More than half a century after Cygnus X-1 helped convince astronomers that black holes really exist, this famous object is still full of surprises.
Better measurements have shown that the black hole is considerably more massive than astronomers once believed. It is also spinning at an extraordinary rate, making Cygnus X-1 one of the most interesting stellar-mass black holes ever studied.
Located thousands of light-years away in the constellation Cygnus, the system has played a central role in black hole astronomy for decades.
The Black Hole That Made History
Cygnus X-1 first attracted attention in 1964, when astronomers detected it as a powerful source of X-rays.
At the time, black holes were still largely theoretical objects. Scientists knew that Einstein's general theory of relativity allowed such objects to exist, but convincing observational evidence was another matter.
That began to change in the early 1970s.
Astronomers discovered that Cygnus X-1 was part of a binary system containing a massive visible star and an unseen companion. By studying the motion of the visible star, they found that its invisible partner appeared too massive and compact to be an ordinary star or neutron star.
The evidence pointed toward a black hole.
Cygnus X-1 consequently became the first object widely accepted as a stellar-mass black hole.
A Black Hole Heavier Than Expected
For years, astronomers estimated that the black hole in Cygnus X-1 had a mass of roughly 15 times that of the Sun.
More precise observations later changed that picture.
Researchers used radio observations to improve measurements of the system's distance and motion. Those results placed Cygnus X-1 roughly 7,000 light-years from Earth and led scientists to revise the black hole's mass upward to about 21 times the mass of the Sun.
That made it roughly 50 percent more massive than some earlier estimates.
The revision was more than just a change to one number. It also forced astronomers to reconsider how such a massive stellar black hole could have formed.
How Did It Become So Massive?
Stellar-mass black holes form from massive stars, but exactly how much mass a star loses before its collapse depends on several factors.
Powerful stellar winds can strip material away over the star's lifetime. If those winds are weaker than expected, the star can retain more of its mass and eventually leave behind a heavier black hole.
Cygnus X-1 therefore provides astronomers with a valuable test of models describing the lives and deaths of massive stars.
Understanding systems like this is also important for studying other heavy stellar black holes, including those detected through gravitational waves.
Cygnus X-1 Is Spinning Extremely Fast
Its mass isn't the only remarkable feature.
Measurements of radiation coming from material near the black hole indicate that Cygnus X-1 is rotating extremely rapidly.
Astronomers describe black hole rotation using a dimensionless quantity called the spin parameter. A non-rotating black hole has a value of zero, while a maximally rotating black hole approaches one.
Studies of Cygnus X-1 have placed its spin very close to that upper theoretical limit.
That makes it an example of what physicists call a rapidly rotating, or Kerr, black hole.
It is safer, however, to describe Cygnus X-1 as one of the fastest-spinning stellar-mass black holes measured rather than simply declaring it the fastest black hole known. Spin measurements depend on observational methods and assumptions about the system.
A Black Hole Feeding on a Giant Star
Cygnus X-1 is not alone.
Its companion is a massive blue supergiant star known as HDE 226868.
The two objects orbit one another closely, and the black hole captures some of the gas flowing away from its companion.
As that material falls toward the black hole, it forms an accretion disk. Friction and other processes heat the gas to extremely high temperatures, causing the region to shine strongly in X-rays.
Those X-rays are what first brought Cygnus X-1 to astronomers' attention.
The black hole itself remains invisible. What scientists observe is the behavior of matter and radiation around it, along with the gravitational influence it has on its companion star.
Hawking's Famous Bet
Cygnus X-1 even has a place in one of the most famous stories in modern physics.
In 1974, physicists Stephen Hawking and Kip Thorne made a bet over whether the system really contained a black hole.
Hawking bet against the idea, although he later explained that the wager gave him something of a consolation prize if his own work on black holes turned out to be wrong.
As observational evidence became increasingly convincing, Hawking eventually conceded the bet.
Today, there is overwhelming evidence that Cygnus X-1 contains a stellar-mass black hole.
Why Astronomers Still Study Cygnus X-1
Cygnus X-1 may be an old name in astronomy, but it is far from an outdated research target.
Modern X-ray, radio and other observations allow scientists to study how matter behaves under some of the most extreme gravitational conditions in the universe.
The system can help researchers investigate black hole spin, accretion disks, high-energy plasma, stellar evolution and the interaction between a black hole and a m
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