New findings reveal the makings of magnetars. Astronomers have figured out how to make the universe’s most powerful magnet. All you need is two massive stars orbiting close to each other so that one swipes gas from the other, causing the thief to spin so quickly that its magnetic field dwarfs that of Earth by 100 trillion-fold. The finding offers fresh insight into how some of the galaxy's smallest but most extraordinary stars arise.
Magnetars are a special breed of pulsars, which are fast-spinning neutron stars that form when a massive star explodes as a supernova: The star's outer layers shoot off into space, while its core collapses to become the pulsar. Magnetars are as rare as they are extraordinary. Known pulsars number in the thousands; known magnetars, only a couple of dozen.
Astronomer Simon Clark of the Open University in Milton Keynes, U.K., and his colleagues observed a young star cluster named Westerlund 1, which sports one of the few known magnetars. The cluster is only 5 million years old and lies 16,000 light-years from Earth in Ara, a constellation just south of Scorpius.
The astronomers identified a peculiar blue supergiant—a star much hotter and more luminous than the sun—that they believe once orbited the star that later became the magnetar. Named Westerlund 1-5, the blue supergiant dumped large amounts of gas onto its partner, speeding up its spin the way falling water makes a water wheel twirl. As Clark's team reports online this week in Astronomy & Astrophysics, this spin-up amplified the star's magnetic field so that when it exploded and collapsed, it became a magnetar rather than an ordinary pulsar.
Furthermore, the blue supergiant saved its partner from a bleak fate. The premagnetar star was so massive that it should have collapsed into a black hole. But before it exploded, it began to expand, as aging stars do, and its partner grabbed enough gas back that the premagnetar star slimmed down, becoming a magnetar rather than a black hole. This removal of material also kept the premagnetar star spinning fast; normally, expanding stars spin more slowly, just as spinning ice skaters do when they extend their arms.
The evidence? First, the blue supergiant is racing away from the cluster, suggesting that another star recently kicked it away when it exploded. Second, the blue supergiant has odd abundances of carbon, nitrogen, and oxygen.