The “big patriarch” of the galaxy

Discovery of “Big Parents”

　　Since ancient times, people have wondered about a white area in the starry sky. It is like a long chain, extending from one side of the sky to the other, with different orientations as the seasons change, it is called the “Milky Way”. The Milky Way is the galaxy where our solar system is located, and the sun revolves around the center of the Milky Way. For a long time, the question of “what forces force the solar system to revolve around the center of the Milky Way” has puzzled astronomers. It was later known that there is a black hole in the center of the Milky Way, and it is the strong gravitational force of the black hole that forces all celestial bodies to revolve around it. The black hole is like a “big patriarch”, managing the operation of stars.
　　The black hole at the center of the Milky Way is in the constellation Sagittarius and is called “Sgr A* Sagittarius”. It is huge, about 4.4 million solar masses. Scientists believe that it may work together with dark matter to exert its huge ability to drive stars around it.
　　Sgr A* Sagittarius was discovered in the early days of radio telescopes, but at that time it was thought to be radio waves from space. Later, scientists determined that it was located in the center of the Milky Way, so it was called “Sagittarius A”. After that, scientists further clarified its location and called it “Sgr A* Sagittarius”.
　　In 2002, Reinhard Genzel’s research team discovered that a star orbited Sgr A* next to Sagittarius. By measuring the relationship between the two, the researchers knew the approximate mass of the black hole. In 2009, another research team published more observations of stars orbiting, conclusively confirming the existence of a massive black hole at the center of the Milky Way. Because of this, the 2020 Nobel Prize in Physics is awarded to the scientist who discovered the black hole.

Sagittarius Sgr A* as seen by the Very Large Array radio telescope
“messy” place

　　Where there is a black hole, it must be a “place of right and wrong”, and the black hole will swallow the surrounding stars. It first pulls the stars to its side, making the stars surround itself. At this time, the star moved faster and faster, and was finally torn into pieces by the black hole.
　　Scientists discovered G2 in the center of the Milky Way. They initially thought that G2 was a nebula, but later realized that they had made a mistake. G2 was not a nebula, but two stars. Maybe they are not binary stars themselves, but when they are close to the black hole, they will merge into a larger star, so that they can resist the gravity of the black hole and avoid the fate of being swallowed. They became long strips as they were pulled by the black hole and were mistaken for nebulae.
　　G2 is about 25 billion kilometers away from the black hole. If this distance is placed in the solar system, it should be near the orbit of Neptune. This distance is too close, extremely close to the black hole, but G2 has not been swallowed by the black hole. Maybe it takes a long time to devour, and the long nebula shape is only a temporary performance.

Black hole at the center of the Milky Way as seen by the Event Horizon Telescope

　　The Milky Way’s black hole is not violent. Typically, a violent black hole that absorbs too many stars produces jets, the most spectacular sights in the universe. Currently Sgr A* Sagittarius is still in the quiet phase, but it has a lot of restless performances. On January 5, 2015, NASA reported that they observed an X-ray flare 400 times brighter than usual, which is the manifestation of a celestial body falling into a black hole. Magnetic field disturbances also occur when black holes absorb objects. When the magnetic fields are entangled together, some other changes will also be brought about, which are manifested in various bands of electromagnetic waves.

3D rendering of a black hole
take pictures of black holes

　　Astronomers are preparing to take a picture of the black hole at the center of the Milky Way to let the public know what it looks like. Taking pictures of Sgr A* Sagittarius was no easy feat.
　　If a black hole does not emit light, it cannot be observed by an optical telescope. But it emits electromagnetic waves, so radio telescopes can observe it. A single radio telescope does not have such a large resolution. For this reason, scientists have created the technology of virtual telescopes, which combine multiple telescopes to integrate a large telescope. The telescope that takes pictures of the black hole consists of 8 millimeter/submillimeter-wave radio telescopes. These telescopes are distributed all over the world. The combined aperture is equivalent to the diameter of the earth. It is called the “Event Horizon Telescope” (EHT). Its resolution can reach 1000 times that of Hubble. With such powerful resolving power, it is possible to take pictures of black holes.
　　The Event Horizon Telescope has carried out observations of Sagittarius Sgr A* for several nights, collecting data for several hours each time, and then obtaining thousands of images through different calculation methods, and finally averaging various factors to generate the main image. On May 12, 2022, the Event Horizon Telescope finally produced a picture of the black hole at the center of our galaxy. This is a ring-shaped circular spot, and the bright part in the picture is the surrounding gas cloud. Because Earth is on the edge of the Milky Way, this photo was taken at an angle (probably less than 30 degrees).
　　People have finally “seen” what the black hole at the center of the Milky Way looks like, and this is the second time the Event Horizon Telescope has released photos of the black hole. Previously released was an image of the black hole at the center of the Virgo galaxy M87. Although the two photos don’t look much different, they consume a huge amount of device resources. Building the Event Horizon Telescope is an amazing feat. In the next step, scientists will build a more advanced Event Horizon Telescope to make a documentary that reveals how the outline of the black hole changes over time, so that we can learn more about the black hole at the center of the Milky Way, and more Learn more about the surroundings of a black hole.