One method that astronomers have used to search for black holes is by X-ray binary detection. The method is based on a similar detection of a neutron star in a binary system. A neutron star in such a system forms an accretion disk due to the star’s strong gravitational pull from the binary system’s companion star. A black hole has a much stronger gravitational pull than a neutron star, and because of this a black hole should be surrounded by a hot, X-ray emitting accretion disk. So you may be wondering how we know that the object is a black hole and not just in fact a neutron star. The trick to deciphering between the two lies in measurement of the objects mass. A neutron star is confined to a limit of 3MSun. Astronomers have detected binary systems where large luminous stars of masses in excess of 18MSun orbit an unseen companion with a mass of about 10MSun. Although there is discrepancy in the estimates of the objects mass, it is clear that the invisible objects mass exceeds that of the neutron star limit of 3MSun. These observations lead astronomers to believe they have they have begun to detect actual physical evidence of black holes. 



Below is a clip of an artistic rendering of a black hole pulling matter from a star. This forms the accretion disk which would allow us to detect the black hole's existence by observing the x rays and visible light emitted from the extremely hot, moving material.

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     There are hundreds of black holes theorized to be scattered throughout our Milky Way Galaxy, and over the past several years scientists have begun simulations of possible collisions between two “intermediate mass” black holes. The simulations predict that as two black holes come together to form an even more massive black hole, it should receive a tremendous "kick" because of the conservation of momentum. The new more massive black hole should actually get kicked right out of the globular cluster in a random direction as fast as 4,000 kilometers a second. Since the escape velocity of a globular cluster is only about 100 kilometers per second, that black hole won't ever come back to its original position. If this research is true, then the approximated 200 globular clusters in the Milky Way might have created many black holes, and then sent them in random directions into the galaxy.

     There are probably several hundred black holes wandering invisibly through our galaxy.As previously stated black holes are defined as objects of extreme intense gravity that attracts everything near it and allows nothing to escape it. Because of these characteristics many astronomers are led to believe that the mysterious center of our own Milky Way Galaxy may contain a super massive black hole. Our galaxy is blanketed with clouds of dust and gas that prevent us from seeing visible light emitted from the center of the galaxy. Radio, infrared and X-ray telescopes allow us to peer though these dust clouds and into the heart of the Milky Way. In the exact central region of the galaxy astronomers have found a strong source of radio emission named Sagittarius A* (Sagittarius A-star or Sgr A*). This source is unlike any other radio source in the galaxy, because several hundred stars are stuffed into the region within only 1 light-year of Sgr A*. The motion of the stars and gas near Sgr A* show that it contains a few million solar masses in this tiny region of space. Observations show that there are not enough stars in this region to account for all this mass, even with the high numbers of stars packed together. Because of the strong radio emission and high mass, astronomers believe that Sgr A* contains a super massive black hole.


Below is a short exerpt from an interview with Astronomer Andrea Ghez about how to find a super massive black hole.

 







(To the left is a long term X-ray exposure of the region known as Sagittarius A)


        
     Over the years black holes have evolved from an idea that was deemed to be impractical to an object that has gained relevance and proof. An object of such high mass and intense gravitational pull that emits no light seems that it would be impossible to detect, but as the use of X-ray, radio and infrared telescopes have shown, these objects may, and most likely do exist. Not only do they exist, but our own galaxy is scattered with hundreds of black holes that could potentially collide to form even more massive black holes known as rogue black holes. These super massive black holes are so powerful that one of them is thought to be sitting at the center of our own galaxy.


Cain, Fraser. “There May Be Hundreds of Rogue Black Holes in the Milky Way.” 6 December 2008. <http://www.universetoday.com/2008/01/09/there-may-be-hundreds-of-rogue-black-holes-in-the-milky-way/>

http://www.pbs.org/wgbh/nova/blackhole/

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