Weekend Feature: The Cygnus X-1 –The Black Hole that Lost Stephen Hawking a Bet



Stephen Hawking lost a big bet: Several thousand light-years from Earth, near the "heart" of Cygnus, the swan, two stars are locked in their gravitational destiny. One star is a blue supergiant, known as HDE 226868 –about 30 times as massive as the Sun and 400,000 times brighter. The other star is 5 to 10 times the mass of the Sun, but it's extremely small. But its mass is too great to be a white dwarf or a neutron star. Recently astronomers have proven its existence as a black hole.

The system is called Cygnus X-1, indicating it was the first source of X-rays discovered in the constellation Cygnus. Discovered by the Uhuru X-ray satellite in the early 1970s, it was also one of the first suspected black holes.  Since its discovery 45 years ago, Cygnus X-1 has been one of the most intensively studied cosmic X-ray sources. About a decade after its discovery, Cygnus X-1 secured a place in the history of astronomy when a combination of X-ray and optical observations led to the conclusion that it was a black hole, the first such identification.

In 1971, cosmologists from across the world said that Cygnus X-1, a strong X-rays source, was in fact a black hole. The intensity of the X-ray emissions was off the charts, given its estimated distance of 6050 light years. Cygnus was soon realized to be a double star system – a dark star and a blue star orbiting one another.

Cygnus X-1, since then, has been an object of intensive studies for astrophysicists all around the globe. Most experts believed that it was a black hole and all evidence pointed to that.

Stephen Hawking thought otherwise. In 1974, Hawking and Caltech astrophysicist Kip Thorne made a friendly wager. Hawking claimed that the compact object emitting the X-rays was a neutron star, in spite of evidence that the intensity was too much to account for that way. Hawking described the famous wager in his record shattering best seller, A Brief History of Time:

"This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist. But in that case, I would have the consolation of winning my bet, which would win me four years of the magazine Private Eye. If black holes do exist, Kip will get one year of Penthouse. When we made the bet in 1975, we were 80% certain that Cygnus was a black hole. By now [1988], I would say that we are about 95% certain, but the bet has yet to be settled."

Astrophysicists from Harvard-Smithsonian Institute measured the distance and the mass of the stars using direct methods. The reason is simple. If we know the radiation intensity we receive from a star in a certain small band of the electromagnetic spectrum, then, by measuring its distance and mass, we can figure out how powerful a source the star is. However, X-rays are much more difficult to study than radio waves and, fortunately, Cygnus X-1 is also a strong radio wave emitter.

The Smithsonian team, led by Mark Reid, took to the Very Large Array (VLA) radio telescope, scattered around from Hawaii to New England, and focused it on Cygnus X-1. The resolution was a hundred times better than Hubble and was crucial in measuring the distance using the parallax method. The distance was pegged at 6050 light years, give or take 400 light years.

The mass of Cygnus X-1’s dark star is 14.8 solar masses and the orbiting blue star, slowly getting its mass ripped apart by the compact dark star, weighs in at a heavier 19 solar masses. This is way above the mass required for a compact object to become a black hole – it is much too heavy to remain a neutron star. 

The team further measured the orbital speed (the spin) of the gas falling into the star. Measuring the temperature of the gas, using radiation emission data, the team found that it is so hot the innermost gas must be spinning at 670 revolutions per second, or at 50 % the speed of light.

The Daily Galaxy via chandra.harvard.edu and techie-buzz.com

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