Perhaps nothing symbolizes the terror and wonder of space more than a black hole.
The first-ever photo of a black hole, taken in 2019, offered a glimpse of a dark center encased in a fiery ring that can bend space and time with its incredible gravitational pull. Our knowledge of these strange beings is constantly evolving— the black hole from the 2019 photograph, for example, has since been found to be wobbly.
In a study published Thursday in the journal Science, scientists have added another twist to this tale. A new measurement of a specific kind of black hole reveals it is so massive it calls into question fundamental aspects of stellar evolution.
Here’s the background — When dying stars collapse, they can form black holes. After they are created, black holes continue to gain mass from their environment. As a result, the size of a black hole can give scientists an indication of the mass of its original star, but it isn't an exact measure.
Earth’s Sun rules over our Solar System all by its lonesome. But there are also binary star systems out in the cosmos, in which two stars orbit around a shared center of mass. If a black hole interacts with one of a binary system’s stars, then the star emits X-rays. These X-rays, in turn, can form radio jets. Radio jets emit powerful radio waves — powerful enough for scientists to detect them.
Radio jet observations are crucial to the scientific study of black holes — including imposing limits on what a black hole can, and cannot, be characterized by. From these measurements, for example, scientists thought black holes in binary systems do not have a mass larger than 20 solar masses. A single solar mass, for context, is roughly equivalent to the mass of the Sun, and 330,000 times the mass of Earth.
The most massive binary black holes observed had clocked in between 15 and 17 solar masses — but this wasn't the full picture. Scientists weren’t ready for Cygnus X-1. This binary star system has a black hole with a mass of 21 solar masses, according to the new study.
Digging into the details — In the world of astronomy, Cygnus X-1 is about as famous as a binary star system can be. First discovered via observations of X-rays emanating from the system in the 1960s, in 1973 it became the first scientifically accepted proof of a black hole. The system entered popular culture, too, when Stephen Hawking revealed in his book, A Brief History of Time, he once placed a bet with a fellow astronomer over the possibility of black holes existing in this region of space. Hawking bet against the idea, and (eventually) conceded defeat.
James Miller-Jones is the Science Director of the Curtin Institute of Radio Astronomy in Perth, Australia and a co-author of the study. He tells Inverse that when a binary star system becomes one star and one black hole, determining their sizes can be achieved by looking at “how the optical light from the companion star changes as it moves around its orbit.”
Previous studies “measured the distance to Cygnus X-1 by measuring its apparent shift against distant objects when viewed from different vantage points in the Earth’s orbit around the Sun,” Miller-Jones says.
While this earlier work was “groundbreaking,” Miller-Jones says, it “did not fully sample the black hole’s orbit around its companion star.”
What’s new — Miller-Jones and his team were able to go further than these past studies by using the Very Long Baseline Array, a network of 10 radio telescopes located across the United States.
Using data from the array, the team realized the black hole in Cygnus X-1 was positionally shifting as it moved around its orbit. Through a full sampling of the orbit over six days, Miller-Jones explains the team was able to “correct for these orbital effects, and measure a more accurate distance, and hence black hole mass” than was previously possible.
Why it matters — To understand why this new mass information is so crucial, we can look to our own galaxy, the Milky Way. In our galactic home, generations of stars have synthesized heavy elements, which in turn allow today’s stars to “drive more powerful winds than was possible for earlier generations” of stars, Miller-Jones explains.
These powerful winds should impose restrictions on black holes' masses — basically, the black hole could only steal so much mass from their companion star. But Cygnus-X1 is not playing by those rules. It has “forced us to revise downwards our estimates of how much material the most massive stars lose in winds,” Miller-Jones says.
Incredibly, the black hole at the center of this study is actually just a little longer than 73 miles in diameter.
As Miller-Jones says: “combined mass of the two objects is over 60 times the mass of the Sun, all contained in a region of space less than a quarter of the distance between the Earth and the Sun.”
Sure, it has a mass of about 7 million times that of Earth, but “it would take you an hour to drive across it!”
What’s next — Now that Cygnus X-1 has the title of “most massive electromagnetically detected stellar-mass black hole currently known,” the big question is what is next for the binary star system?
Both its star and black hole spin around each other in a rapid 5.6 day orbit. But one day, the star will die out. What happens then is unknown, but the researchers note in the study the system's orbit suggests the two members of the system will not one day become a merged, far larger black hole. As the team write in their paper, they do not “expect Cygnus x-1 to undergo a binary black hole merger in a timescale equal to the age of the Universe.”
Abstract: The evolution of massive stars is influenced by the mass lost to stellar winds over their lifetimes. These winds limit the masses of the stellar remnants (such as black holes) that the stars ultimately produce. We use radio astrometry to refine the distance to the black hole X-ray binary Cygnus X-1, which we find to be 2.22 kiloparsecs. When combined with archival optical data, this implies a black hole mass of 21.2 ± 2.2 solar masses, higher than previous measurements. The formation of such a high-mass black hole in a high metallicity system (within the Milky Way) constrains wind mass loss from massive stars.