Sagittarius is a constellation, a region of the celestial sphere, which happens to include the center of the Milky Way galaxy (red target):

To the naked eye, Sagittarius is where the visible Milky Way is brightest. Deeper images (as in your question) reveal vast numbers of stars and dense clouds of gas and dust, some of which are in the process of forming new stars. The center of the galaxy itself is obscured in visible light by 26,000 light-years of dust and gas, but observations at both shorter and longer wavelengths have revealed a tiny but intense source of radiation known as Sagittarius A* ("Sagittarius A-star," or Sag A*), embedded within a larger area of radio emission called Sagittarius A.

At the heart of Sag A* is a supermassive black hole with a mass more than 4 million times that of the sun as determined from the orbits of nearby stars:

http://www.astro.ucla.edu/~ghezgroup/gc/

A black hole's diameter is proportional to its mass, and a solar-mass black hole would have a diameter of about 6 km (just under 4 miles), so a 4-million-solar-mass black hole will have a diameter of about 24 million km (15 million miles). The black hole itself is invisible, but infalling gas and dust forms an accretion disk around the black hole, and it is radiation from the superheated matter in the disk that we observe as Sag A*. The numerous massive stars orbiting the black hole beyond the accretion disk energize the surrounding gas and dust at even greater distances, resulting in the surrounding radio source known as Sag A.
Source(s):
http://www.starrynight.com/sntimes/2006/2006-08-full.html
http://en.wikipedia.org/wiki/Sagittarius_A*
http://en.wikipedia.org/wiki/Sagittarius_A
http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html

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