Clues to the starry riddle that is the magnetar

Astronomers said 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 with magnetic fields millions of times greater than that of 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 it 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 probe. "Stars this massive are expected to collapse to form black holes after their deaths, not neutron stars."