Key Takeaways:

  1. The first black hole ever detected, at the heart of the Cygnus X-1 system, is 50% more massive than previously thought.
  2. The black hole is estimated to be 21 times the mass of our sun and spinning faster than any other known black hole.
  3. The mass of a black hole depends on the properties of its parent star, including the star’s mass and its metallicity.
  4. Cygnus X-1’s black hole is slowly devouring its bright blue companion star, forming a bright disk rotating around the black hole.
  5. The discovery underscores how improvements in the sensitivity and accuracy of telescopes can unveil mysteries in even some of the most studied parts of our universe.
The black hole Cygnus X-1 is pulling material from a massive blue companion star. That "stuff" forms an accretion disk around the black hole.
The black hole Cygnus X-1 is pulling material from a massive blue companion star. That “stuff” forms an accretion disk around the black hole. (Image credit: NASA/CXC)

Six decades after its discovery, the first black hole ever detected continues to puzzle astronomers. The cosmic giant at the core of the Cygnus X-1 system turns out to be 50% more massive than previously believed, making it the heaviest stellar-mass black hole ever directly observed.

New observations by an international team of researchers estimate that the black hole is 21 times the mass of our sun and spins faster than any other known black hole. This recalculated mass is prompting scientists to reconsider how bright stars that transform into black holes evolve and how quickly they shed their outer layers before their demise.

The mass of a black hole is dependent on its parent star’s properties, such as the star’s mass and its metallicity (the proportion of it composed of elements heavier than helium). Throughout a star’s lifetime, it sheds its outer layers through stellar wind blasts. Scientists believe that larger stars rich in heavy elements shed their mass faster than smaller stars with lower metallicity.

“Stars lose mass to their surrounding environment through stellar winds that blow away from their surface. But to create a black hole this heavy and rotating so rapidly, we need to reduce the amount of mass that bright stars lose during their lifetimes,” said study co-author Ilya Mandel, an astrophysicist from Australia’s Monash University.

In this new study, researchers estimated Cygnus X-1’s mass using a well-established method for measuring star distances from Earth, known as parallax. As Earth orbits the sun, astronomers measure the visible movement of stars relative to more distant stars’ background. With some trigonometry, they can use this movement to calculate a star’s distance from Earth.

Furthermore, Cygnus X-1’s black hole is gradually consuming its bright blue companion star by drawing in the star’s outer layers, forming a luminous disk rotating around the black hole. As matter falls into the black hole, it heats up to millions of degrees and emits brilliant X-ray radiation. Some of this matter narrowly escapes the black hole and is ejected in powerful jets emitting radio waves detectable on Earth.

The research team tracked these signature bright jets using observations from the Very Long Baseline Array (VLBA), a continent-sized network of 10 radio telescopes spread across the United States. Over six days, they followed the black hole’s full orbit around its companion star and determined how much space the black hole shifted.

They discovered that Cygnus X-1 is approximately 7,200 light-years from Earth, surpassing the previous estimate of 6,000 light-years. The updated distance suggests that the blue supergiant companion star is brighter and more massive than previously thought, at 40 times more massive than our sun. Given the black hole’s orbital period, they were able to provide a new estimate for the black hole’s mass — an astonishing 21 solar masses.

“Using updated measurements for the black hole’s mass and its distance from Earth, we confirmed that Cygnus X-1 is spinning incredibly quickly — very close to light speed and faster than any other known black hole,” said study co-author Lijun Gou, a researcher at China’s National Astronomical Observatories (NAOC).

This discovery highlights how advancements in telescopes’ sensitivity and accuracy can reveal mysteries in even some of our universe’s most studied regions.

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