By Paul Rincon Science reporter, BBC News, Austin

Astronomers have weighed the biggest known black hole in the Universe.

The monster celestial object is 18 billion times more massive than our own Sun, says a team from Finland - six times larger than the previous record.

The object, called OJ287, is orbited by a smaller black hole, which allowed its mass to be measured very accurately.

The finding also enabled the researchers to test Einstein's theory of gravity for the first time in a strong gravitational field.

Details of the finding were presented at the 211th meeting of the American Astronomical Society (AAS) in Austin, Texas.

The binary black hole system powers a quasar - a compact halo of matter which radiates enormous amounts of energy.

It is relatively close for a quasar - about 3.5 billion light-years away from us in the constellation of Cancer - making it one of the most studied objects of its type.

It emits a pulsing light signal, with two major pulses every 12 years. From this, astronomers were able to construct models to predict the arrival of the pulses.

Beating disc

The team, led by Mauri Valtonen of Turku University in Finland, set up their observations for a predicted pulse on 13 September 2007.

Its detection on cue confirmed the model, suggesting that OJ287 was a black hole binary.

The researchers think that, as the smaller black hole orbits the larger one, it hits the disc of matter surrounding it twice. That impact releases hot gas from the disc which comes out from both sides. This is the source of the optical pulse which can be seen from Earth.

Ascertaining the binary nature of the system also allowed the astronomers to make their accurate measurement of the bigger black hole's mass. It had previously been suggested that black holes up to this mass might exist in quasars, but this is the first direct confirmation.

The gravitational field between the two black holes was so strong that it had never been possible to test Albert Einstein's theory of General Relativity in such an extreme situation.

The team managed to show that Einstein's theory could give the correct orbital behaviour of the binary orbit. They measured this at a 10% level of accuracy, though the researchers said this could be improved with future observations.