What is the galactic center?

The central bulge of the galaxy is a bright sphere with a diameter of about 20,000 light years and a thickness of 10000 light years. This region consists of high-density stars, mainly red stars over10 billion years old. Many evidences show that there is a huge black hole in the central region, and the activity of galactic nuclei is very intense. The center of the Milky Way, that is, the intersection of the axis of rotation of the Milky Way and the galactic plane.

The center of the Milky Way is in the direction of Sagittarius, and the coordinates of 1950 calendar elements are: right ascension 174229, declination -28 59 18. Besides being a geometric point, the galactic center also means the central region of the Milky Way. The sun is about 100000 parsec from Yin Xin, which is about 8 parsec to the north of galactic plane. There is a lot of interstellar dust between the galactic center and the solar system, so it is difficult to see the galactic center in visible light band with optical telescopes in the northern hemisphere. After the rise of radio astronomy and infrared observation technology, people can detect the information of the center of the Milky Way through interstellar dust in the band of 2 microns to 73 centimeters. The observation of neutral hydrogen 2 1 cm spectral line reveals that there is an expanding arm of hydrogen flow in O, which is 4,000 parsec away from Yin Xin, that is, the so-called "3,000 parsec arm" (the distance was mistakenly set at 3,000 parsec at first, and then revised to 4,000 parsec, but the old name is still used). About 1 0,000,000 neutral hydrogens with solar mass rush towards the solar system at a speed of 53 kilometers per second. On the other side of the silver core, there is a neutral hydrogen expansion arm with roughly the same mass, which leaves the silver core at a speed of 135 km per second. They should have been driven out of the silver heart in an asymmetric way from100000 to150000 years ago. In the sky area 300 parsec away from Yin Xin, there is a hydrogen disk rotating rapidly around Yin Xin, expanding outward at a speed of 70 ~ 140 km per second. There is a hydrogen molecular cloud with an average diameter of 30 parsec in the disk. 70 parsec away from Yin Xin, there is a violently disturbed ionized hydrogen region, which also expands outward at a high speed. It has been known that not only a large amount of gas gushes from the center of the Milky Way, but also there is a strong radio source in the center of the Milky Way, Sagittarius A, which emits strong synchronous radiation. The detection of very long baseline interferometer shows that the central area of the central radio source of the Milky Way is very small, even less than 10 astronomical unit, that is, it does not exceed Jupiter's orbit around the sun. According to the infrared observation data of 12.8 micron, the mass of the silver core with the diameter of 1 parsec is equivalent to millions of solar masses, of which about 1 10,000 solar masses exist in the form of stars. Xing has a title? O There is a massive dense nucleus in the center of the Milky Way galaxy, which may be a black hole. Relativistic electrons flowing into the compact core accretion disk are accelerated in a strong magnetic field, thus generating synchrotron radiation. The motion state of galaxy gas, the strong radio source in the center of the galaxy and the existence of special galaxies with strong core activity (such as seyfert galaxy) make us think that there has been core excitation activity in the evolution history of galaxies, including the Milky Way, and it has not stopped yet.