An international team of astronomers says it has found the first direct proof that a supermassive black hole exists at the centre of our galaxy.

The dark heart (not even light escapes it) of the Milky Way has a mass up to 3.7-million times that of the sun, researchers reported in today's edition of the British journal Nature. Its powerful gravitational pull sucks in dust and gas and even stars that come too close.

The researchers reached that conclusion after using new imaging technology to document the extraordinarily fast orbit of a star, known as S2, that appears close to being drawn into the black hole and disappearing.

For years, researchers have been amassing indirect evidence that a black hole lurked 26,000 light years from us at the galaxy's centre. Viewed from Earth, that area of the Milky Way appears in the southern constellation Sagittarius. They called the dark area Sagittarius A. Although they suspected it was a black hole, they weren't sure. It might have been a collection of dense stars or a large concentration of particles.

Direct proof came when the team led by researchers from the Max-Planck Institute for Extraterrestrial Physics in Germany used new optical-imaging technology to document the fast-moving star, S2, whipping around the dark centre of the galaxy in a 15-year orbit.

The researchers were able to see the star and measure its orbit because of the new technology now in use at the world's largest optical telescope in Chile (at the European Southern Observatory's Paranal site) provided them the sharpest and deepest pictures ever taken of the Milky Way's centre.

They say their measurements rule out the possibility that anything but a black hole could be exerting that kind of gravitational pull on the orbiting star.

"The new measurements . . . leave little doubt of the presence of a supermassive black hole at the centre of the galaxy in which we live," the researchers said in a press release. "No event like this has ever been recorded."

Karl Gebhardt, an astronomy professor at the University of Texas, agrees that the only compelling explanation for the star's orbit is the existence of a gigantic black hole. He was not part of the team that made the discovery, but wrote a commentary about it for Nature.

"It is the most remarkable thing. We've been talking about black holes for a couple of decades without having any direct proof. About five years ago, we had a name, MDO, or massive dark object, because we didn't want to use the term black hole . . . it wasn't a sure thing." The MDO term never caught on, and now astronomers can abandon it.

Black holes are one of the least understood forces in the universe, even though they may be at the heart of all the galaxies in the universe. How they form is still a mystery.

They may start as a "seed black hole," formed when a star like our sun runs out of fuel and collapses. The hole may then grow bigger as it swallows dust and gas.

The discovery announced today could lead to a clearer and more detailed picture of the universe, and our galaxy in particular.

While black holes do absorb matter that comes too close, it is a mistake to imagine them as the vacuum cleaners of the universe, Prof. Gebhardt said.

"If you fall [too close to] . . . the black hole, you are lost forever. But if you are outside . . ., you can orbit it happily."

That means that S2, although close to the Milky Way's black hole, is safe for now, unless it gets knocked off its orbit by a collision with another star.