The first black hole discovered still has some surprises.
New observations of the black-star hole pair called Cygnus X-1 indicate that the black hole weighs about 21 times heavier than the sun, almost 1.5 times heavier than past estimates. The updated mass causes astronomers to rethink how some stars that form black holes evolve. For a black hole the size of a star or stellar to exist in the Milky Way, its parent star must have poured less mass through stellar winds than expected, researchers reported on Feb. 18 in Science.
Knowing how many massive stars you lose from stellar winds throughout your life is important to understanding how these stars enrich their environment with heavy elements. It is also key to understanding the masses and compositions of these stars when they explode and leave black holes behind.
The updated mass measurement of Cygnus X-1 is “a big change for an old favorite,” says Tana Joseph, an astronomer at the University of Amsterdam who is not involved in the work. Stephen Hawking bet on the famous physicist Kip Thorne that the Cygnus X-1 system, discovered in 1964, did not include a black hole and admitted the bet in 1990, when scientists accepted that Cygnus X-1 contained the first known black hole in the universe (SN: 10/10/19).
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Astronomers have taken a fresh look at Cygnus X-1 using the Very Long Baseline Array or VLBA. This network of 10 radio dishes extends across the United States, from Hawaii to the Virgin Islands, collectively forming a continental-sized radio dish. In 2016, the VLBA tracked jets of bright material that came out of the black hole of Cygnus X-1 for six days (the time it took the black hole and its companion star to orbit each other). These observations provided a clear view of how the position of the black hole in space changed along its orbit. This, in turn, helped researchers refine the estimated distance to Cygnus X-1.
New observations suggest that Cygnus X-1 is about 7,200 light-years from Earth, rather than the previous estimate of about 6,000 light-years. This implies that the star of Cygnus X-1 is even brighter and therefore larger than astronomers thought. The star weighs about 40.6 soles, according to researchers. The black hole must also be more massive to explain its gravitational pull on such a massive star. Black holes weigh about 21.2 suns, much heavier than the 14.8 solar masses previously estimated, scientists say.
The new mass measurement for Cygnus X-1's black hole is so large that it challenges astronomers' understanding of massive stars collapsing to form black holes, says study co-author Ilya Mandel, an astrophysicist at Monash University in Melbourne. , Australia.
"Sometimes stars are born with fairly high masses; there are observations that stars are born with masses of more than 100 solar masses," says Mandel. But these huge stars are thought to shed much of their weight through stellar winds before turning into black holes. The larger the star and the heavier elements it contains, the stronger its stellar winds. Thus, in galaxies rich in heavy elements such as the Milky Way, large stars, regardless of their initial mass, are supposed to be reduced to about 15 solar masses before collapsing into black holes.
Cygnus X-1's black solar mass hole 21 undermines that idea.
LIGO and Virgo gravitational wave detectors have discovered black holes that weigh dozens of solar masses in other galaxies (SN: 1/21/21). But that’s probably because LIGO observes distant galaxies that existed before in the universe, Joseph says. Then there were fewer heavy elements, so the stellar winds were weaker. With the new Cygnus X-1 measurement, “now we have to say, wait, we’re in a (heavy element) rich environment compared to the primitive universe … but we still manage to make this black hole really huge” she says, “so maybe not we lost as much mass from stellar winds as we initially thought. "