KAILUA-KONA, Hawaii, Nov. 8 (UPI) -- Massive stars moving at blazing speeds in eccentric orbits around the center of the Milky Way galaxy prove there is a black hole there, astrophysicists reported Friday.
However, this black hole -- which is about a million times more massive than the sun -- consumes very little surrounding matter, only about the equivalent of one-tenth of one percent of the mass of the Earth each year, scientists said.
"Rather than being a powerful monster in the Galactic Center, this black hole is more like the Cowardly Lion," University of California-Berkeley staff scientist Geoffrey Bower told United Press International.
Black holes are massive objects whose gravitational forces are so strong not even light can escape them. Matter that enters a black hole essentially disappears from the universe.
"We've been able to watch stars make complete orbits around the galactic center," University of California, Los Angeles, physics and astrophysics professor Andrea Ghez told UPI. "We've measured the radio velocity for some of these high velocity stars. It makes the case for a black hole four orders of magnitude stronger, which is a remarkable improvement."
Ghez has observed 30 stars with masses about 15 times that of the sun revolving around the galactic center at speeds of about 9,000 kilometers per second. The closest ones pass within 60 astronomical units of the black hole -- about 5 billion miles or the distance of the planet Pluto from the sun. One astronomical unit is the distance between the sun and Earth.
"The important point is that these stars are (in) very small orbits," Ghez told UPI. "How did they get there? They are very massive and young. The environment where they are today is inhospitable to star formation. Where did they come from, and how did they make it down to the small (orbital) radii?"
The jury is still out on the answer, Ghez said, but collisions probably were responsible. "It's a crowded party and they kept bumping into one another," she said.
The black hole at the center of the galaxy consumes far less material than previously thought, and much less than black holes at the center of other galaxies. Heino Falcke, a scientist with the Max Planck Institute for
Radio Astronomy in Bonn, Germany, told UPI.
"People often ask, 'Is it dangerous? Do we get eaten or something?' and that has always been a matter of discussion. How much is going in? ... It is a surprisingly small percentage of material. You think it is a monster, but somehow it is being kept on a starvation diet."
Falcke added, "I would say that stars are fighting back and keeping it from consuming everything. We see a lot of material going toward the center, and it seems that this material is turning into stars rather violently ... It is a cauldron of creation."
The stars apparently gather up the gas and material before it enters the black hole, he said. He compared these stars to ants that bond together and dominate the region, rather than feeding the black hole.
"We had this huge debate on how much the thing is swallowing," Falcke said. "The only firm conclusion we reached is that there is really only a very small amount of matter going inwards. That's fairly safe to say."
Berkeley's Bower said, "We've known for some time that black hole produces a lot less light and a lot less radiation than we would normally expect. It's a very massive object, a million times as massive as the sun, yet it only produces about as much radiation as the sun does. This is quite mysterious."
In other galaxies, there are black holes a million to a billion times as big as the sun, he explained. "In some of these objects, we see a hundred billion times as much radiation as we see coming from the sun."
Black holes generate light by stripping the gravitational energy from material that enters it. This gravitational energy heats up and turns into radiation. "It's a little like a meteor coming into the Earth's atmosphere," Bower said. One hypothesis about the galaxy's black hole being so relatively unenergetic was that a lot of material was falling in, but through some trick of the mechanism wasn't getting heated up.
"We've shown that is definitely not what's happening," Bower said. "There is very little matter that is accreting on (falling into) the black hole. It's glowing with reasonable efficiency, there just is very little of it."
The next mystery is why there is not much material there to accrete. "Potentially, the area could have been cleared out by a supernova explosion," Bower said.
The scientists are presenting their research at the seminar, "The Central 300 Parsecs of the Galaxy" (three hundred parsecs is equal to approximately 1,000 light-years across). The international gathering is sponsored by the Gemini Observatory, the National Science Foundation and several other observatories on Hawaii's Mauna Kea.
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(Reported by Dan Whipple, UPI Science News, in Broomfield, Colo.)
