Dark heart: Artist's impression showing a supermassive black hole at the centre of a galaxy.
Credit: NASA/JPL-Caltech
FROM THE SUMMIT of Mauna Kea, more than four kilometres above the Pacific Ocean, the Milky Way tilts luminously across the night sky, an edge on view of our galaxy.
Parts of the great disc are obscured by dust, and beyond one of those dusty blots, near the teapot of the constellation Sagittarius, lies the centre of the Milky Way. Hidden there is a deeply mysterious structure around which more than 200 billion stars revolve.
Behind me, atop the craggy rocks of this dormant volcano on the U.S. island of Hawaii, are the twin domes of the W.M. Keck Observatory. Each dome houses a telescope with a giant mirror that is more than 10 m wide and, like a fly's eye, is made of interlocking segments.
The mirrors are among the world's largest for gathering starlight, and one of the telescopes has been equipped with a dazzling new tool that greatly increases its power. Fewer than 100 people have seen this technology in action. I gaze at the nearest of the Milky Way's graceful spiral arms as I wait for technicians to flick the switch.
Then suddenly and with the faint click of a shutter sliding open, a golden orange laser beam shoots from the open dome into the sky. The ray of light, almost 46 cm wide, appears to end inside one of the blackest spots in the Milky Way. It actually ends some 89 km above the surface of the Earth.
The signal it makes there allows the telescope to compensate for the blur of Earth's atmosphere. Instead of jittery pictures smeared by the constantly shifting rivers of air over our heads, the telescope produces images as clear as any obtained by satellites in space.
Keck was one of the first observatories to be outfitted with a laser guide; now half a dozen others are beginning to use them. The technology provides astronomers with a sharp view of the galaxy's core, where stars are packed as tightly as a summer swarm of gnats, and swirl around the darkest place of all: a giant black hole.
WITHOUT QUESTION, the Milky Way's black hole is the strangest thing in our galaxy – a three-dimensional (3-D) cavity in space just 10 times the physical size of our Sun but with four million times the mass; a virtual bottomless pit from which nothing can escape.
Every major galaxy, it turns out, has a black hole at its core. Now, for the first time, scientists have the chance to study the havoc these mind-boggling entities wreak.
For the next decade, Keck astronomers will track the movements of thousands of stars caught in the gravity of the Milky Way's black hole. They will try to figure out how stars are born close to the black hole and how it distorts the fabric of space itself.
"I find it amazing that we can see stars whipping around our galaxy's black hole," says astronomer Taft Armandroff, director of the Keck Observatory. "If you had told me as a graduate student that I'd see that during my career, I'd have said it was science fiction."
To be sure, the evidence for black holes is entirely indirect; astronomers have never actually seen one. Albert Einstein's general theory of relativity predicted that the gravity of an extremely dense body could bend a ray of light so severely that it could not escape.
Something the mass of our Sun, for instance, could trap light if it shrank into a ball just 2.4 km across. For Earth to become a black hole, its entire mass would have to fit into a sphere no bigger than a pea.
