The black hole is around 600 light years from the centre of galaxy M82. "This black hole might eventually sink to the centre of the galaxy where it may grow to become a supermassive black hole", says Hironori Matsumoto of the Massachusetts Institute of Technology, lead author on one of three papers about the discovery.

The Chandra team compared new high-resolution images with optical, radio and infrared maps to establish that the X-rays were coming from a single strong source. Eight months of observations revealed that the signal intensity was fluctuating slowly over a period of months, and also flickering with a 600-second oscillation. "This flickering is similar to the well-studied characteristics of black holes swallowing gas from a nearby star or cloud", explains Philip Kaaret of the Harvard-Smithsonian Center for Astrophysics. "Explanations other than a massive black hole for this object are implausible".

The next challenge for astronomers is to establish how the newly discovered black hole formed. "This opens a whole new field of research", says Martin Ward of the University of Leicester in the UK. "Nobody was sure that such black holes existed, especially outside the centres of galaxies". Some scientists believe that it could be a 'cannibal' that has reached its present size by swallowing many other stars. Astronomers may also need to modify current theories of stellar evolution in the light of the discovery.