Few objects of such mass could fit in such a small space, Reynolds said.
The likeliest candidates are either a supermassive black hole or a cluster of several million stellar remnants. Such a cluster would survive for about 20,000 years before eventually becoming a black hole itself, he said.
The latest evidence that Sgr A* is indeed a black hole comes from new measurements that constrain the object to a size of one astronomical unit (AU), a span equal to Earth's distance from the sun.
This is half the size measured in any previous observation of Sgr A*.
"This is what makes it interesting," Lo said. "We are really probing the size of the black hole."
Lead study author Zhiqiang Shen of the Shanghai Astronomical Observatory says the new data also suggest that Sgr A* has a mass density at least a trillion times greater than any known cosmic object.
"With such a mass density, the lifetime for a dark cluster of stellar remnants would be less than a hundred years," Shen said in an e-mail interview.
According to Reynolds of the University of Maryland, astronomers are very unlikely to witness such a short-lived phenomenon.
Shen, Lo, and colleagues made their new measurements of Sgr A* with an array of ten radio telescopes strung across the U.S. from Mauna Kea, Hawaii, to St. Croix in the U.S. Virgin Islands.
Known as the Very Long Baseline Array (VLBA), the array can view radio waves in the finest detail of any telescope on Earth. Refined observation techniques allowed the team to view radio waves emitted just beyond the edge of Sgr A*.
Further refinements and new instruments may allow astronomers to eventually capture an image of a distinctive shadow around the black hole.
The shadow is caused by radiation from sources that cross the so-called event horizonthe point of no return surrounding a black hole.
"That would really put the nail in the coffin," Lo said.
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