What is Sagittarius A?
[[File:A monster in the Milky Way.jpg|thumb|right|300px|Astronomers have observed stars spinning around the [[supermassive black hole]] in [[Sagittarius A*]].<ref>{{cite news|title=A monster in the Milky Way|url=http://www.spacetelescope.org/images/potw1340a/|accessdate=8 October 2013|newspaper=ESA/Hubble Picture of the Week}}</ref> ]] [[File:SgrAWest BEAR.jpg|thumb|right|300px|Surface brightness and velocity field of the inner part of Sagittarius A West]] '''Sagittarius A''' (or '''Sgr A''') is a complex [[radio]] source at the [[galactic centre]] of the [[Milky Way]]. It is in the [[Sagittarius (constellation)|Sagittarius]] constellation, but hidden from view by large clouds of [[cosmic dust]] in the spiral arms of the Milky Way. It consists of three components: the remains of a [[supernova]] '''Sagittarius A East''', the [[spiral]] structure '''Sagittarius A West''', and a very bright compact radio source at the center of the spiral, '''[[Sagittarius A*]]'''. These three overlap: Sagittarius A East is the largest, West appears off-center within East, and A* is at the centre of West. == Sagittarius A East == This feature is about 25 [[light-years]] wide. It looks like a remnant from a supernova explosion that happened between 35,000 and 100,000 years ago. However, it would take 50 to 100 times more energy than a standard supernova explosion to create a structure of this size and energy. Sgr A East might be the remains of a star that exploded as it was [[gravitation]]ally squashed approaching the central black hole.<ref name=Melia>Melia, Fulvio 2003. ''The black hole in the center of our galaxy''. Princeton University Press. {{ISBN|978-0-691-09505-9}}</ref> == Sagittarius A West == Sgr A West looks like a three-arm spiral from the point of view of the Earth. Actually it is made of several dust and gas clouds, which [[orbit]] and fall onto Sagittarius A* at velocities as high as 1,000 kilometers per second. The surface layer of these clouds is [[ionization|ionized]]. The source of ionisation is the populati