Cygnus X-1 Is Indeed a Black Hole; Stephen Hawking Was Wrong

Stephen Hawking has been proved wrong, but he knew this was coming. The irony is that a team from Harvard Smithsonian for Astrophysics from Cambridge proved him wrong; Stephen Hawking is the Lucasian Professor of Mathematics at the University of Cambridge.

The famous wager

The story harks back four decades. In 1971, cosmologists from across the world said that Cygnus X-1, a strong X-rays source, was in fact a black hole. (You know what a black hole is, right?) 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. Everyone believed that it was a black hole and all evidence pointed to that. Stephen Hawking disagreed.

Both smiling: Stephen Hawking (left) And Kip Thorne (right)

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 high to account for that. What was the bet? Hawking described it 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.

Hawking enjoying weightlessness

Hawking conceded defeat in 1998.

Getting a bit more serious: The modern perspective

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 harder to study than radio waves and, fortunately, Cygnus X-1 is also a strong radio wave emitter. This is a common feature in many compact objects. They are generally bright in both the X-ray and radio frequencies.

Cygnus X-1

The Smithsonian team, led by Mark Reid, took to the Very Large Array (VLA) Very Large Baseline Array (VLBA)  radio telescope, which is scattered 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. (If you’re not into astronomy, you’ll probably not be able to appreciate the fact that this is really a small margin of error.)

The mass of Cygnus X-1’s dark star is 14.8 solar masses and the orbiting blue star, slowly getting its mass torn 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. It must be a black hole.

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 really fast. They even put a number on it – 670 revolutions per second, or at 50 % the speed of light!!

The final words

The findings of the team are not reported in any paper as yet, but the Astrophysical Journal has acknowledged receiving three papers on this work. I’d imagine that both Kip Thorne and Stephen Hawking are happy – Thorne for being proved right and Hawking for being proved wrong.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.