Star exposes black hole in Milky Way

By LIDIA WASOWICZ, UPI Senior Science Writer

After a decade of painstaking calculations, astronomers have calibrated unprecedented measurements of a unique star on the cusp of a monstrous black hole -- and have provided the most direct and convincing evidence yet a ravenous beast lurks at the heart of the Milky Way galaxy.

The new clues have been brought to light by recently installed state-of-the-art star-gazing technologies. They present a seemingly air-tight case for what scientists have long suspected and other research has recently supported: a gargantuan black hole -- with a mass of nearly 3 million suns -- exists at the center of our galaxy. The proof lies in the direct observation of a nearly complete orbit of the closest star to the enigmatic heavenly body with a fatal attraction, scientists said.


"The Milky Way, like other galaxies, is thought to harbor a black hole at its center," said astronomer Karl Gebhardt of the University of Texas at Austin, who analyzed the findings. "The remarkable observation of a star in close orbit around the galactic center is the first firm evidence that this is so."


"(The finding) nails the lid in the coffin of the case for a black hole," said Andrea Ghez of the University of California at Los Angeles, whose own team has determined the star's complete orbit. She will be presenting those findings Oct. 23 at a symposium on black holes in Pasadena, Calif., and Nov. 7 at a workshop on the galactic center in Kona, Hawaii.

The evidence has accrued more from a process of elimination than of exposure.

Sky watchers, marveling at the dizzying speeds of whirling stars and twirling gas around the galactic center, for years have speculated about the hidden force that drives the dancing dervishes to their doom. They conceptualized matter swirling, like water down a bathtub drain, into the gaping jaws of a giant black hole -- a collapsed star with an escape-proof gravitational pull even light cannot withstand.

The view gained credence from observations made with ground-based and space-bound telescopes that have hinted at an awesome power centered in the southern constellation of Sagittarius, 24,000 light years from Earth. A light year is the distance light traveling at 186,000 miles (300,000 kilometers) per second covers in one year, or 6 trillion miles (9.5 trillion kilometers).


Until now, however, no studies could discount the possibility of other explanations for the eerie effect, such as clumps of extremely small and dense dying neutron stars or clusters of mysterious, invisible dark stellar matter.

Analysis of 10 years of high-resolution images of the nearest neighbor of the intense radio source, dubbed Sagittarius A*, puts such doubts to rest, the investigators told United Press International.

They based their computations on data from the newest observation tools, including instruments implemented late last year at the European Southern Observatory in a remote desert area of northern Chile. The results leave little possibility that forces other than a black hole bear responsibility for the strange goings-on in the middle of the Milky Way, they said. This active region bustles with giant molecular clouds, the remnants of exploding stars and mysterious filaments hundreds of light years long.

The images -- even sharper than those offered by the Hubble Space Telescope sitting above the dust in Earth's atmosphere -- portrayed in brilliant detail thousands of individual stars within the central,1-light-year-wide region, about a quarter of the distance to Proxima Centauri, the star nearest to the solar system.

"Despite decades of research and discovery, no one had found conclusive evidence that supermassive black holes exist," Gebhardt said. "The problem was that astronomers had never been able to observe the centers of galaxies closely enough to rule out other possibilities, such as a collection of neutron stars masquerading as a central black hole."


The new, closer-than-ever probe of the galactic center appears to clear up the confusion.

"The matter density that (the authors) infer from the details of the star's orbit is inconsistent with the presence of neutron stars, or other more exotic objects," Gebhardt noted. "The only compelling explanation is that there is a supermassive black hole lurking there. These results are the best evidence yet that supermassive black holes are not just theory, but fact."

The rigorous review revealed the small star, dubbed S2, completes its elliptical orbit around Sagittarius A* in 15.2 years.

"The sheer size of galaxies normally makes the detection of such movement impossible within a human lifetime," Gebhardt pointed out. "For example, our own sun takes 230 million years to circle the Milky Way. The (study) star ... will complete its orbit around the central black hole of our galaxy in a mere 15 years -- lightning speed on the grand, slow scale of the universe."

The space pictures also showed that at its closest approach to the black hole last spring, S2 came within a scant 17 light hours -- 11.3 trillion miles (18 trillion kilometers) -- only three times the distance between the sun and the planet Pluto.


Just as the Earth's steady run around the sun prevents the star from ensnaring the planet, so S2's stable orbit safeguards it from being swallowed by the supermassive black hole, at least until a collision or gravitational encounter with another heavenly body throws it off course, scientists said.

To keep from falling into the black hole, S2 speeds along at more than 3,100 miles (5,000 kilometers) per second, the researchers from Germany, France, Israel and the United States calculated. If it happened to pass by Earth, the star would traverse the distance from Los Angeles to New York in six-tenths of a second.

"We see the star move at an incredible speed -- 200 times that of Earth around the sun," Reinhard Genzel, director of the Max-Planck Institute for Extraterrestrial Physics in Garching, Germany, said in a telephone interview. "No one has ever seen such a star."

The extraordinary images allowed the space sleuths to trace two-thirds of the orbit of S2, currently the closest observable star to Sagittarius A*. Ghez and company have finished the job, reconstructing the entire orbit.

"It is the only star that has ever been observed in an orbit around a massive black hole," said Rainer Schoedel of the Max-Planck Institute, lead author of the study that will be published in the Oct. 17 issue of the British journal Nature. "Other galaxies are too far away for this kind of observation. This is what makes it so unique."


With new and improved instrumentation, astronomers expect to find more stars orbiting the black hole in periods ranging from a few years to a century, he told UPI.

The findings were made possible with cutting-edge observation tools -- the Nasmyth Adaptive Optics System and the Near-Infrared Imager and Spectrograph adaptive optics camera, collectively known as NOAS/CONICA -- which were mounted Nov. 25, 2001, on the 27-foot (8.2-meter) Yepun telescope. This fourth and final unit completed the Very Large Telescope at the ESO observatory, which sits atop Cerro Paranal, an 8,700-foot (2,635-meter) high mountain in Chile's pollution-free Atacama Desert. Each unit -- hooked into three different instruments -- has the ability to see objects 4 billion times fainter than those visible to the naked eye.

NAOS/CONICA -- which features the largest single mirrors of any telescope in the world -- counters the distorting effects of Earth's atmosphere that make stars appear to twinkle when viewed from the ground. The system uses a guide star as a reference to correct the smearing caused when starlight travels through the turbulent terrestrial atmosphere. It measures the deformation, then sends commands -- 500 times a second -- to a computer-controlled flexible mirror to counter the distortion. The mirror readjusts to remove the blurring effects.


"Using this new technology, we are now able to see images that are up to 20 times sharper than they once appeared, making it possible to differentiate individual stars in the crowded stellar regions at the center of the Milky Way," Gebhardt said. "Faint stars that were practically invisible can now be isolated, and more stars in orbit at the very center of our galaxy could be found."

Space watchers believe Sagittarius A* formed billions of years ago, perhaps in the death throes of gargantuan primordial stars that collapsed in a supermassive heap. Despite a mass 2.6 million times the sun's, it is considered of meager proportions by supermassive black hole standards, which range from 1 million to 3 billion solar masses. Its rank as second-smallest among the 37 known supermassives is in keeping with an emerging connection between a black hole's mass and a galaxy's "bulge," as astronomers call it.

"Our galaxy has a small bulge fitting in nicely with this relation," Ghez told UPI.

Yet, it might not be among the smallest of black holes universe-wide, researchers added.

"Scientists are just beginning to learn about possible medium-sized black holes, which are about 1,000 times smaller than (ours)," Gebhardt told UPI.


At one time viewed merely as fantastic fodder for science fiction writers, black holes increasingly are seen as significant sources for scientific inquiry into galactic secrets.

"Scientists now realize black holes are not just items to pique curiosity but very important sources of information on how galaxies came about," Genzel said.

With its firmed-up status, Sagittarius A* likely will attract a galaxy of inquiring eyes seeking an observable model for other black holes, scientists predicted.

"This (finding) makes even more likely the supermassive black hole interpretation for the enormous concentration of dark mass detected at the center of many other galaxies," said Alvio Renzini, a scientist at ESO.

"The existence of massive black holes at the centers of galaxies has become a standard paradigm of astronomy over the last decades," Schoedel said. "By means of our straightforward and direct approach we could show that there does indeed exist a massive black hole in the center of our own galaxy. This provides a solid foundation for claiming the existence of massive black holes in other galaxies and thus for a fundamental problem of modern astronomy."

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