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How the Universe's Most Intense Magnetic Fields Are Born

A runaway star was "the smoking gun" that explains why some dying stars form black holes, while others become magnetars.
Artist's depiction of Westerlund 1's mysterious magnetar. Image via ESO/L. Calçada.

Magnetars are among the most bizarre objects in the universe, which if you haven't noticed, is a pretty competitive category. They are a special class of neutron stars capable of generating magnetic fields one thousand trillion times stronger than Earth's. Occasionally, they undergo traumatic “starquakes” that release the same amount of energy in one millisecond as the Sun emits in 150,000 years. They are cosmic weirdos, through and through.

Now, thanks to a study published in Astronomy and Astrophysics, one of the biggest open questions about magnetars may have been solved: how they form in the first place. Astronomers led by Simon Clark of the Open University have puzzled over this question for years, focusing in particular on a magnetar located in the star cluster Westerlund 1 about five kiloparsecs (roughly 16,000 light years) away.


The team had previously calculated that this magnetar, named CXOU J164710.2-455216, was the corpse of a massive star about 40 times larger than the Sun. But a star that gargantuan should have formed a black hole when it died. Why then did it leave behind a magnetar instead? Astronomers floated the idea that the star may have been in a binary system, but found no companion star near the magnetar.

Undeterred, they used the Very Large Telescope to track down any nearby stars that might have been forcefully ejected by the supernova that forged the magnetar. Sure enough, they discovered that Westerlund 1-5 was just such a “runaway star.”

“We originally made the observations back in 2008, and after we carefully analyzed the raw data we realized we had something unusual with Westerlund 1-5 in 2011,” Simon Clark, the study's lead author, told me. “Then it was a case of really carefully analyzing the data with sophisticated computer codes to determine what the star was made from and writing up our results. So it's been a long process.”

But the hard work paid off, because Westerlund 1-5 was a perfect match in more ways than one. “Not only does this star have the high velocity expected if it is recoiling from a supernova explosion, but the combination of its low mass, high luminosity, and carbon-rich composition appear impossible to replicate in a single star—a smoking gun that shows it must have originally formed with a binary companion,” said the study's co-author Ben Richie in a statement.

Westerlund 1 captured by the Very Large Telescope. Image via ESO/VPHAS+Survey/N. Wright.

So here's what Clark's team think happened: when Westerlund 1-5, the more massive star in the binary system, ran out of fuel, it started to transfer its stellar material to the smaller star. This caused the smaller star to spin extremely rapidly, which generated the insanely strong magnetic fields characteristic of a magnetar. Then, as the magnetar-in-training beefed up on its companion's runoff, it became the larger star, and the transfer was reversed.

“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 team member Francisco Najarro in a statement.

This ingenious reconstruction of the early relationship between Westerlund 1-5 and CXOU J164710.2-455216 may explain the formation of all magnetars, and Clark's team is eager to hunt for runaway stars nearby other magnetars.

“There are about two dozen [magnetars] that we know about, but to date, no one's actually looked for any runaway objects,” Clark told me. “There are another two examples of magnetars being found in star clusters where we might also hope to identify the companion being kicked out by the supernova that produced the magnetar so we can look at these.”

“Also, we hope to go back to Westerlund 1-5 and take very high resolution pictures of it over a number of years,” he added. “By comparing how the position of Westerlund 1-5 changes on the sky over time, this will allow us to determine the direction it's moving and hence work out if it was born in the same location as the magnetar.”

Whether all magnetars form by swapping material back and forth in this mind-boggling game of stellar hot potato “really still has to be determined,” said Clark. Even so, the decades-long mystery of magnetar formation is well on its way to a concrete solution—and one that is, not surprisingly, every bit as outlandish as the objects themselves.