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| Pulling material from a massive blue companion star is the black hole Cygnus X-1. An accretion disc around the black hole is created by that "stuff". (Image credit: NASA/CXC) |
The first
black hole ever found, six decades after its discovery, is still causing
astronomers to scratch their heads. The celestial behemoth at the core of the
Cygnus X-1 system turns out to be 50 percent more massive than previously
believed, making it the heaviest black hole stellar-mass ever directly
observed.
An
international team of researchers estimates that the black hole is 21 times the
mass of our sun and that it spins faster than any other known black hole, based
on recent observations. The recalculated weight leads scientists to reconsider
the evolution of bright stars turning into black holes and how easily they lose
their skin before they die.
A black
hole's mass depends on its parent star's properties, such as the mass of the
star and its metallicity (how much of it is made up of elements heavier than
helium). It sheds its outer layers by blasts of stellar winds over a star's
lifespan. Scientists think that larger stars rich in heavy elements shed their
mass faster than smaller stars with less metallicity.
Stars, by
stellar winds that blow away from their surface, lose mass to their surrounding
atmosphere. But we need to turn down the amount of mass that bright stars lose
over their lifetimes in order to make a black hole this heavy and spinning so
fast, research co-author Ilya Mandel, an astrophysicist said in a statement.
Distance
matters
In the new
research, scientists measured the mass of Cygnus X-1 using a tried-and-tested
approach called parallax to calculate the distances of stars from Earth. As
Earth orbits the sun, astronomers measure the apparent motion of stars relative
to the backdrop of more distant stars, and they can use that motion to
determine the distance of the star from Earth with a bit of trigonometry.
Moreover,
the black hole of Cygnus X-1 is slowly devouring its bright blue companion star
by pulling in the outer layers of that star, creating a bright disc revolving
around the black hole. It gets heated to millions of degrees and releases
brilliant X-ray radiation as the matter falls into the black hole. The black
hole barely avoids some of this matter and is spit out in strong jets that emit
radio waves observable on Earth.
Using
observations from the Very Long Baseline Array (VLBA) , a continent-size
network of 10 radio telescopes distributed throughout the United States,
ranging from Hawaii to the Virgin Islands, the research team monitored these
signature bright jets. They observed the complete orbit of the black hole
around its companion star over a period of six days and calculated how the
black hole moved in space.
They
discovered that Cygnus X-1 is roughly 7,200 light-years from Earth, surpassing
the previous 6,000 light-year estimate. The revised distance shows that the
blue supergiant companion star, 40 times more massive than our sun, is brighter
and more massive than previously assumed. And they were able to give a new
estimate of the mass of the black hole, a whopping 21 solar masses, given the
orbital length of the black hole.
Using the
revised measurements for the mass of the black hole and its distance from Earth,
we were able to confirm that Cygnus X-1 spins extremely fast, very close to the
speed of light and faster than any other black hole discovered to date, the
study co-author Lijun Gou, a researcher at (NAOC) National Astronomical
Observatories, said in the release.
The
discovery is a testament to how advances in telescope sensitivity and accuracy
can uncover mysteries in even some of our universe's most studied pieces.
As the
next generation of telescopes comes online, their enhanced sensitivity shows
the universe in increasingly more detail, said in a statement study co-author
Xueshan Zhao, a NAOC researcher. Becoming an astronomer is a great time.
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