This was based on the assumption that SMBHs form in a similar way to stellar-mass black holes, marking the final phase in the lifespan of massive stars that have exhausted all their fuel for fusion.

One question, Tan says, is how efficiently the process works for massive versus light stars, because that's likely to be the mechanism that determines stellar mass.

This means that the lowest mass black holes may be very rare, short-lived or non-existent, creating a conspicuous gap between the most massive neutron stars and the least massive stellar black holes.