The discovery of a rare magnetic star - or magnetar - is challenging theories about the origin of black holes.

Magnetars are a special type of neutron star with a powerful magnetic field.

They are formed by gravitational collapse after the original, or progenitor star, dies and forms a catastrophic supernova.

For this newly discovered magnetar, astronomers calculated that the mass of the progenitor must have been at least 40 times greater than that of our Sun.

The researchers' arguments are based on robust measurements, not just theory Professor Mike Cruise, University of Birmingham

Collapsing stars of this size should form a black hole. The fact that this one resulted in a neutron star challenges established theory.

The study, led by Dr Ben Ritchie of the Open University, is published in the journal Astronomy and Astrophysics.

The new magnetar was found in an extraordinary star cluster known as Westerlund 1, located 16,000 light years away in the southern constellation of Ara (the Altar). This region contains numerous massive stars.

Dr Ritchie remarked that if the Earth was "located at the heart of this remarkable cluster, our night sky would be full of hundreds of stars as bright as the full Moon".

Live fast, die young

To calculate the mass of the progenitor star, the research team estimated its lifespan. Massive stars collapse earlier than small stars because the pressure on their core is greater, causing them to burn up their hydrogen fuel more rapidly.

Image caption The missing black hole might be explained if the original star lost mass to a neighbouring star

The astronomers assumed that this star formed at the same time as others in the same cluster.

So the fact that this star had already collapsed shows that it must have been more massive than the other stars that still exist there.

Stars that are more than 25 times more massive than our Sun normally collapse to form black holes.

Dr Negueruela of the University of Alicante in Spain, a co-author on the study, said that the mystery of the missing black hole might be explained if the progenitor star got rid "of nine tenths of its mass before exploding as a supernova".

One way of achieving this "diet plan" would be if the progenitor was part of cosmic double-act known as a "binary star", and its companion pulled off some of its mass, Dr Clark, another co-author, told the BBC. This would have allowed it to avoid the fate of becoming a black hole.

Professor Mike Cruise, an astrophysicist at the UK's University of Birmingham, who was not involved in the study, told BBC News that the new research was "a brilliant piece of detective work".

He commented: "What is especially attractive about this paper is the way the researchers' arguments are based on robust measurements, not just theory."