Astronomers believe that a supermassive black hole exists at the centre of every galaxy in the universe. Recent X-ray observations and measurements of the orbits of stars around Sagittarius A* - a powerful radio source at the centre of our galaxy – confirm that it is a black hole that is 3.6 million times more massive than the Sun.

Now, for the first time, Genzel and colleagues have detected periodic bursts of infrared radiation from Sagittarius A* with the Very Large Telescope (VLT) in Chile. Moreover, they found that these bright flares arrived periodically - roughly every 17 minutes. This can only happen if the accretion disk of hot gas around the black hole is rotating, which means that the black hole itself must be rotating.

Genzel and colleagues have calculated that the black hole is spinning at roughly half the maximum rate allowed by theory. Moreover, they have shown that the infrared signals are coming from within a few thousandths of an arcsecond - or few light hours – of the event horizon.

“Such measurements will provide a fundamental test of general relativity,” says Genzel. The team says that it will now confirm the periodicities of the flares. It also plans to study them more closely and investigate if they are associated with events already detected at X-ray and sub-millimetre wavelengths.