Astronomers said Wednesday they may have found the answer to a cosmic riddle called the magnetar -- a star so dense that just a teaspoonful of it would have a mass of about a billion tonnes. Magnetars are mysterious phenomena whose magnetic fields are millions of times greater than that of the Earth. They also erupt with storms of gamma radiation when their crust undergoes sudden modification, a change called a starquake. How these oddities are formed, though, has until now been unclear. They are considered to be a type of neutron star, which is one of two potential outcomes when a massive star collapses under its own gravity and rips apart to form a supernova. Of some two dozen known magnetars in the Milky Way, a favoured target for astronomers is called CXOU JI64710.2, located in Westerlund 1, a star cluster about 16,000 light years away in the constellation Ara (The Altar). Previous work determined this magnetar was born from the supernova of a mega-star 40 times as massive as the Sun -- but that finding posed a headache in itself. "We did not understand how it could have become a magnetar," said Simon Clark of the European Southern Observatory (ESO), who led the latest probe into CXOU J164710.2. "Stars this massive are expected to collapse to form black holes after their deaths, not neutron stars." Using ESO's Very Large Telescope, located in the arid highlands of the Chilean desert, Clark's team found a clue for the conundrum in a massive star called Westerlund 1-5 in the same star cluster. It is travelling at ultra-high velocity out of the cluster, expelled by the force of the supernova. Its trajectory and speed provide evidence that it somehow played a part in creating the magnetar CXOU J164710.2, the astronomers said. According to their simulation, Westerlund 1-5 was once a nearby companion to another massive, though slightly smaller, star. - Cosmic pass-the-parcel - The bigger of the two started to run out of fuel and transferred its outer layers to the other -- the future magnetar -- causing it to rotate fiercely and develop a powerful magnetic field. The transfer caused the smaller star to become so big that it shed a large chunk of its newly acquired mass, according to the theory. Gravitational pull transferred this mass back to the original star, which is shining today as Westerlund 1-5. The companion star exploded, becoming a magnetar-type neutron star, and Westerlund 1-5 was kicked into the great beyond, the reconstruction suggests. Spanish astrophysicist Francisco Najarro described the process as "a game of stellar pass-the-parcel with cosmic consequences." He explained: "It is this process of swapping material that has imparted the unique chemical signature to Westerlund 1-5 and allowed the mass of its companion to shrink to low enough levels that a magnetar was born instead of a black hole." The explanation may apply to all magnetars, ESO said. "It seems that being a component of a double star may... be an essential ingredient in the recipe for forming a magnetar," it said in a press release. "The rapid rotation created by mass transfer between the two stars appears necessary to generate the ultra-strong magnetic field, and then a second mass transfer phase allows the magnetar-to-be to slim down sufficiently so that it does not collapse into a black hole at the moment of its death." The study will be published in the journal Astronomy and Astrophysics.