As physicist Stephen Hawking said in the Brief Answers to the Big Questions, "Fact is sometimes stranger than fiction, and nowhere is that more true than in the case of black holes. Black holes are stranger than anything dreamed up by science-fiction writers."
Chinese astronomers have discovered a stellar black hole. On November 28, 2019, the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) announced in Beijing that Liu Jifeng and Zhang Haotong's Research Group has discovered a supermassive stellar black hole by means of the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST, also known as Guo Shoujing Telescope), the major national science and technology infrastructure independently developed by China. The black hole has a mass of about 70 times that of the sun, far exceeding the upper limit of the theoretical prediction, and can be called the King of Stellar Black Holes.
In the early hours of the same day, a paper on this achievement was published online in Nature, an internationally renowned academic journal.
Where are hundreds of millions of black holes?
A black hole is a small but supermassive astronomical object, like a swallowing mouth in the universe. After Einstein proposed general relativity in 1915, the German physicist Karl Schwarzschild deduced the exact solutions of Einstein's field equations, showing the existence of black holes.
Since then, human beings have never stopped imagining and exploring mysterious black holes.
In 2015, gravitational waves were detected by astronomers for the first time, providing more specific evidence for the existence of black holes. In 2019, astronomers captured the first-ever image of the black hole at eight observation sites across four continents — the first visual evidence of the black hole over the past 10 years, giving human beings a glimpse of the once invisible astronomical object.
Why do black holes fascinate astronomers?
According to Liu Jifeng, Deputy Director of NAOC and Executive Vice President of the School of Astronomy and Space Science, University of Chinese Academy of Sciences, one of the major features of black holes is their extremely high density. How dense a black hole can be? When a star 10 times the mass of the sun is compressed into a sphere with the diameter equivalent to the diameter of the sixth ring road of Beijing, the density is equivalent to the density of a black hole. As a result, black holes have so strong gravitational attraction that not even light can escape their grasp.
According to the masses of black holes, black holes are roughly classified into three types: stellar black holes with the mass of less than 100 times that of the sun, intermediate-mass black holes with the mass of 100-100,000 times that of the sun, and supermassive black holes with the mass of more than 100,000 times that of the sun.
Stellar black holes are formed from the gravitational collapse of massive stars, peppered throughout the universe. According to theoretical predictions, there should be hundreds of millions of stellar black holes roaming in the galaxy, but only about 20 have been discovered in the past few decades.
"Where are the remaining hundreds of millions of black holes?" Liu said that these black holes themselves do not give off any light, so it is not easy for astronomers to discover them in the vast universe.
Liu drew an analogy: Although a black hole itself does not give off any light, when it forms a binary system with a normal star, it swallows up the gases on the star's companion stars and forms an accretion disk that emits bright X-rays. These X-rays are like the backlight of these gases before they are swallowed up by the black hole. It is the backlight that has become a powerful clue for astronomers to search for black holes.
Previously, almost all stellar black holes in the galaxy were identified by the X-rays emitted from the gases when these black holes accreted the gases on their companion stars. In the past 50 years, scientists have used this method to discover about 20 black holes with the masses of 3 ~ 20 times the mass of the sun.
"However, when a black hole is far away from its companion star, it behaves quietly and gently. How to search for these calm and gentle black holes?" said Liu.
Chinese astronomers gave their answers in the process of discovering the King of Black Holes.
A great star
At the beginning of 2016, Zhang Haotong, a research fellow of the NAOC and Director of the LAMOST Scientific Survey Department, and Han Zhanwen, an academician of the Chinese Academy of Sciences, proposed the program of observing the binary spectrum and researching the binary system with the LAMOST, and selected more than 3,000 astronomical objects for more than 700 days of spectral monitoring.
Among these astronomical objects, a great type-B star attracted the research fellows' attention, since this blue star always moves periodically around an invisible astronomical object. The unusual spectral features suggest that the invisible astronomical object is most likely a black hole.
According to Zheng Chuanjie, a member of Liu Jifeng's Research Group and an undergraduate student of the UCAS enrolled in 2014, the B-type star spectrum taken by the LAMOST carries abundant information. In addition to important information such as its effective temperature, surface gravity and metal abundance, the spectrum also has an almost stationary bright line, making itself more mystery.
"Is there a story behind this B-type star? Is it really moving around an invisible black hole?" said Zheng.
To get to the bottom of this particular star, the Chinese research group immediately confirmed the spectral properties of the binary system through 21 observations by the Spanish 10.4-meter Gran Telescopio Canarias (GTC) and 7 high-resolution observations by the 10-meter Keck telescope in the U.S. The B-type star has a mass of about 8 times that of the sun and is 14,000 light-years from earth, while the invisible astronomical object has a mass of about 70 times that of the sun.
"An astronomical object with such a large mass cannot be anything but a black hole." said Liu Jifeng.
That's how the black hole behind the B-type star was discovered by astronomers. Liu said that the group was initially surprised at this result and could not believe it. The reviewer of Nature was also very cautious, asking many questions and repeatedly confirming with the Chinese research group. "In the end, it is the result, which means that the theory of how stars evolve to form black holes will probably be rewritten, or there is some kind of black hole formation mechanism that has been ignored."
"Without the casting of a wide net in the vast sea of stars two years ago, this protagonist would not have emerged today." According to Bai Zhongrui, a senior engineer at NAOC, in memory of the LAMOST's contribution to the discovery of the giant stellar black hole, astronomers named the binary system containing the black hole LB-1.
Unlike other known stellar black holes, LB-1 has never been detected in any X-ray observation, and has very weak accretion on its companion star. It is therefore deemed to be the quiet and gentle stellar black hole champion.
Stepping into the no-go area of the theory that overturns the perception
Indeed, the international astronomy community has never stopped capturing black holes.
Since 2015, the Laser Interference Gravitational Wave Observatory (LIGO) has discovered black holes that are dozens of times the mass of the sun by detecting gravitational waves. In 2017, the three main members of LIGO, Reina·Weiss, Kip·Stephen·Thorne, and Barry·Clark·Barish, were awarded the Nobel Prize in Physics for their contributions to the development of LIGO and the detection of gravitational waves.
However, the current mainstream stellar evolutionary model predicts that only black holes up to 25 times the mass of the sun can be formed at solar metal abundance. Today, the discovery of the black hole, 70 times the mass of the sun, marks a step into the no-go area of the theory, overturning the perception of stellar black hole formation.
Commenting on the Chinese achievement, David·Reitz, a Nobel Prize winner and Director of LIGO, said: "This extraordinary achievement will drive astronomers to rewrite the formation model of stellar-mass black holes and revitalize the astrophysical research of black holes."
It is worth mentioning that the discovery of the king of stellar black holes has once again demonstrated LAMOST's powerful capability of spectral acquisition.
According to Zhang Haotong, LAMOST has 4,000 optical fibers, equivalent to 4,000 eyes, which can observe up to 4,000 astronomical objects simultaneously. In March this year, LAMOST released 11.25 million spectra, making itself the telescope to undertake the world's first spectral survey project to obtain more than 10 million spectra. It has therefore been hailed by astronomers as the king of spectra with the highest spectral acquisition rate in the world.
Liu said that this heaviest stellar black hole so far is the first black hole discovered by LAMOST, marking the beginning of a new era of searching for black holes with LAMOST.
According to Liu, the Chinese research group will launch a black hole hunter program, which is expected to discover and measure nearly 100 black holes and capture an image of the black hole population in the next five years.
Liu said that astronomers are expected to discover a number of deeply hidden quiet black holes, gradually unveiling the family of black holes, and ushering in a new era of bulk discovery of black holes.
Editor's note: The co-authors of the major achievement also include five students of UCAS: undergraduates Zheng Chuanjie enrolled in 2014 and Mou Xiaoyong enrolled in 2015, and doctoral students Cui Kaiming, Niu Zeqian and Wang Yilun.
