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Final ‘Proof’ Provided for Milky Way’s Central Black Hole
Thu Oct 17, 9:38 AM ET
"We could not believe our eyes."
-- Astronomer Thomas Ott on the surprising discovery of a star swinging around the very heart of the Milky Way.
Surprising observations of a star swiftly orbiting the cloudy heart of the Milky Way Galaxy have verified with near certainty the existence of a central black hole, a theoretical object that still eludes direct detection.
Astronomers watched the star for a decade, tracking two-thirds of its path around the galactic center. No object has ever before been seen so close to the center of any galaxy, nor has any other object previously been observed making more than a small fraction of its orbital trek around a galaxy.
"Our work proves that there is indeed a supermassive black hole in our own galaxy," said Rainer Schoedel, a PhD student at the Max-Planck Institute for Extraterrestrial Physics (MPE) in Germany.
An international team of astronomers photographed the star as it zoomed around the galactic center at speeds ultimately exceeding 11 million mph (5,000 kilometers per second). Early this year, the star flitted precariously close to the black hole, coming within 17 light-hours, or just three times the distance from the Sun to Pluto.
The observations rule out nearly all other possible explanations for the tremendous amount of matter -- equal to some 2.6 million suns -- packed into a tight spot at the center of our galaxy.
"If one accepts the universal validity of the laws of physics," Schoedel told SPACE.com, "it is extremely hard to avoid the conclusion that the supermassive black hole in the Milky Way does indeed exist."
Case nearly closed
As always with these enigmatic gravity sinks, however, this one refuses to give up its innermost secrets, allowing itself to be observed only indirectly by its effect on another object. Other astronomers will continue to hem and haw -- though just a little now -- over whether theory has finally been resolved into hard fact.
"These results are the best evidence yet that supermassive black holes are not just theory, but fact," said Karl Gebhardt, a University of Texas at Austin astronomer who was not involved in the work. Gebhardt said most astronomers believe a black hole to be the only viable model to explain the star’s motion and other data gathered on objects and structures near the galactic center.
"Now, to actually say that it is a black hole is going to be very tough, since in order to do that we have to witness an event horizon -- the defining property of a black hole," Gebhardt told SPACE.com. An event horizon is a theoretical sphere surrounding a black hole through which objects pass into oblivion. "We are not there yet with this black hole or any other."
Schoedel echoed the latter statement.
"We are far from being able to image the event horizon," Shoedel said, adding that the star’s closest brush with the black hole equates to a radius about 2,100 times larger than that calculated for the event horizon.
The results, to be published tomorrow in the journal Nature, also lend significant support to the theory that even more gargantuan black holes anchor other galaxies, Schoedel and other astronomers said.
A black hole by theoretical definition is so dense that nothing, not even light, can escape its gravitational clutches once trapped inside. Scientists suspect their existence based on the rapid motions of stars and, sometimes, by energetic expulsions from the jowls of dark objects that are feeding on matter that spirals inward, ever faster, like water going down a drain.
The swift-moving star in the new study, called S2, is a relatively normal star about 15 times the mass of our Sun. Its path took it to a record-setting close distance to the location of known radio signals thought to mark the center of the galaxy and the presumed black hole. The star’s elongated orbit also takes it 10 light-days from the central object.
The astronomers witnessed enough of the star’s 15.2-year orbit to project the entire path. The elliptical orbit is fairly stable, they say, and the star would have to come 70 times closer to the black hole to be destroyed by gravitational tugging.
Gebhardt, who wrote an analysis of the work for Nature, said the observations are remarkable in and of themselves.
"This is the first time that an orbit has been detected on the galactic scale," Gebhardt writes. "The sheer size of galaxies normally makes the detection of such movement impossible within a human lifetime."
The Sun, for example, takes 230 million years to go around the galaxy. Stars closer to the center move faster. Hundreds have been observed travelling swiftly under the influence of the black hole. But watching them has never been easy, as a thick haze shrouds the galactic center, preventing many observing schemes.
Schoedel and his colleagues were aided by adaptive optics, a modern technique that distorts telescope mirrors on purpose to counteract distortions applied to light as it passes through Earth’s atmosphere. Gebhardt said the setup provided images 20 times sharper than otherwise possible.
The star’s path was monitored for a decade by infrared and radio telescopes. Schoedel said his team knew the star was approaching the presumed black hole, but their estimates for when the closest approach would occur ranged from imminently to decades or more, he said.
Then in May of this year, a new instrument on the European Observatory’s Very Large Telescope in Chile provided a surprising near-infrared view of S2.
"We could not believe our eyes," said Thomas Ott, an MPE researcher who co-led the study along with Schoedel and MPE director Reinhard Genzel. "We suddenly realized that we were actually witnessing the motion of a star in orbit around the central black hole, taking it incredibly close to that mysterious object."
Ruling out other objects
While black holes can’t be seen, few theorists have doubted their existence in recent years. Still, a handful of other exotic explanations have been put forth to account for the heavy center of the Milky Way, based on what scientists understand about physics: a cluster stellar mass black holes; a cluster of neutron stars made entirely of tightly packed neutrons; or perhaps a big ball of heavy neutrinos.
These alternatives can all be excluded, according to the astronomers who made the new observations.
One other viable explanation remains: a hypothetical star made of heavy elementary particles called bosons. But there’s one big strike against this possibility.
"Even if such a boson star is in principle possible, it would rapidly collapse into a supermassive black hole anyhow," Genzel said.
Beyond the Milky Way
The Milky Way’s black hole is small by comparison to others in the universe, and it is relatively inactive. Other galaxies are known to harbor central objects containing the masses of billions of stars. These theoretical black holes are generally said to be active, consuming matter voraciously, through a process called accretion, and converting some of this matter to X-rays and other radiation that signal their presence.
While it’s not clear why the Milky Way’s black hole is relatively quiet, astronomers have long suspected that a black hole is a black hole, and that in a general sense what’s observed in the Milky Way can be applied to the more massive objects seen elsewhere. The Milky Way’s black hole may be simply taking a culinary break after billions of years of high consumption.
The new finding "makes even more likely the supermassive black hole interpretation for the enormous concentration of dark mass detected at the centers of many other galaxies," said Alvio Renzini, a program scientist with the Very Large Telescope and another participant in the new study.
Gebhardt said new techniques like adaptive optics might make it possible to figure out how material is funneled into a black hole and why the Milky Way seems to be on such a conservative diet right now.
"One exciting possibility is to measure the winds coming off of the stars that orbit close to the center," Gebhardt said. "If we can do this accurately enough, we might be able to provide the available mass that is floating around there that the black hole can accrete. We can then use this information to infer that an event horizon is present."
Copyright © 2002 SPACE.com.