The first black hole ever discovered still has some surprises in store.
New observations of the black hole star pair Cygnus X-1 show that the black hole weighs about 21 times as much as the Sun – almost 1.5 times heavier than previous estimates. In the updated mass, astronomers are rethinking how some black hole-forming stars evolve. For a star-sized or stellar black hole so massive in the Milky Way to exist, its parent star must have lost less mass than expected due to stellar winds, researchers reported online on February 18 science.
It is important to know how much mass stars lose in the course of their life to stellar winds in order to understand how these stars enrich their surroundings with heavy elements. It is also important to understand the masses and compositions of these stars as they explode and leave black holes.
Cygnus X-1’s updated mass measurement is “a big change from an old favorite,” says Tana Joseph, an astronomer at the University of Amsterdam who is not involved in the work. Stephen Hawking bet physicist Kip Thorne that the Cygnus X-1 system, discovered in 1964, did not contain a black hole – and conceded the bet in 1990 when scientists had generally accepted that Cygnus X-1 contained the first known black hole in the universe (SN: 4/10/19).
Astronomers got a fresh look at Cygnus X-1 with the Very Long Baseline Array (VLBA). This network of 10 radio dishes spans the United States from Hawaii to the Virgin Islands and together forms a continent-sized radio dish. In 2016, the VLBA tracked beams of radioactive material bubbling from the black hole of Cygnus X-1 for six days (the time it took for the black hole and its companion star to orbit one another). These observations provided a clear picture of how the position of the black hole in space has shifted during its orbit. This in turn helped the researchers refine the estimated distance to Cygnus X-1.
The 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 in Cygnus X-1 is even brighter and therefore larger than astronomers thought. According to estimates by the researchers, the star weighs around 40.6 suns. The black hole also has to be more massive to explain its gravitational tug on such a massive star. The black hole weighs about 21.2 suns – much heavier than its previously estimated 14.8 solar masses, the scientists say.
The new mass measurement for the Cygnus X-1 black hole is so large that astronomers’ understanding of the massive stars collapsing into black holes is being questioned, says study co-author Ilya Mandel, an astrophysicist at Monash University in Melbourne, Australia.
“Sometimes stars are born with fairly high masses – there have been observations that stars are born with masses well over 100 solar masses,” says Mandel. However, it is believed that such giant stars lose much of their weight in stellar winds before turning into black holes. The larger the star and the heavier the elements, the stronger its stellar winds are. In galaxies rich in heavy elements such as the Milky Way, large stars – regardless of their initial mass – are said to shrink to around 15 solar masses before falling into black holes.
Cygnus X-1’s 21 solar mass black hole undermines this idea.
The gravitational wave detectors LIGO and Virgo have discovered black holes with a weight of several tens of solar masses in other galaxies (SN: 01/21/21). But that’s likely because LIGO is looking at distant galaxies that existed earlier in the universe, says Joseph. At that time there were less heavy elements, so the stellar winds were weaker. With the new Cygnus X-1 measurement: “Now we have to say, wait a minute, we’re in one [heavy element]- A rich environment compared to the early universe … but we still managed to create this really massive black hole, “she says.” Maybe we don’t lose as much mass from stellar winds as we originally thought. “