In the constellation Cygnus
21 times the mass of the Sun
Diameter 78 miles (126 km), equal to the size of a large American county
Several thousand light-years away, near the "heart" of Cygnus, the swan, two stars are locked in a gravitational embrace. One star is a blue supergiant, known as HDE 226868. It is more than 40 times as massive as the Sun and 300,000 times brighter. The other star is more than 20 times the mass of the Sun, but it's extremely small. The object must be the collapsed core of a star. Its mass is too great to be a white dwarf or a neutron star, though, so it must be a black hole -- the corpse of a star that once resembled the supergiant.
The system is called Cygnus X-1, indicating it was the first source of X-rays discovered in the constellation Cygnus. Discovered by a small rocket launched from New Mexico in 1964 and studied extensively by the Uhuru X-ray satellite in 1971, it was the first suspected black hole.
The X-rays come from a disk of gas that's spiraling into the black hole. As the two stars orbit each other once every 5.6 days, the black hole's gravitational pull causes the blue supergiant to "bulge" toward it. In profile, the supergiant would resemble an egg, with the small end aimed at the black hole. But this egg doesn't have a smooth edge. Instead, hot gas flows away from the star toward the black hole. The gas forms a wide, flat accretion disk that encircles the black hole. Friction heats the gas to a billion degrees or more, causing it to emit a torrent of X-rays -- enough to fry any living thing within millions of miles.
But the X-ray glow isn't steady. Instead, it flickers, which is one bit of evidence that identifies the dark member of the binary as a black hole. Gas enters the outer edge of the accretion disk then spirals closer to the star. If the center of the disk contained a normal star, or even a superdense neutron star, then the disk would get hotter and brighter all the way in to its center, with the brightest X-rays coming from the middle. Instead, the X-ray glow cuts off well outside the center of the disk. Observations with Hubble Space Telescope reveal that the central region occasionally flares up as blobs of gas break off the inner edge of the disk and spiral into the black hole.
These blobs are accelerated to a large fraction of the speed of light, so they circle the black hole hundreds of times per second. This causes the system's X-rays to "flicker." If the blobs of gas were orbiting a larger object, they would not move as fast, so their high-speed revolution is one bit of circumstantial evidence that identifies the dark companion as a black hole.
The black hole's strong gravitational field shifts the energy emitted by this gas to longer and longer wavelengths. As the gas approaches the event horizon the redshift becomes so great that the material disappears from view -- just before it spirals into the black hole.
In 2021, a team of astronomers produced the most complete dossier on Cygnus X-1 to date. The key to the team's findings was a precise measurement of the system's distance from Earth.
Using the Very-Large Baseline Array, a group of radio telescopes spread across several thousand miles of Earth's surface, the team measured the system's parallax more precisely than ever before. In essence, the astronomers measured the direction to the system when Earth was on different sides of the Sun, then calculated the angle between those two directions. (It's like holding your finger at arm's length and looking at it with first one eye, then the other. The finger's back-and-forth shift against the background of more-distant objects is its parallax.) The team compared VLBA measurements in 2020 to those made in 2011 to produce the most precise distance to date: 7,200 light-years.
With an accurate measurement of the distance, the team determined the brightness of the blue supergiant star, which led to a better determination of its mass. That, in turn, led to the best measurement of the black hole's mass to date -- 21 times the mass of the Sun. That makes it the heaviest stellar-mass black hole measured by its electromagnetic radiation to date. (Black holes up to a few times that mass have been discovered through the gravitational waves they produced as they merged with companion black holes.)
It is also one of the most rapidly rotating black holes yet discovered. It spins about 800 times per second, which means that a point at the equator of its event horizon is moving at close to the speed of light. This high-speed rotation drags the space around the black hole, creating a spacetime vortex.
The new measurements also helped the astronomers piece together the system's history.
The star that became the black hole probably was born about 60 times as massive as the Sun. Strong "winds" of hot gas from its surface blew away about two-thirds of that mass. As nuclear reactions in the star's core shut down, the star didn't explode as a supernova, as is the case with many supergiant stars. Instead, the entire star collapsed in a matter of hours, so the star simply vanished from view.
First Black Hole Ever Detected is More Massive Than We Thought (Chinese Academy of Sciences)
New observations reveal that Cygnus X-1 is more distant than thought, which makes it several times more massive.
Black Hole Forges Invisible Bubble (Space.com)
Astronomers may have to revise their notion of how much energy stellar-mass black holes put back into the space around them after discovering a huge invisible bubble of energetic gas surrounding the black hole known as Cygnus X-1.
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This document was last modified: February 19, 2021
No animations available for this black hole.