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Black holes are thought to form after the collapse of a large star at least ten to fifteen times the mass of our sun, but the discovery of a rare magnetic star in the Westerlund 1 star cluster is questioning those theories.

A magnetic star or Magnetar is a type of neutron star with a hugely powerful magnetic field. Neutron stars form after the original star collapses, the current theory was that the original star must be of at least 1.5 times the mass of our sun but no more than five times its mass.

Neutron stars are small, only around 20 kilometers in diameter but they have a mass of almost one and half times that our sun, one teaspoon of a neutron star would weigh hundreds of billions of kilos.

The reason this discovery of this Magnetar has been so surprising is that its original mass was at least 40 times the mass of our sun. Collapsing stars with a mass as large as this usually form into a black hole.

One explanation given for this anomaly is that the original star was actually part of a binary system, the other star could have removed the majority of its mass leaving it with only around four times the mass of our sun before it exploded as a supernova.

The magnetar was discovered in a star cluster 16,000 light years away by a study led by the Open University (UK) and the University of Alicante (Spain).