Below is an example of how relatively routine the identification of black holes has become.  The page below is from the Chandra X-Ray Observatory Center.  It is a picture of the Andromeda galaxy taken by the orbiting Chandra X-Ray Observatory.  Massive black holes cause the surrounding gas to produce X rays as the gas tunnels into the black hole.   The Chandra X-ray telescope reveals the X-ray sources that are hidden from optical telescopes.  Compare this image with the optical image of Andromeda shown in Chapter 1.  With the latest data from the Chandra Observatory astronomers now estimate there are approximately 300 million black holes in our universe!
 
 
 
Andromeda Galaxy (M31)

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Chandra X-ray
Observatory Center		 Harvard-Smithsonian Center for Astrophysics
60 Garden St. Cambridge, MA 02138 USA
http://chandra.harvard.edu

Andromeda Galaxy (M31): Our nearest neighbor spiral galaxy at a distance of two million light years. 
(Credit: NASA/CXC/SAO)

Caption: This X-ray image shows the central portion of the Andromeda Galaxy. The blue dot in the center of the image is a "cool" million degree X-ray source where a supermassive black hole with the mass of 30 million suns is located. The X-rays are produced by matter funneling toward the black hole. Numerous other hotter X-ray sources are also apparent. Most of these are probably due to X-ray binary systems, in which a neutron star or black hole is in a close orbit around a normal star. 
Chandra X-ray Observatory ACIS Image

CXC operated for NASA by the Smithsonian Astrophysical Observatory