The orbiting Chandra X-ray Observatory has discovered two supermassive black holes lurking in a single galaxy that eventually will spiral together in a catastrophic merger, sending gravity waves rippling through the cosmos.

"The detection of a binary black hole supports the idea that black holes can grow to enormous masses in the centers of galaxies by merging with other black holes," said Stefanie Komossa, of the Max Planck Institute for Extraterrestrial Physics in Germany, in a statement. "This is important for understanding how galaxies form and evolve."

The black holes were discovered in galaxy NGC 6240, an exceptionally bright star swarm 400 million light-years away. The galaxy is thought to be the result of a merger of two smaller galaxies -- a collision that triggered widespread starbirth.

Previous studies using smaller X-ray, infrared, optical and radio telescopes showed two concentrations of energy in the galaxy, but astronomers were unable to determine their nature.

The Chandra X-ray Observatory showed that both galactic nuclei were, in fact, two supermassive black holes separated by about 3,000 light-years.

The black holes themselves cannot be seen. But as surrounding matter is sucked into their gravitational maws, friction causes enormous heating and the release of torrents of energy in a variety of wavelengths. Because X-rays can penetrate intervening clouds of gas and dust, Chandra was able to solve the riddle of the two bright nuclei in NGC 6240.

"The breakthrough came with Chandra's ability to clearly distinguish the two nuclei and measure the details of the X-radiation from each nucleus," said Guenther Hasinger of the Max Planck Institute. "These cosmic fingerprints revealed features characteristic of supermassive black holes -- an excess of high-energy photons from gas swirling around a black hole, and X-rays from fluorescing iron atoms in gas near black holes."

The black holes are orbiting each other in a gravitational ballet that will cause them to spiral together and merge into a single, truly monstrous black hole in a few hundred million years. Toward the end of that merger, gravity waves will be generated, stretching and compressing space in a measurable, ripplelike fashion.

Future space-based gravitational wave detectors, such as NASA's proposed Laser Interferometer Space Antenna, or LISA, should be able to detect such ripples from similar black hole collisions.

"We certainly aren't expecting LISA to be around to detect these particular waves, but this system is still very important because it tells us there are many such black hole mergers taking place in the universe," astrophysicist Joan Centrella of NASA's Goddard Space Flight Center said at a briefing.

"The fact that this system is so close to us means there are many of these, and we expect to detect several of these per year with the LISA detector," she said.

While 400 million light-years is not "close to us" in any earthly sense, Steinn Sigurdsson of Pennsylvania State University told reporters that that was just around the corner in cosmic terms. If the universe were the size of the Earth, he said, and if the Milky Way were in Washington, NGC 6240 would be about as far away as Boston.

"It's very exciting that we're actually seeing an example of this in our neighborhood," he said. "This is something that's been speculated about theoretically, but until now we haven't actually known of a pair of black holes that are bound to each other and, we think, therefore irrevocably committed to running into each other and coalescing."

The picture of NGC 6240 revealed by Chandra provides a "snapshot of an evolutionary process," Sigurdsson said. "We think the mergers of galaxies and, in particular, the roles black holes play during the merger, is the process by which galaxies transform from one type to the other."

And it may explain how star-forming galaxies such as Earth's Milky Way, which is shaped like a flattened disk, evolve into more passive, more massive elliptical galaxies that do not exhibit the same rates of star formation.

At left, the galaxy NGC 6240, as seen by the Hubble telescope. At right, Chandra shows two black holes.