Black hole riddles: the more we find, the less we know

SourceKristof Wesely/Wikimedia Commons

New data on the object found 57 years ago in Cygnus X-1 forces us to review the models of mass loss by stellar winds

Einstein first predicted the existence of black holes when he published his theory of general relativity in 1916, in which he described how gravity shapes the fabric of space-time. But astronomers did not detect any until 1964, about 6,070 light-years away in the constellation Cygnus.

Geiger counters launched into space detected cosmic X-rays from a region called Cygnus X-1. (We now know that cosmic rays are produced by black holes. But, back then, scientists had no idea what it was. Stephen Hawking’s wager to physicist Kip Thorne even became famous that this signal did not come from a black hole, until he admitted his mistake in 1990).

Some 57 years later, scientists have discovered that the black hole in Cygnus X-1 is much larger than originally believed, forcing us to rethink, once again, how black holes form and develop. . This time, the observations were made from the surface of the Earth.

Researcher at the International Center for Radio Astronomy Research at Curtin University in Australia and lead author of the recent study, published in Science, James Miller-Jones admits: “To some extent, it was a serendipitous result. At first we had not set out to go back. to measure the distance and mass of the black hole, but after analyzing the data, we realized its full potential. “

Black holes are objects so massive that not even light is supposed to escape their gravitational pull, much less physical matter. However, they sometimes inexplicably eject jets of radiation and ionized matter into space. Miller-Jones and her team wanted to investigate how matter is absorbed and ejected from black holes, so they decided to take a closer look at Cygnus X-1.

They observed the black hole for six days using the network of 10 Very Long Baseline Array radio telescopes, located in North America from Hawaii to the Virgin Islands (both in the US).

Its resolution is comparable to what it would take to detect a 10-centimeter object on the Moon, and it is the same technique that the Event Horizon Telescope (EHT) used to take the first photo of a black hole. .

Using a combination of measurements involving radio waves and temperatures, the team modeled the precise orbits of 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, it was 21 solar masses, which is about 50% more than previously thought.

The mass of black holes depends on several factors, especially the size of the star that collapsed into the black hole and the amount of mass that is eroded away in the form of a stellar wind. The hottest and brightest stars tend to produce more volatile stellar winds and also tend to weigh more. So the more massive a star is, the more likely it is to lose mass through the stellar wind before and during its collapse, resulting in a clearer black hole.

But overall, scientists thought that the Milky Way’s stellar winds were strong enough to limit the mass of black holes to less than 15 solar masses, regardless of the initial size of the stars. The new findings clearly alter those estimates.

Miller-Jones confirms: “Finding a black hole significantly larger than this limit indicates that we need to review our models of how much mass the largest stars lose in stellar winds over their lifetime.”

The stellar winds moving through the Milky Way may be less powerful than we think, or the stars may lose mass in other ways. Or it could mean that black holes behave in a more erratic way than we can anticipate.

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