Zero Genie

More than 60 years ago, astronomers realized about 10 percent of massive stars have powerful magnetic fields bursting from their surfaces. But the exact origins of these magnetic fields —which can reach hundreds to thousands of times the strength of the Sun’s — has so far remained a mystery.

The answer, it turns out, may be due to a collision between two normal stars.

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A team of scientists recently used cutting-edge simulations to uncover an evolutionary path they think explains the formation of extremely magnetic stars. And as a cherry on top, their findings may also shed light on the origins of a slew of other astronomical oddities. These mysteries include magnetars (a rare type of hyper-magnetic neutron star), blue stragglers (massive stars that appear too young for their age), and maybe even enigmatic cosmic events like fast radio bursts and super-luminous supernovae.

The research was published October 9 in the journal Nature.

Magnetic mergers

When two stars collide, it sends their surfaces spinning and simultaneously kicks off enormous amounts of turbulence. This dramatically boosts the final star’s magnetic field.

As the star spins, its inner layers rotate faster than its outer layers — a process called differential rotation. Running through and connecting each of these layers are magnetic field lines, Fabian Schneider of Heidelberg University and author of the new study told Astronomy. Because each layer rotates at a different speed, the magnetic field lines connecting the layers get twisted and tangled up, Schneider says. This serves to amplify the overall strength of the magnetic field.  

“Now comes the turbulence,” Schneider explained. During a merger event, stellar material gets violently sloshed around. This turbulence further stirs the magnetic field lines, exponentially increasing the star’s magnetism.  

But the new research doesn’t just describe how colliding stars can form insanely magnetized stars, it also may explain the origins of a bizarre class of strange objects called blue stragglers.

Making a blue straggler

Blue stragglers are a unique class of stars that masquerade as stars younger than they truly are. These “rejuvenated” stars are much hotter — making them bluer — and brighter than your average main-sequence (or middle-aged) star of a similar apparent age.

But what is the fountain of youth that keeps blue stragglers looking so fresh? A leading theory is that merging with another star will do the trick. And this new research supports that notion.

First off, typical main-sequence stars power themselves by fusing hydrogen into helium in their cores. But when the hydrogen in their cores runs out, they move on to fusing concentric shells of hydrogen around their now-inert cores. This causes the star to balloon up into a red giant, moving it off the main-sequence and into the so-called red giant branch.

But if two main-sequence stars collide, their material gets mixed together. The resulting merged product now has a restocked reservoir of hydrogen in its core, which allows it to chug along as a more massive — yet still main-sequence — blue straggler instead of evolving into a red giant.

“This just means that post-merger stars have more nuclear fuel to then live longer,” Schneider says. “In other words, their internal clock has been set back.”

But that only makes the newly formed star appear younger. “The point is simply that the blue straggler could have lived for a long time as lower-mass stars and then merged to become this more massive blue straggler,” Schneider says. “It’s high mass fooling us into thinking it must be younger.”