posted by @ulaulaman about #astronomy #SagittariusA #BlackHole #MilkyWay
Sagittarius A* (pronounced "Sagittarius A-star", standard abbreviation Sgr A*) is a bright and very compact astronomical radio source at the center of the Milky Way Galaxy, near the border of the constellations Sagittarius and Scorpius. It is part of a larger astronomical feature known as Sagittarius A. Sagittarius A* is believed to be the location of a supermassive black hole,(1, 2) like those that are now generally accepted to be at the centers of most spiral and elliptical galaxies. Observations of the star S2 in orbit around Sagittarius A* have been used to show the presence of, and produce data about, the Milky Way's central supermassive black hole, and have led to the conclusion that Sagittarius A* is the site of that black hole(3).
In the image there is an x-ray photo of Sgr A* from the paper by Wang et al. published on Science(4).
the x-ray emission from Sgr A* can be described as the superposition of a pointlike source from the black hole itself, and a much larger extended cloud of emission about 2″ across. Within this cloud, we can identify over a hundred individually resolved bright stars, and infer thousands more that are too dim to detect.(5)They also infer that
the temperature and density profile of the gas cloud surrounding Sgr A*. They show that over 99% of the gas never reaches the central black hole, but rather is ejected from the system(5)There are also some unresolved questions: for example if the observed accretion rate is dued exclusively by Sgr A* or if there is another source for the data; or his low luminosity, orders of magnitude below its theoretical potential(5).
(1) Christopher S. Reynolds (2008). Bringing black holes into focus, Nature, 455 (7209) 39-40.
(2) Doeleman S.S., Weintroub J., Rogers A.E.E., Plambeck R., Freund R., Tilanus R.P.J., Friberg P., Ziurys L.M., Moran J.M. & Corey B. & (2008). Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre, Nature, 455 (7209) 78-80. DOI: 10.1038/nature07245 (arXiv)
(3) Hamaus N., Paumard T., Müller T., Gillessen S., Eisenhauer F., Trippe S. & Genzel R. (2009). Prospects for testing the nature of Sgr A*'s near-infrared flares on the basis of current very large telescope - and future very large telescope interferometer - observations, The Astrophysical Journal, 692 (1) 902-916. DOI: 10.1088/0004-637X/692/1/902 (via Mark Henderson)
(4) Wang Q.D., Nowak M.A., Markoff S.B., Baganoff F.K., Nayakshin S., Yuan F., Cuadra J., Davis J., Dexter J. & Fabian A.C. & (2013). Dissecting X-ray-Emitting Gas Around the Center of Our Galaxy, Science, 341 (6149) 981-983. DOI: 10.1126/science.1240755
(5) Jeremy D. Schnittman (2013). The Curious Behavior of the Milky Way's Central Black Hole, Science, 341 (6149) 964-965