History of the search for a central black hole in the Milky Way

1770’s
An astronomer named Charles Messier discovered what he called “spiral nebula,” and made a catalog of these objects.  Many of these objects were actually distant galaxies.

1920’s
It was not until 150 years later that the many of the mysteries of “spiral nebula” would be answered through the debate of Heber Curtiss and Harlow Shapley.  Both contributed important information.  Curtiss argued that the “spiral nebula” were large distant galaxies similar to ours, while Shapley argued that the “spiral nebula” were large gas clouds within our galaxy and that the universe is composed of only one giant galaxy.  Although Shapley was wrong about this part of the debate, he contributed other vital information.  Shapley was the first to realize that the Sun is not the center of the Milky Way Galaxy, and was actually able to discover the center of the galaxy by observing the distribution of globular clusters.  The center lies in the direction of the constellation Sagittarius.  

Late 1920’s
The debate was later solved when Edwin Hubble determined the distance of a globular cluster in M31, the Andromeda Galaxy, concluding that it is outside of our galaxy and there are other galaxies in the universe besides ours.

With all this information, newer questions were developed.  What is actually at the center of the galactic nucleus?

1930's
X rays detected by radio telescopes, but the sources could not be studied on account of the large amounts of dust material in the way.

1960's
Studies began at Caltech searching for answers of what is in the center of the Milky way. They used measured levels of infrared light coming from the Milky Way disk and found the point from which they received the maximum light, then reasoning this to be the center of the galaxy. Here, they traced paths of several stars around an invisible central point. Doubts existed until the traced paths and speeds confirmed the mass of the central object. Any object that massive had to be a black hole.

1979
The National Radio Astronomy Observatory constructs 27 massive radio telescopes in New Mexico, later used to provide more detailed photos and data about the centers of galaxies.
 

1980s
Gamma rays are measured in enormous quantities coming from Sagittarius A, indicating the black hole was feeding. The gamma rays soon began to deplete, eventually disappearing, showing the black hole had stopped temporarily.

2002
For 10 years Rainer Schodel observed the motion of the star S2 near Sagittarius A (Sgr A), which is the name of the Milky Way’s central black hole.  Schodel concluded that it is a highly massive compact object with a mass of about 3.7 million solar masses.  The object is too massive to be a neutron star, which is why it is believed to be a black hole.

2004
Astronomers discover a potential intermediate mass black hole orbiting 3 light years from Sgr A.  Other stars have been observed orbiting this dark matter as well.

2008
Just recently astronomers discovered the edge of the event horizon of Sgr A.  CLICK HERE TO READ MORE!


Sources:

“The Galactic Centre.” Max-Planck-Institut für extraterrestrische Physik. November 17, 2008. http://www.mpe.mpg.de/ir/GC/index.php.

“The Scale of the Universe.” NRC Transcripts of the Great Debate. May 1921. http://antwrp.gsfc.nasa.gov/diamond_jubilee/1920/cs_nrc.html.