NP - A Galactic Center Mystery

[David Morris]

http://science.nasa.gov/headlines/y2002/21feb_mwbh.htm?list649552

February 21, 2002: In the most suspenseful detective stories, the mystery 
deepens even as the plot reveals more clues. So has it been in real life for 
astrophysicists investigating the center of our Milky Way galaxy. They hoped 
that NASA's Chandra X-ray Observatory would reveal a long-suspected black 
hole there -- and indeed it did. But Chandra's revelations have raised new 
questions that baffle scientists perhaps even more than before.
[...]
In 1974, British astronomer Sir Martin Rees proposed that supermassive black 
holes -- ones with a million or even a billion solar masses -- might exist 
within the centers of some galaxies. The galaxies he had in mind have 
impressively active nuclei (centers) that shine as brightly as 30 billion or 
more Suns. They glow, unsteadily flickering, at all wavelengths from radio 
to gamma rays, and they spew powerful jets of charged particles into space. 
Rees reasoned that black holes gobbling matter were the sources of such 
turmoil.
[...]
In 1974, even as Rees was speculating about black holes in active galaxies, 
American radio astronomers Bruce Balick and Robert Brown were observing the 
relatively quiet center of our own galaxy. There they discovered a compact 
and variable radio source that looked much like a faint quasar -- a type of 
far-away AGN that astronomers normally find near the edge of the observable 
Universe. But this object was "only" 26,000 light-years away, in our own 
cosmic backyard! Because it appeared to be inside a large, extended radio 
source already known as Sagittarius A, they named it Sagittarius A* 
(pronounced "A-star").

Over the next two decades, astrophysicists painstakingly observed 
Sagittarius A* at radio, optical, and near-infrared wavelengths. The 
breakneck speed (up to 1400 km/second) of gas and stars swirling around in 
the center of the Milky Way began to convince them that something small yet 
massive -- some 2.6 million solar masses -- was indeed lurking in our 
galaxy's center. Was it a supermassive black hole, or just millions of 
closely packed more-or-less ordinary stars?

[follow the link to find out]







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