Merritt and colleagues studied images of the spiral galaxy M33 that were taken by the Hubble Space Telescope, and had a resolution ten times better than previous Earth-based observations. The team measured the velocities of stars circulating in the nucleus of M33. In other galaxies, the gravity of the central black hole makes the stars closest to it move very rapidly. But Merritt's team found that stars close to the centre of M33 were moving slowly.

Merritt and colleague Laura Ferrarese recently discovered how the mass of the central black hole in a galaxy relates to the velocities of the stars in orbit around it. This allowed them to calculate that the black hole in M33 must be less than 3000 times the mass of the Sun, even accounting for the orbital eccentricities and gravitational interactions of the stars. This means the black hole at the centre of M33 is of 'intermediate' mass at most.

Astronomers previously thought that all galaxies were powered by super-massive black holes, and believed that large and small black holes evolved differently. But the search for the black hole at the centre of M33 suggests that these ideas may be flawed.

"The upper limit on the mass is still consistent with the relation that we discovered a year ago, and implies that small black holes - if they exist - formed in much the same way as very massive ones", says Ferrarese.