For years astronomers have suspected that deep in the heart of our galaxy lurks a dark secret - a colossal black hole.
Now an international team of scientists has gathered the strongest evidence yet that an object in the center of the Milky Way called Sagittarius A is not only a black hole, but one roughly the size of 3 million of our suns.
"I think it's settled now: Black holes really exist," said Rainer Schoedel of the Max Planck Institute for Extraterrestrial Physics in Germany, who led the international team that made the discovery.
The research, published today in the British journal Nature, is expected to open the door to a new understanding of these enigmatic objects first imagined in the 18th century.
"They have a definite mystical fascination," said Roeland van der Marel, an astronomer who studies black holes at the Space Telescope Science Institute in Baltimore. "Both science fiction writers and theoretical physicists like to speculate about them."
According to theory, black holes are regions of space where the force of gravity is so strong that not even light can escape. They form when a large quantity of matter is concentrated into a small enough space, such as when a large star exhausts its fuel and collapses upon itself.
Despite popular belief, black holes are not giant Hoovers in space. "A black hole does not suck things up," said astronomer Karl Gebhardt at the University of Texas.
Instead, he said, they gobble up gas and other stellar snacks only if those approach or cross a mysterious boundary - called the "event horizon" - that surrounds the black hole.
Scientists now think that black holes lie at the center of most galaxies. Sagittarius A - first discovered at the center of the Milky Way in the 1970s - has long been a prime suspect.
But scientists were unable to rule out other possible explanations, such as a dense cluster of stars or more exotic particles masquerading as a black hole.
To find the answer, Schoedel and his team spent a decade carefully tracking a star dubbed S2, which orbits Sagittarius A.
The observation itself is a landmark feat, notes Gebhardt. It's the first time astronomers have observed a star making a near-complete lap around the galaxy, the sheer size of which normally makes this impossible in any scientist's lifetime.
After observing nearly three-quarters of S2's orbit, the astronomers calculated that it circles Sagittarius A in 15.2 years - "lightning speed on the grand, slow scale of the universe," said Gebhardt. At times, the star was clocked at more than 3,100 miles per second.
Earth's sun, by contrast, pokes along at 135 miles per second and requires 230 million years to make a complete orbit.
By determining how quickly S2 was orbiting Sagittarius A, astronomers could calculate how massive the object is - between 2.2 million and 3.7 million times the size of the sun.
But more important, the scientists were able to determine that all that mass was crammed into a relatively small space.
"This leaves us with almost no other explanations other than a black hole," said Schoedel. "We have never had as simple and straightforward evidence as we have now."
The center of the galaxy has become a hot area of study, said astronomer Mark Morris of the University of California at Los Angeles. "It's the place where you see the most extremes of nature."
The discovery of a black hole there will likely make it even more important, he said. In the coming years, scientists hope to answer such vexing questions as which came first, galaxies or the black holes that reside there?
One thing astronomers will never know is what goes on inside the darkness.
"That's always going to be mystery," said van der Marel.