The black hole in the center of the Milky Way galaxy is 27000 light-years away from the earth. Its size is about the same as the doughnut on the moon from the earth. To image it, we must ensure that the telescope is sensitive enough and the details that can be distinguished are small enough, so as to ensure that we can "see" and "see clearly".
After the first black hole photo of mankind was released in 2019, another "true face" of black hole was photographed! On May 12, Beijing time, the Shanghai Observatory of the Chinese Academy of Sciences released the first picture of the black hole Sagittarius A (SGR a ) in the center of the Milky Way galaxy. Five other cities around the world (Brussels, Belgium, Santiago, Chile, Chinese Taipei, Tokyo, Japan, Washington, USA) also released this exciting photo with Shanghai.
This black hole photo is "photographed" by the cooperative organization of event horizon telescope (EHT) through the network of radio telescopes distributed all over the world. Its release gives an empirical evidence that Sagittarius A * in the center of the Milky Way galaxy is a black hole, which provides valuable clues for understanding this "giant beast" widely considered to reside in the center of most galaxies.
"Combination" global radio telescope takes pictures of black holes
Because the black hole does not emit light, we cannot see the black hole itself, but the luminous gas around the black hole gives a signal of its existence.
General relativity predicts that black holes exist, the surrounding space-time bends, and the gas is attracted to fall. As the gas falls to the black hole, gravitational energy is converted into light and heat, so the gas is heated to billions of degrees Celsius. A black hole is like being immersed in a bright area similar to luminous gas. It looks like a shadow surrounded by a new moon shaped halo caused by accretion or jet radiation.
"The completion of the imaging of black holes will provide direct visual evidence to prove the existence of black holes." Lu Rusen, a researcher at the Shanghai Observatory of the Chinese Academy of Sciences, said that the black hole in the center of the Milky Way galaxy is 27000 light-years away from the earth, and its size is about the same as the doughnut on the moon from the earth. To image it, we must ensure that the telescope is sensitive enough and the details that can be distinguished are small enough, so as to ensure that we can "see" and "see clearly".
In order to meet the above requirements, the aperture of the telescope needs to reach the length of the earth's diameter. However, at present, the maximum aperture of a single telescope on earth is only 500 meters. Without a telescope as big as the earth, how can we take a qualified picture of a black hole? Astronomers came up with a good idea - the combination of strong and strong. The EHT Cooperation Organization "combines" eight radio telescopes distributed in six geographical locations around the world to form a "virtual" telescope as large as the earth, with unprecedented sensitivity and resolution.
In 2017, the eight telescopes from Spain in the north and Antarctica in the South threw a "big net" at the selected targets and "fished" back massive data to outline the shape of black holes for us. But the observation window for scientists is very short, only about 10 days a year.
In addition to the limitation of observation time, the requirements of shooting black holes on weather conditions are also extremely harsh. "Because water in the atmosphere has a great impact on this observation band, water will affect the intensity of radio waves, which means that precipitation will interfere with the observation." Shen Zhiqiang, director and researcher of the Shanghai Observatory of the Chinese Academy of Sciences, said.
Therefore, the eight telescopes are located in areas with high altitude, little rainfall and very high probability of sunny days.
In addition, to successfully image, all telescopes must be fully synchronized in time. During the observation, each radio telescope collects and records radio wave signals from near the target black hole. These data are integrated to obtain images of the event horizon. "In order to ensure the stability of the signal, eht uses an atomic clock to ensure that the signals it collects and records are synchronized in time," Shen Zhiqiang said
Provide the first direct visual evidence for Silver Core black hole
Scientists have previously observed that many stars orbit an invisible, dense and massive object in the center of the Milky Way galaxy. This has strongly suggested that the object called Sagittarius A * is a black hole, and the released photos provide the first direct visual observation evidence.
In 2019, the first black hole photo released by mankind captured the central black hole M87 , the more distant galaxy M87. Although M87 is more than 1500 times larger and 1500 times heavier than the black hole Sagittarius A * in the center of the Milky Way galaxy, the two black holes look very similar.
"They come from two different types of galaxies and have very different black hole masses, but when we focus on the edges of the two black holes, they look magically similar." Professor Sarah Markov, a theoretical astrophysicist from the University of Amsterdam in the Netherlands and co chairman of the EHT Scientific Committee, said: "this tells us that objects close to the black hole are completely dominated by general relativity. The different appearances we see in the distance are caused by the differences in the matter around the black hole."
Although Sagittarius A is closer to the earth, taking pictures of it is much more difficult than M87 . From steward Observatory Chi Kwan Chan, a scientist at the Department of astronomy and the Institute of data science at the University of Arizona, explains: "The surrounding gas revolves around Sagittarius A and M87 at a speed almost close to the speed of light. It takes days to weeks for the gas to revolve around M87 but for a much smaller Sagittarius A the gas can revolve around it in a few minutes. This means that when eht observes Sagittarius A , the brightness and pattern of the gas around the supermassive black hole also change rapidly from time to time. Taking a picture of Sagittarius A is a bit like taking a positive animal Take a clear picture of the dog chasing its tail. "
The realization of this achievement has gathered the wonderful ideas of more than 300 researchers from 80 research institutions around the world. In addition to developing complex tools to overcome the challenges faced by Sagittarius A * imaging, eht cooperation also spent five years synthesizing and analyzing data with supercomputers, compiling an unprecedented black hole simulation database and strictly comparing the observation results.
Eht will make a black hole "movie" in the future
"Now we can study the difference between these two supermassive black holes." Keiichi Asada, a scientist from the Institute of astronomy and astrophysics, said, "using the existing photos of two black holes with a mass difference of more than 1500 times, we will be able to further test the gravity in extreme environments."
The photos of Sagittarius A * are made from a combination of many photos extracted from eht observation data in 2017. In 2018, 2020 and 2021, the EHT cooperation organization did not stop the pace of observation and research. Moreover, the number of EHT stations is increasing. In 2018, three new stations including Greenland telescope were added, and the coverage of the observation array was further expanded.
In future plans, more telescopes will be used to expand the EHT observation array, such as the Owens Valley Radio Observatory that has been built and the African millimeter wave telescope that will be built soon. In addition to the increase in the number of telescopes, from 2023, eht will conduct observations in the 0.87mm band. Compared with the observations in the 1.3mm band under the same conditions, the resolution is improved by nearly 50%. Sagittarius A and M87 are still the most important scientific observation targets of EHT cooperation. Astronomers are exploring how they change over time and studying the magnetic field around them.
The continuous expansion of EHT number and technological innovation will enable scientists to share more eye-catching astronomical photos and even black hole "movies". "Using the next generation eht to shoot such a 'film' of the black hole in the center of the Milky Way galaxy is the pursuit of astronomers." "We are planning to build China's submillimeter wave VLBI telescope to participate in the 24-hour continuous relay observation of Sagittarius A *," Shen Zhiqiang said
Chinese scientists have long paid attention to the research of high-resolution black hole imaging. In this eht cooperation, Chinese scientists actively participated in the joint promotion of early eht international cooperation, the joint application of EHT observation time, the observation operation of Hawaii Maxwell Telescope, and later data processing and analysis.
"We will also aim the telescope at other target sources and make more 'Donuts'. Because other' Donuts' are smaller, they need higher angular resolution. Building a space telescope may achieve this goal," Shen said
Lu rushen
Researcher, Shanghai Observatory, Chinese Academy of Sciences