University of Toronto: The Bulletin.
November 10, 1997


Articles

The First Black Hole

Tom Bolton found it 25 years ago this week
By Bruce Rolston (art by Tim W. Kuzniar)

No known force can destroy it. Over time, it only grows larger as it swallows everything that comes close. Eating away at the known universe, it consumes mass, heat, light, structure, information: it is a one-way exit door out of our universe.

�It� is the peculiar form of collapsed star that scientists call a �black hole.� The gravitational pull of a black hole is so powerful that nothing, not even radiation � light, heat, energy � can escape it. Black holes are born in the death throes of the largest stars as the sheer weight of their huge masses collapses upon itself, punching through the very fabric of space.

Since light cannot escape a black hole, it follows that finding one would be rather difficult. Around 25 years ago, the first confirmed sighting was truly the astronomical prize. That is, until a 28 year-old U of T astronomer named Thomas Bolton stepped forward to say he had found an invisible mass that was eating away at a giant blue star in the constellation Cygnus.

Working at the David Dunlap Observatory�s 74-inch reflector, Canada�s largest telescope, Bolton attracted worldwide attention as the first astronomer to state without doubt that the X-ray source known as Cygnus X-1 was actually a black hole, circling a blue star known only as HDE 226868, 11,000 light years away from Earth. It may be the most significant stellar observation ever made in this country.

Bolton began work on his signal accomplishment in 1971. Theorists had suspected strong stellar X-ray sources might indicate stellar gases being sucked off a star into a nearby black hole but they had to depend on stargazers like Bolton, using optical telescopes, to actually search the area where the X-rays came from. Although the black hole itself would be invisible, Bolton knew he could detect the �wobble� of an orbiting star if it was circling around such an invisible companion. As he focused in on his chosen blue star in the constellation Cygnus through the autumn nights, Bolton realized this star was indeed wobbling around an incredibly massive but invisible partner.

It was a breathless time for the young astronomer: due to deliver a paper at the American Astronomical Society convention in Puerto Rico, he was still processing the data he�d received upon his arrival. �Five minutes before I gave my paper I was revising my paper on the fly. I was sitting at the back of the room trying to get the latest data in my diagram.�

Understandably, in his actual paper Bolton hadn�t claimed to have discovered the first black hole, yet. But to those attending his presentation at the San Juan conference, he fleshed out the implications: this object giving off X-rays that could well be the first black hole.

Bolton knew one thing his listeners may not have. He was not the only explorer in this hunt. Shortly before the conference he had seen a preprint of a paper for the journal Nature by the Royal Greenwich Observatory team of Louise Webster and Paul Murdin. Established astronomers, they had also spent the fall looking at Cygnus X-1, with a larger telescope than Bolton was using. But curiously they too had hedged in their conclusions: Cygnus X-1 could be a black hole, they wrote, but they couldn�t rule out other explanations based on their observations. It seemed no one was willing to stake their credibility yet: they had seen how other astronomers who had claimed they had seen black holes before had been proven wrong. The discovery would have to wait for another observing season.

Looking back Bolton admits he was naive to think he would be able to develop his observations at leisure. The idea of a race against other scientists never really entered his mind, he says. In the summer of 1972, Bolton was back at the telescope, gathering a clearer picture of Cygnus X-1, data point by data point. Calculations showed the dark companion was too massive to be anything but a black hole; Bolton even thought he could see faint emissions from stellar gas being sucked off the nearby blue star and accelerating to incredible speeds before suddenly vanishing.

And he had learned his lesson about competition. This year Bolton had a paper into the journal Nature Physical Science in the first week of November; it was published before the year was out. Cygnus X- 1 was a black hole, he wrote, without a doubt. He would stake his entire reputation on it.

Bolton worked and wrote alone. He didn�t have tenure. And what he claimed to have found would be such a strong piece of evidence for a whole line of astrophysical thinking dating back to Einstein, that it was inevitable his claim would be debated, criticized and challenged by others in the astronomical community. In the next year alone five major papers were published, probing for flaws in his conclusions. Everyone had an opinion, it seems. At the California Institute of Technology, leading American astrophysicist Kip Thorne bet renowned black hole theorist Stephen Hawking a subscription to their favourite magazines that Bolton was right. (Hawking, who later conceded the wager, actually believed Bolton had found a black hole too but said that if black holes, the subject of his life�s work, were ever found to be a chimera, he at least wanted to have something to read afterwards.)

Bolton had had some indication of what was to come shortly after the Puerto Rico conference when he was invited to speak at Princeton University�s Institute for Advanced Study. There in a grand wood-lined hall, waiting to hear him, was �a sizable fraction of the biggest names in astronomy,� he recalls. Among his interrogators was the institute�s John Bahcall, later one of his strongest critics. �It was a bit intimidating but I survived,� Bolton recalls. �But by that time I was completely confident in my observations.�

Even Bahcall accepted that Bolton�s discovery, if not a black hole, was certainly something unique. Other scientists were more critical: another paper that year argued that Bolton and other observers had got things totally wrong, that all their measurements were skewed because they had muffed their distance calculations. Although Bolton maintains, �I don�t have any emotional stake in Cygnus X-1 being a black hole,� that criticism in particular still irks him, 25 years later. �That paper was such a piece of.... There was no basis for making such an argument.�

Nothing is ever certain in stellar astronomy. Like so much of science it is built on a complex framework of theories and assumptions, stacked more or less precariously on top of each other. By the end of 1973 it was generally conceded that Cygnus X-1, barring the discovery of contradictory evidence, was a black hole. Bolton�s observation became another piece in the framework and the astronomical debate moved on. Star explorers, unlike those on Earth, are easily forgotten, it seems � perhaps because they never have an opportunity to plant a flag.

Bolton continues to work at the David Dunlap Observatory and plans to return to teaching at U of T after a recent leave. In the two-and-a-half decades since the heady days of the early 1970s, he has accomplished much, not all of it printed in academic journals. He was a key figure in the fight to pass bylaws in his home town of Richmond Hill to keep the night sky dark, which will greatly increase the Observatory�s useful life. He is involved with plans for a Canadian ultraviolet telescope. And he occasionally works with his lifelong partner in exploration: that 74-inch reflector. On some autumn nights you can still see them there, patiently mapping one more corner of the galaxy, looking for the significant, the beautiful and occasionally, just occasionally, the truly bizarre.


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