Galactic Center


May 17, 2022

The galactic center is the barycenter of the Milky Way. Seen from Earth, it lies in the constellation Sagittarius, where the visible band of the Milky Way appears most dense (right ascension α 17h 46m and declination δ −29° 00′). The galactic center contains the supermassive black hole Sagittarius A* with a mass of 4.1 million solar masses. The orientation of the galactic coordinate system is determined (for historical reasons only approximately) by the position of the galactic center (galactic longitude is ≈0° here), but for practical reasons the zero point of the galactic coordinate system is the sun. At 8.12 kpc, the galactic center is close enough to Earth (8.178±0.035 kpc as measured by the Hipparcos and Gaia spacecraft) that the motions of individual stars can be studied. In the constellation Auriga, directly opposite the galactic center, lies the region of the galactic disc with the lowest observable star density - the galactic anticenter.

Hidden behind dark clouds

The constellation Sagittarius (Latin sagittarius) contains a particularly large number of stars and nebulae, but the galactic center itself cannot be observed in visible light, since it is weakened by dark dust clouds of interstellar matter by about 30 magnitudes (factor 1012) on the way to Earth will. However, observations are possible with longer-wave radiation such as infrared radiation and radio waves as well as with shorter-wave hard X-ray radiation, since such parts of the electromagnetic spectrum penetrate the regions of interstellar dust clouds much better. In addition, the galactic center is the center of the galactic rotation of all bodies present in the Milky Way system and as such can be inferred indirectly.

Radio, infrared and X-ray radiation

Already at the beginning of the development of radio astronomy, Karl Guthe Jansky succeeded in 1931 in proving radio radiation from the direction of the galactic center. Later observations resolved this emission into different radio sources of different nature. One such source, Sagittarius A (West), is a roughly helical structure of ionized gas about 2 pc in size. It is surrounded by a ring of colder molecular interstellar matter. Within Sagittarius A is the very compact radio source Sagittarius A*. This source at α 17h 45m 40s.04 and δ −29° 00′ 28″.2 (J2000.0) lies at the center of the Milky Way. Since the 1960s, as infrared astronomy has advanced, the galactic center has become one of its preferred targets. A star cluster S-star cluster, which becomes increasingly dense inwards, was shown, the center of which lies at Sagittarius A*. Surprisingly, many stars in the innermost 0.5 pc are young, hot stars. It is not yet fully understood how they were able to form under the extreme conditions there, or how they got there during their lifetime of only a few million years. Towards the end of the 1990s, X-ray emission from Sagittarius A* was detected for the first time with images from the X-ray satellite Chandra. Earlier X-ray telescopes had already detected emissions from the area of ​​the galactic center, but their assignment was not clear due to the poorer angular resolution.

Central black hole

Supermassive black holes are widely accepted in astronomy as the energy source of active galactic cores and are now suspected to be at the core of every bright elliptical galaxy and spiral galaxy bulge. However, at least in individual cases, direct evidence of the gravitational effect of the black hole is necessary in a way that excludes other explanations. The galactic center probably offers the strongest evidence today. The properties of