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First photo of black hole at the center of the Milky Way released

  At 21:07 on May 12, at a simultaneous press conference held all over the world, including Shanghai, astronomers showed people the first photo of the supermassive black hole at the center of our galaxy!
  This photo was “taken” by an international research team organized by the Event Horizon Telescope (EHT) collaboration through a network of radio telescopes distributed around the world.
  Direct imaging of the black hole at the center of the Milky Way is a breakthrough astronomical observation. Combined with the photos of the black hole (M87*) at the center of the giant elliptical galaxy M87 released in 2019, scientists are very excited to have obtained images of two black holes of very different sizes. This invaluable photo raises more questions for us to explore and discover.
Why is the first black hole photo not a galactic black hole?

  On April 10, 2019, researchers released the first ever human-captured photo of a black hole, the only time we have ever seen a black hole clearly. And since seeing this black hole photo (M87*), people have always remembered the photo of the black hole at the center of their own Milky Way galaxy.
  Many people see this photo and think it looks like a “doughnut”, so what is the real physical size of this “doughnut”? There are 100 billion kilometers! But because the black hole in the photo is 55 million light-years away, it looks very small, only 41 micro-arcseconds.
  This size is equivalent to dividing one degree on the protractor into 100 million parts, and that 100 millionth of a degree is the size of this “doughnut”, which shows that this black hole is very small.
  In order to photograph the black hole, more than 200 scientists from around the world formed a team. Eight sets of precious millimeter-wave telescope arrays scattered all over the world were formed into a telescope with an equivalent aperture as large as the diameter of the earth. After scientists collected the information about the black hole, it took nearly two years to get this photo. .

The images are of various wavelengths of the Milky Way, with increasing energy from top to bottom, from millimeter waves to gamma rays. Pictures | Scientific Rumors

  Compared with the black hole of the Milky Way, the M87 black hole has a great advantage. Its rotation axis is only 17 degrees, and it is almost seen along the direction of its rotation axis, so there is almost no obstruction, so we can see M87 relatively easily. A photo of a black hole.
  And the Milky Way’s supermassive black hole, located at the center of the Milky Way, is the supermassive black hole of our own galaxy. Some people must think that since they are by our side, isn’t it easier to shoot?
  In fact, as the poem says, “I don’t know the real face of Mount Lu, I just live in this mountain.” Although the black hole of our galaxy itself (called Sgr. A*) is very close, but because of the occlusion, the data It’s more difficult and time-consuming to process, so it takes more time to “take a picture”. In addition, the black hole at the center of the Milky Way has a relatively small mass, so the surrounding matter is much more likely to change.
  Direct observation of black holes has been difficult for a long time. The photo of the black hole at the center of the Milky Way we saw this time is the result of the research team spending a lot of time extracting different photos and averaging them. Scientists used supercomputers to synthesize and analyze data, and rigorously compared the black hole simulation database with the observations, so that we can see the first picture of the black hole at the center of the Milky Way.
  Still, the wait also made the release of this photo even more exciting, as this is a photo of a black hole at the center of our own Milky Way! This is also another major breakthrough after the EHT Collaboration released the first black hole photo of mankind in 2019, capturing the central black hole (M87*) located in the more distant galaxy M87.
How was this galactic black hole photo taken?

  As we all know, M87 is almost in the direction of the axis of rotation, and we are above the galactic disk, so compared with M87, the galactic black hole (the black hole at the center of the Milky Way) will be subject to a lot of occlusion during imaging. For example, when we observe the Milky Way in the optical waveband, we will see a large amount of dust and other gases blocked. At this time, we must use the infrared or radio wavebands with longer wavelengths.
  At present, the millimeter wave and submillimeter wave bands are mature, which is the event horizon telescope. It is worth mentioning that it uses different submillimeter and millimeter wave telescopes around the world to form an array with an aperture of tens of thousands of kilometers.

Image of the black hole at the center of galaxy M87, outlined by the emission of hot gas swirling around it.

  Whether it is the first photo of a black hole, the effect of a black hole on the surrounding gas and stars, or its light emission and gravitational waves, these direct or indirect evidences tell us that black holes exist.

  This photo is very similar to the photo of M87 taken in 2019, which was taken using eight different millimeter-wave telescopes around the world, or event horizon telescopes for short.
  This huge telescope combination is: ALMA (Atacma Large Millimeter/Submeter Array) in Chile, SPT (South Pole Telescope) in South Pole, SMA (Submillimeter Array) in Hawaii, USA, LMT (Large Millimeter Array, Large Millimeter Wave Telescope) in Mexico, JCMT (James Clerk Maxwell Telescope, James Clerk Maxwell Telescope) in Hawaii, IRAM (IRAM 30-m telescope) in Spain, APEX (Atacama Pathfinder EXperiment) in Chile , Atacama Pathfinder Experiment Telescope), SMT (Submillimeter Telescope), Arizona, USA.
  It is worth mentioning that the JCMT telescope in Hawaii, USA, is a telescope operated by China, and many Chinese scientists should conduct observations here. It is a pity that the maximum diameter that can be achieved by infrared observation at present is hundreds of meters. For example, the VLT/gravity of the European Southern Observatory, the observation diameter can reach 130 meters, but the caliber of the distance in kilometers is still very different. Pictures of black holes can be seen in infrared wavelengths.

  In terms of size, the black hole at the center of the Milky Way is obviously slightly smaller than that of M87, but it is more difficult to photograph the black hole at the center of the Milky Way. This is because the mass of the black hole at the center of the Milky Way is much smaller than that of M87, and the distance is closer. A lot, so the possibility of the surrounding material changing is much greater.
  Compared with the situation of observing the black hole of M87, the changes that originally took several days have now occurred in a few minutes or so, so the observation is more difficult. For this photo, for example, the scientists developed new sophisticated tools specifically to consider the gas of Sgr A*.
  We can recall when the last photo was taken: it started in 2017, and in 2019 we got a photo of the black hole at the center of M87*.
  However, it was not until five years later, when scientists synthesized and analyzed data with supercomputers and rigorously compared the black hole simulation database with the observations, we were able to see the first picture of the black hole at the center of the Milky Way.
Why can galactic black holes bind hundreds of billions of stars?

  The galactic center black hole is less than 0.0005% of the Milky Way. Why can it bind hundreds of billions of stars?
  From the perspective of the structure of the Milky Way, the structure of the Milky Way can be divided into three parts: the galactic core (including the black hole), the galactic disk and the galactic halo; from the perspective of mass, the mass of the large black hole at the center of the Milky Way is less than 0.0005 of the mass of the Milky Way. %; and from the perspective of the Milky Way’s core, the Milky Way’s black hole is only a part of the Milky Way’s bulge.
  So, what kind of force is holding the galaxy’s hundreds of billions of stars within a limited range? How did all visible matter come together?
  In fact, this question has been raised since the beginning of the last century.
  Astrophysicist Fritz Zwicky measured the stars of the Coma cluster and found the presence of dark matter. Because Zwicky’s character is not liked by everyone, although the concept is correct, it is not taken seriously by everyone.
  Until 1970, the young Rubin (Verin Rubin) and her mentor Ford (Kent Ford) have made research on the rotation speed of stars in Andromeda. Using high-precision spectroscopic measurements, they were able to detect the relationship between the speed and distance of the rotation of outer stars around the galaxy far from the galaxy’s core region. According to Newton’s laws, if the mass of a galaxy is concentrated mainly on the visible stars in the galaxy’s core region, the speed of the stars at the outer periphery of the galaxy will decrease with distance.
  But the observations show that over a considerable range, the velocities of stars outside the galaxy are constant. This means that there may be a large amount of invisible matter in the galaxy that is not only distributed in the core area of ​​​​the galaxy, and its mass is much greater than the sum of the masses of the luminous stars.
  We now know that invisible matter (dark matter) is about 10 times heavier than visible mass, and that’s true for almost the vast majority of galaxies.
  This is the answer to the previous question. Although the black hole in our galactic center is only so small, with the help of dark matter, it can bind hundreds of billions of stars!
Why do we study black holes?

  Whether it is the first photo of a black hole, the effect of a black hole on the surrounding gas and stars, or the light of a black hole and gravitational waves, these direct or indirect evidences tell us that black holes exist.
  Why do we study black holes? “The first reason that popped into my mind was curiosity. After all, a lot of times we want to do research because of curiosity. But obviously this reason doesn’t convince everyone. There is such a supermassive black hole in our galaxy. , why don’t we understand it? What does this supermassive black hole have to do with us? Will it affect our daily life?” said Zuo Wenwen, an associate researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences.
  Zuo Wenwen analyzed that, on the one hand, the mass of this black hole is 4.1 million times the mass of the sun, and it is 26,000 light-years away from us. At such a distance, we experience little gravitational pull from it.
  Active black holes, on the other hand, emit light, and the light is very strong. Interestingly, the black hole in the middle of the Milky Way is not active, it is very quiet, so the light and energy it emits is relatively weak. And because we’re so far away, by the time the light from this black hole reaches the Earth’s surface, the intensity is even weaker. Moreover, the earth comes with two protective layers, one is the atmosphere and the other is the magnetic field, which protects us from high-energy particles and high-energy photons. Therefore, the light emitted by the supermassive black hole at the center of the Milky Way has negligible effect on us.
  In addition to the supermassive black hole at the center of the Milky Way, in theory, there should be hundreds of millions of stellar-mass black holes in the Milky Way. Since every massive galaxy has a supermassive black hole at its center, what is the relationship between a black hole and the galaxy it’s in?
  Zuo Wenwen said that a black hole in the center of a large galaxy that is relatively close to us has a part called the bulge in the galaxy, and the mass of the black hole and the mass of the bulge are positively correlated. That is, the more massive the black hole, the more massive the bulge at the center of the galaxy it inhabits. Does this mean that the growth of black holes is related to the growth of galaxies? At present, this problem is still an unsolved mystery, so the study of black holes can help us understand galaxies and the relationship between black holes and galaxies. In addition to being of great help to the study of galaxies, black holes are also credited with the study of the history of the entire universe.
  Although supermassive black holes and stellar-mass black holes have very little impact on us in the Milky Way, the study of black holes is very crucial for the study of black holes themselves, black holes and galaxies, and black holes and the universe. There are still many secrets of black holes that have not been solved, which all prompts us to study black holes.
  The future research road is still very long, and exploring the ultimate mystery of the universe is the sea of ​​stars and stars in black hole research. Maybe the effect will not be seen overnight, but it must be worth our expectation.

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