Sagittarius A Sagittarius A star (Sgr A*) is like a "small" behemoth. It is said to be huge because the shadow diameter of Sgr A* is 43 times that of the sun and its mass is as high as 4 million times that of the sun. It lives on Earth. unimaginable to us. However, compared with the M87 black hole, Sgr A* appears very "small", because the shadow diameter of the M87 black hole is 2000 times that of Sgr A*, and the mass is 2000 times that of Sgr A*! (As shown below). ho-paul-01 Source: Institute of Astronomy.
Academia Sinica Comparing the size of Sagittarius phone database A star (Sgr A*) and M87 black hole, the diameter of M87 black hole is 2000 times that of Sgr A*, and the mass is also 2000 times that of Sgr A*. The amazing thing is that if you observe Sagittarius A and the M87 black hole from the earth, the two black holes will look about the same size! WHY? This is because Sagittarius A is about 2,000 times closer to the Earth than the M87 black hole. Seen from the earth, these two black holes just form an opening angle of the same size in the sky (Note 1). As can be seen from the picture, Sagittarius A star and M87 black hole are very similar in structure, with luminous ring structures (accretion disks) around them, and the central shadow is also very obvious. But how do you see black holes on Earth?
First of all, you cannot use an optical telescope, you must use a radio telescope to see the black hole. The main difference between radio waves and visible light is the wavelength. The wavelength of visible light is about 0.5 microns on average, and the observation wavelength of EHT's radio telescope is about 1 mm. The difference between the two is about 2000 times. Academician He emphasized that in order to receive signals from distant galaxies, it is necessary to choose a wavelength that is not affected by dust. The wavelength of radio waves is much larger than that of dust, so it can penetrate dust and receive signals from the center of the Milky Way. Conversely, visible light is easily blocked by dust.