of the discoveries made by modern science come not as "Eureka!" moments, but as the culmination of many observations, models, and discussions. Such was the case with Cygnus X-1, a binary system that was the first object identified as a likely stellar-mass black hole.

It was first seen as a source of X-rays by a small rocket launched from White Sands Missile Range, New Mexico, in June 1964. (Earth's atmosphere absorbs X-rays, so the only way to see astronomical X-rays source is to loft a telescope to a great altitude.) The rocket's scientific instruments could identify points of X-ray energy but couldn't see any details, so the nature of the object remained a mystery.

In 1971, though, the first dedicated X-ray satellite, Uhuru, discovered that the X-ray energy from Cygnus X-1 flickered quickly, indicating that the object was quite small. Follow-up observations from the ground found that its X-ray light varied over as little as a thousandth of a second.

Uhuru also provided a more precise location for the object, allowing astronomers to search the area for sources of both visible light and radio waves. They discovered both in 1971, and found that the X-rays were coming from a binary system containing a blue supergiant star and a tiny but heavy companion.

Map of the X-ray objects seen by Uhuru, with Cygnus X-1 highlighted [NASA]

The fast X-ray flickering indicated that the X-ray emitting object was only about 40 miles (60 km) in diameter, while the brightness of the X-rays indicated that the entire object was flickering, not just a spot on its surface. And from the radio and optical observations, astronomers found that the small companion was at least six times as massive as the Sun, making it too heavy to be a neutron star. From all of these bits of information, the conclusion became clear: the small companion in the Cygnus X-1 system must be a black hole that is stealing gas from the surface of the supergiant.

The X-rays come from a disk of gas that is spiraling into the black hole. As the two stars orbit each other once every 5.6 days, the black hole's gravitational pull causes the blue supergiant to "bulge" toward it. In profile, the supergiant would resemble an egg, with the small end aimed at the black hole. But this egg doesn't have an edge. Instead, hot gas flows away from the star toward the black hole. The gas forms a wide, flat accretion disk that encircles the black hole. Friction heats the gas to a billion degrees or more, causing it to emit a torrent of X-rays — enough to fry any living thing within millions of miles.

The X-rays flickers as gas enters the outer edge of the accretion disk then spirals closer to the star. Although a steady X-ray glow cuts off well outside the center of the disk, the central region occasionally flares as blobs of gas break off the inner edge and spiral into the black hole. These blobs are accelerated to a large fraction of the speed of light, so they circle the black hole hundreds of times per second, creating the flickering first seen by Uhuru.

Resources

Cygnus X-1 Fact Sheet

Black Holes

The Hundred Greatest Stars, by James Kaler, 2003.

Timeline