Though scientists have suspected for a while that a giant black hole lurks at the center of our galaxy, they still can't say for sure it's the explanation for the strange behavior observed there. Now researchers are closer than ever to being able to image this region and probe the physics at work â€“ potentially shedding light on the great conflict between the theories of general relativity and quantum mechanics.
At the heart of the Milky Way, astronomers see some wacky things. For example, about a dozen stars seem to be orbiting some invisible object. One star has been found to make a 16-year orbit around the unseen thing, moving at the hard-to-imagine speed of about 3,000 miles (5,000 kilometers) a second. By comparison, the sun moves through space at a comparatively glacial 137 miles (220 kilometers) a second.
Based on the laws of motion, these dozen stars' orbits should be caused by the gravitational pull of some massive object in the center of the galaxy. Yet telescopes observe nothing there.
"The really important thing is that all the orbits have a common focus," astrophysicist Mark Reidof the Harvard-Smithsonian Center for Astrophysics said during the recently concluded April 2012 meeting of the American Physical Society."There's one point on the sky, and there's nothing you can see on images at this position."
Plus, all this is happening in a region only about 100 times as wide as the distance between the Earth and the sun â€“ very tiny in the galactic scheme of things. [Photos: Black Holes of the Universe]
There is, however, a very faint emission of radio waves coming from this area, which scientists call Sagittarius A* (pronounced "Sagittarius A-Star"). By comparing it against the sun's movement around the Milky Way, researchers have been able to determine that this object is barely moving at all â€“ less than 1 kilometer (0.62 miles) a second, much slower even than the rate that the Earth revolves around the sun.
If Sagittarius A* were any moderate-mass object, it likely would be pulled by the gravity of nearby objects and experience some motion.
Reid said of the object's apparent stillness: "The only way that this can happen is if Sagittarius A* is tied to a very massive object. When you do the analysis, you get a lower limit of 4 million solar masses."
The density limit of a black hole
Astronomers can't see the galactic center well enough to measure exactly how large Sagittarius A* is, but they can say for sure that its radius is no larger than about two-tenths the distance between the Earth and the sun.
The means that in the center of the Milky Way, something packing about 4 million times the mass of the sun is sitting within an area that could fit inside the orbit of Mercury and is basically invisible, producing much less light than any of the stars orbiting it.
Right now, that puts this object's density at about an eighth of the theoretical limit for a black hole. So while scientists can't say for sure the object is a black hole, it's looking mighty likely.
"Although there are alternative explanations, they would actually be even much more fantastic than the rather mundane supermassive black hole that almost certainly is there," Reid said.
One of these other, exotic explanations is that there exists a ball made of an unidentified variety of heavy fermion particles. But even such a ball would be unlikely to have the density required to explain all the evidence.