An international team of astronomers have provided compelling evidence that we have a supermassive black hole at the centre of our galaxy.

The astronomers, led by Zhi-Qiang Shen of the Shanghai Astronomical Observatory, used an array of 10 radio telescope across the US to measure the radio emissions from around the edges of Sgr A.

Earlier research, which estimated the size of Sgr A by tracking the speed of orbit of nearby stars, had suggested that the object must be no larger than Pluto's orbit around the sun.

These new observations mean that the object must fit in a space no larger than that described by Earth's orbit. With an estimated mass equivalent to that of around four million suns, the new constraints on the size of Sgr A place serious restrictions of the kind of object it could be.

It has been suggested that Sgr A could be a collection of neutron stars, but if the conclusions are correct, it is the densest object known in the universe. Zhi-Qiang Shen says that the sheer density of the object means that if it was a collection of neutron stars, it would have a life expectancy of around a century, NationalGeographic.com reports.

The next step in nailing down the exact nature of Sagittarius A is to attempt to directly observe the event horizon - the point beyond which nothing, not even light, can escape from a black hole.

The research team says new equipment and some adjustments to the Very Long Base Array of radio telescopes could make it possible to collect evidence of things crossing the event horizon. The radiation emitted each time this happens would create a shadow that could be detected with radio telescopes. ®

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