Galactic Center

As galactic center, the center of mass of the Milky Way is called. It lies in the constellation Sagittarius, where the visible band of the Milky Way appears thickest. The galactic center contains the nearest known to us supermassive black hole and shows other unusual astrophysical phenomena. The galactic coordinate system here has its starting point ( α = 17h 42.4min and δ = -28.92 ° ), but does not exactly agree with this for historical reasons.

Seen from Earth directly opposite the galactic center is located in the constellation Auriga, the region of the galactic disk with the lowest observed density of stars - the galactic anti center.

Hidden behind dark clouds

Although the constellation of Sagittarius (Latin sagittarius ) contains many stars and nebulae, but the galactic center itself is hidden behind dark clouds of dust of the interstellar matter. In the visible light, it can not therefore be observed. Visible light is attenuated on the way from the galactic center to the earth by about 30 mag (factor of 1012). With longer wavelength radiation (infrared, and radio waves ) and in the hard X-ray observations, however, are possible because these regions of the electromagnetic spectrum dust penetrate much better. Moreover, the galactic center is the center of the galactic rotation of all present in the Milky Way system and body can be developed as such indirectly ( To build up our Milky Way see the Milky Way ).

The distance from the Earth to the center of the Milky Way (about 8 kpc = 26,000 light years) is 100 to 1000 times smaller than the next comparable to the cores of galaxies. It can therefore be examined much more closely. For example, the properties and motions of individual stars can be determined.

Radio, infrared, and X-radiation

Back in the early development of radio astronomy succeeded in 1931 by Karl Guthe Jansky, the detection of radio emission from the direction of the galactic center. Later observations solved this issue into different radio sources of different nature. One of these sources, Sagittarius A (West), one approximately spiral- shaped structure ionized gas of about 2 pc 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 right ascension 17 h 45 m 40.04 s and declination -29 ° 00 ' 28.1 " ( J2000.0 ) is located in the center of the Milky Way.

Since the 1960s, was with increasing development of infrared astronomy, the galactic center to one of their favorite destinations. It was an inward toward increasingly dense expectant star cluster whose center is located at Sagittarius A *. Surprisingly, many stars are in the innermost 0.5 pc young, hot stars. It is not yet fully understood how they could be created under the local conditions or extreme but could get there during their lifetime of only a few million years.

Towards the end of the 1990s succeeded with shots of the X-ray satellite Chandra for the first time the detection of X-ray radiation from Sagittarius A *. Earlier X-ray telescopes had indeed already established emission from the region of the galactic center, their assignment was not clarified because of poorer angular resolution.

Black hole in the center of the Milky Way

Black holes are widely accepted in astronomy as an energy source active galactic nuclei and each bright elliptical galaxy and each bulge of a spiral galaxy are now suspected in the core. But necessary, at least in some cases, the direct detection of the gravitational effect of the black hole in a way that excludes other explanations. The galactic center offers today probably strongest evidence here.

Evidence of a black hole resulting from the properties of the radio source Sagittarius A *. It shines very bright from a very small area, which can not be explained by other types of radio sources. However, this evidence is still indirect. The low proper motion of Sgr A * - essentially all you see is the reflection of the revolution of the sun around the galactic center - indicates a very massive object,. An object with less mass should be like the stars in the central star cluster move very quickly in the sky when it is not just randomly moved exactly to the sun.

The best evidence of a black hole comes from the motion of matter under the influence of its gravity. Already in the late 1970s, Charles H. Townes and coworkers had demonstrated rapid gas motions in the galactic center, but here were still lingering doubts, since gas may be subject to other forces than gravity (for example, by magnetic or stellar winds ), and the spatial resolution not full enough. Since the 1990s, therefore, German and American research groups have studied the movement of the stars of the central star cluster with increasingly higher spatial resolution. For the correction of atmospheric turbulence was first speckle interferometry and adaptive optics used. The increase in the rates of movement of the stars near the central mass could be tracked to less than 0.1 arcsec distance. Astronomers at the Max Planck Institute for Extraterrestrial Physics were able to determine the mass of this black hole with relatively high accuracy to about 4.31 million solar masses. Other options than a black hole to pack so much mass in such a small volume would not be stable over the age of the Milky Way.

Since 2001, outbursts of typically been observed to a few hours duration from the immediate vicinity of the black hole in the X-ray and infrared repeated. Your short-term flicker contains - in just a few Schwarzschild radii from the black hole - possibly information about the space-time.

Astronomers had a certain star of Sagittarius for over decades in their sights and watched two thirds of its covered path around the galactic center. Previously, no object had been observed over such a long period of time so close to the center of a galaxy, nor was any other object until then been discovered, which described more than a small part of its orbital orbit around the galaxy. Rainer Schoedel of the Max Planck Institute for Extraterrestrial Physics sees these findings as evidence of a supermassive black hole.

In 2011, astronomers studied the Max Planck Institute for extraterrestrial Physics pictures of the Milky Way center in the 3.8 micron wavelength range. They discovered a gas cloud that moves in the direction of the central black hole. By the end of 2013, this gas cloud is as close approach to the black hole that it is thereby torn apart by tidal forces. It is believed that some of the gas decelerated within the next few years, drawn by the gravity of the black hole and disappear behind the event horizon.

Gallery

Infrared photo from the Spitzer Space Telescope.

A look at the night sky near the constellation Sagittarius. The main stars in Sagittarius are highlighted in red.

The central parts of the Milky Way, as seen in the near infrared with the NACO instrument of the Very Large Telescope of ESO.

Infrared photo of the center of the Milky Way that reveals a new population of massive stars.