Earth is more than just the beautiful blue marble we all know and love; it’s also a giant magnet, with a field extending far out into space generated by the movement of molten elements deep within its core. This magnetic field protects surface life from harmful cosmic radiation, creates the glowing auroras in high-latitude skies, and tells compasses in both survival kits and animal brains which way it is to the poles.
Our sun is an even bigger and more complex magnet, with a field that stretches throughout the solar system and tangles into twisting and snapping knots across its surface. But there are even more powerful magnets out there in the galaxy — unbelievably dense stellar corpses with magnetic fields thousands of millions of millions of times more powerful than Earth’s. Called magnetars, they are created by the collapse of very massive stars but why they don’t become black holes instead has been a question that has long puzzled astronomers… and now they think they have the answer.
Magnetars are actually a rare type of neutron star, the leftover core of a star more than 10 times the mass of our sun that’s gone supernova. Neutron stars are incredibly dense, as they’re nearly an entire star’s worth of stuff collapsed into a ball the size of a small city. A teaspoonful of neutron star would weigh over five billion tons.
Neutron stars spin rapidly, emit powerful radiation — even by stellar standards — have a crazy gravitational pull, and very strong magnetic fields. But magnetars are like neutron stars on steroids; they blast out powerful gamma rays, have quite literally lethal magnetic fields (deadly from even hundreds of miles away), and are even more massive than neutron stars. There have been very few of them found in the galaxy, and why those couple dozen didn’t turn into black holes (what with all that mass) may be due to a little help from their stellar friends. In at least one magnetar’s case, a former friend.
An international team of astronomers studying a young star system 16,000 light-years away (Westerlund 1) known to contain a magnetar have spotted the “smoking gun” of a proposed theory to their formation: a low-mass, carbon-rich star speeding away from the scene, a star that could have only formed as one of a pair but has since been sent packing at high velocity.
The runaway star was once the more massive in a close binary pair, the researchers suggest. As it began to run out of fuel, it shed its outer layers (as stars do), transferring some of that material to its companion. The sharing of matter gave the lower-mass star extra spin, and as it gained the hand-me-down mass it, too, began to kick off its outer layers, eventually exploding in a supernova and flinging its erstwhile partner out into space (with a little extra mass back as a parting re-gift).
The combination of a turbocharged rotation rate and crash diet help to create a magnetar out of the remaining collapsed star, rather than a black hole or garden-variety neutron star.
“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 — a game of stellar pass-the-parcel with cosmic consequences!” said research team member Francisco Najarro of Centro de Astrobiología, Spain.
The observations, made with the FLAMES instrument on the European Southern Observatory’s VLT in Chile, are helping to solve the 35-year-old mystery of the Milky Way’s magnetars. The team’s findings will be published in the journal Astronomy and Astrophysics. Read more in the ESO news release here.