Black hole is an extreme celestial body predicted by Einstein's general theory of relativity. Its gravity is so strong that it can't escape the light of 300,000 kilometers per second, so it is the darkest thing in our universe. However, black holes have only existed in theory for a long time. Although astronomers found many gravitational anomalies in the universe and inferred that they were caused by black holes, it was not until 20 19 that astronomers published the first photo of black holes that the existence and true appearance of black holes were known to most people. This is the first black hole photo taken by the astronomical community with the horizon telescope. It comes from the center of M87 galaxy 55 million light years away, and its mass is 6.5 billion times that of the sun, so it belongs to a supermassive black hole.

After shooting in 20 17, two years later, in 20 19, we got the black hole photos of M87, which is 55 million light years away from us. This is the only time we can see a black hole clearly. Compared with the black hole in the Milky Way, the M87 black hole has great advantages. Its axis of rotation is only 17 degrees, which is almost along its axis of rotation, so it is almost unobstructed, so we can see the photos of M87 black hole relatively easily.

In the early stage of VLBI technology development, due to the limited number of radio telescopes, the "correct" observation wavelength and radio telescope can only be found within an "appropriate" distance. In recent years, due to the continuous development of VLBI technology and observation instruments, a series of high-resolution observations have been made on black holes, especially in the millimeter wave band in the last two decades. EHT cooperative members from Shanghai Observatory also said that the data analysis of the first imaging observation of the black hole in the center of the Milky Way galaxy took the team great efforts to determine the black hole photos!

Although Sagittarius A* is closer to us, shooting it is much more difficult than shooting M87*. The gas near the black hole moves at a speed close to the speed of light, and it takes several days to weeks for the gas to make a circle around the larger M87*, while it takes only a few minutes for the smaller Sagittarius A*. This means that when the EHT cooperation team observes Sagittarius A*, the brightness and pattern of the surrounding gas will change rapidly.