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The first black hole ever discovered is more massive than we thought

Cygnus X-1’s black hole was first discovered more than half a century ago. But it continues to teach us new things.
February 18, 2021
Cygnus X-1 black hole
An artist's illustration of the Cygnus X-1 black hole and star.International Centre for Radio Astronomy Research

Einstein first predicted the existence of black holes when he published his theory of general relativity in 1916, describing how gravity shapes the fabric of spacetime. But astronomers didn’t spot one until 1964, some 6,070 light-years away in the Cygnus constellation. Geiger counters launched into space detected cosmic x-rays coming from a region called Cygnus X-1. (We now know the cosmic rays are produced by black holes. Back then, scientists disagreed about what it was: Stephen Hawking famously bet physicist Kip Thorne that this signal was not from a black hole, but he conceded in 1990.)

Now, some 57 years later, scientists have learned that the black hole at Cygnus X-1 is much more massive than first believed—forcing us to once again rethink how black holes form and evolve. This time, the observations were taken from Earth’s surface.

“To some extent, the result was serendipitous,” says James Miller-Jones of the International Centre for Radio Astronomy Research at Curtin University in Australia, the lead author of the new study, published in Science. “We had not initially set out to remeasure the distance and black hole mass, but when we had analyzed our data, we realized its full potential.”

Black holes are objects so massive that not even light, let alone physical matter, is supposed to escape its gravitational pull. Yet sometimes one inexplicably spews jets of radiation and ionized matter into space. Miller-Jones and his team wanted to investigate how matter is sucked into and expelled from black holes, so they took a closer look at Cygnus X-1.

They observed the black hole for six days using the Very Long Baseline Array, a network of 10 radio telescopes sited across North America from Hawaii to the Virgin Islands. The resolution is comparable to what would be required to spot a 10-centimeter object on the moon, and it’s the same technique that the Event Horizon Telescope used to snap the first photo of a black hole

Using a combination of measurements involving radio waves and temperatures, the team modeled the precise orbits of both Cygnus X-1’s black hole and the massive supergiant star HDE 226868 (the two objects orbit each other). Knowing the orbits of each object allowed the team to extrapolate their masses—in the case of the black hole, 21 solar masses, which is about 50% more than once thought. 

The mass of black holes depends on a few factors, particularly the size of the star that collapsed into the black hole and the amount of mass that erodes away in the form of stellar wind. Hotter and brighter stars tend to produce more volatile stellar winds, and they also tend to be heavier. So the more massive a star is, the more prone it is to losing mass via stellar wind before and during its collapse, resulting in a lighter black hole. 

But in general, scientists thought stellar winds in the Milky Way were strong enough to limit the mass of black holes to no more than 15 solar masses, regardless of how big the stars were originally. The new findings clearly upend those estimates. 

“Finding a black hole that was significantly more massive than this limit tells us that we have to revise our models of how much mass the largest stars lose in stellar winds over their lifetimes,” says Miller-Jones. It may mean the stellar winds that move through the Milky Way are less powerful than we thought, or that stars hemorrhage mass in other ways. Or it could mean black holes behave in more erratic ways than we’re able to anticipate.

The team plans to follow up with more observations of Cygnus X-1. Other instruments, such as the planned Square Kilometer Array in Australia and South Africa, could provide better views of this and other nearby black holes. There could be anywhere from 10 million to a billion black holes in the Milky Way, and studying at least a few more of them might help clear up this mystery.

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