Physicists have long known that the Sun spins, like the Earth. But a few decades ago, they realised the surface of the Sun spins more slowly than their models predicted — not by a lot, but enough to signal that something they didn't understand was going on. This kicked off a solar mystery, and some scientists started to doubt their own understanding of the Sun's behaviour.
A team of astronomers have stared into the Sun long enough that they think they have found the source of the slowdown — and it's a really elegant hypothesis. Other scientists worry that the idea is too speculative. But if the new hypothesis is correct, then the team may have uncovered a behaviour common to all of the stars in the universe.
"You can imagine in the case of bright quasars or stars much heavier than the Sun or many thousands of times brighter," Jeff Kuhn, one of the astronomers behind the hypothesis from the University of Hawaii told Gizmodo, "this slowing down effect could be important to the life history of these objects."
The Sun is a complex beast to study. Most of its interior spins uniformly like a ball, surrounded by a turbulent, bubbling mess called the convection zone. The comparatively tiny 500km-thick outermost region responsible for radiating light flows more smoothly, but the outer 70km spins about five per cent slower than the interior, according to the American Physical Society's Physics. "This is surprising because it isn't understood from the physics of convection and turbulence," said Kuhn, "and it's in a place where the atmosphere is mostly stable." Why should a stable region slow down like this?
The team's theory is almost too simple: If the Sun is spinning, then it should spit out light particles at an angle. Those light particles would produce a rotational force, called torque, which could cause the slowdown over time. Kuhn said to think of spinning sprinkler — the water is angled, so when you turn it on, it causes the stationary fixed sprinkler to rotate in the opposite direction of the water. But if you spun a really heavy sprinkler in the direction the water shoots before turning it on, the water would just slow the spin down. And if the Sun is slowing this way, then other objects in the night sky are, too, said Kuhn.
The astronomers made lots of observations of the Sun to bolster their hypothesis, measured using NASA's Solar Dynamics Observatory satellite. They calculated the rotation speeds by looking at the edges of their solar images, observing how acoustic waves similar to the earthquake-causing waves on Earth travel from the Sun's surface into its core and back. This allowed the researchers to make detailed measurements of the rotational slowdown, which they published last week in the journal Physical Review Letters.
Other physicists thought the observations were strong, but couldn't help but point out that the speculative nature of the theory. "Their method they used was really clever, and with the amount of data they looked at to analyse this, they did a lot of work to get this result. That's pretty impressive," Michael Hahn, associate research scientist in astronomy from Columbia University, told Gizmodo. "Their explanation [of the slowdown], I'm not sure about that too well. Maybe, maybe not." Kuhn and his team say the torque acting over time could increase the slowdown deeper down, but this requires assumptions about just how smoothly the gases flow inside the Sun, or its viscosity. "They make an estimate for what kind of viscosity they'd need for this to be important," said Hahn. "It's quite a bit larger than the actual viscosity," though Kuhn notes in the paper that the turbulence could possibly increase the viscosity.
Kuhn was still sure about his team's observations, given the amount of data and his calculations, and the maths behind how a torque from the exiting photons would sum up to his team's observation over time. But he sort of knew that the ideas would be controversial.
"This is a bit of a firehouse," said Kuhn. "Convection theory is a hotbed of lots different ideas."
Scott McIntosh, director of the High Altitude Observatory at the National Center for Atmospheric Research in Hawaii, offered Gizmodo a more wry take summarising how difficult establishing facts about our Sun can be. "The work presents an interesting hypothesis about how the Sun reached its current condition that only millennia of further observation can truly validate."