Does it describe a new discovery or new methodology that's useful to others?

  How did you become involved in this research?

I was a Ph.D. student of Reinhard Genzel's and the subject of the paper was part of my thesis. Reinhard Genzel has been working on the Galactic Center and the supermassive black hole Sagittarius A* for decades. It is also one of the key projects of his submillimeter/near-infrared group at the Max Planck Institut für Extraterrestrische Physik.

  Could you summarize the significance of your paper in layman's terms?

Black holes are objects so massive and compact that no physical body or signal—not even light—can escape from their gravitational pull once it has crossed the so-called "event horizon" around the black hole (for a black hole of the mass of our sun the event horizon would have a radius of just 3 km, hence black holes are comparably small objects in astronomical terms). Besides the frequent stellar mass black holes (which are thought to be generated at the death of stars about 10 times or more massive than our sun) there is a second class of black holes, which reside in the centers of almost all galaxies and are a million to billion times as massive as our sun. Such so-called supermassive black holes are thought to be the central engine of "Quasars," the most distant and luminous objects known in the universe. Their electromagnetic emission is thought to arise when gas heats up to temperatures of million of degrees before falling beyond the event horizon of a supermassive black hole. In order to find evidence for the existence of such objects, one has to show that so much matter is concentrated in such a small volume that all alternatives to black hole can be excluded—e.g., dense clusters of dark astrophysical bodies such as planets, neutron stars, etc. This is a difficult task in extragalactic systems because of their incredibly large distance. The center of our home galaxy, the Milky Way, however, is about 100 times closer than our next major neighbour galaxy, the Andromeda galaxy. Therefore the undertaking of finding strong direct evidence for the existence of supermassive black holes is most easily done in the Milky Way. For probing the gravitational field at the center of the Milky Way one can use stars as test particles. Imagine the following scenario: If our sun did not give off any light, we would still know of its existence because the gravitational pull of the Sun (along with the angular momentum of the Earth) makes our home planet move around it once a year. In a similar way we have observed the movements of stars close to the supposed supermassive black hole, called Sagittarius A*, in the center of our Milky Way for over a decade. A star was found that moves around Sagittarius A* once every 15 years. By inferring the properties of its orbit with the help of Kepler's laws it was straightforward to show that the (dark) object that forces the star onto its orbit must have about 3.5 million times as much mass as our sun. Furthermore, this mass must be concentrated into a volume just about three times as large as our solar system. Since there is no other known physical object with these properties that can be stable over a significant length of time (compared to the lifetime of our galaxy of about 10 billion years) we concluded that the Sagittarius A* is indeed a supermassive black hole.

Dr. Rainer Schödel and Reinhard Genzel
Albertus-Magnus-Universität
I.Physikalisches Institut
Universität zu Köln
Köln, Germany

Prof. Dr. Reinhard Genzel 
Max Planck Institut für Extraterrestrische Physik, 
Garching, Germany
and University of Berkeley
Berkeley, California, USA