It may not look like it from here on Earth, but the burning ball of plasma in our sky rotates on its axis roughly once a month.

We've known this since 1611, when astronomer Johannes Fabricius watched sunspots track across the surface of the Sun.

But in 1996, researchers made a surprising discovery - the outer layer of the Sun seems to be spinning more slowly than the layers beneath it. Not only that, but that outer layer is actually gradually getting even slower.

This led to a slightly concerning question - what in the vacuum of space could apply a drag to the surface of our Sun? As Derek Muller explains in the latest episode of Sciencium, it's as simple and as complex as sunlight.

But how do we even know what's happening 150 million kilometres (93 million miles) in our Sun in the first place? That's thanks to the solar equivalent of earthquakes.

Turbulence in the convection zone of the Sun causes massive vibrations that actually make our Sun ring like a bell - we can even see traces of these seismic waves in the photosphere, the visible surface of the Sun.

Based on these waves, scientists can infer the internal structure of the Sun, just like an ultrasound. And what researchers have seen is that the outer layer of the Sun rotates slower than the interior.

In particular, the outer 'skin' of our star is moving even more slowly, providing clear evidence that the Sun is slowing down from the outside in.

Now, scientists think they've finally figured out why.

The clue came from an old observation about dust particles orbiting around stars. You might expect those particles to orbit forever, like the planets around the Sun. But that's not what happens.

Instead, the dust particles gradually lose energy and spiral into the star. 

The source of drag on those particles isn't friction - they lose energy even if they never collide with anything. In fact, the only thing they always collide with is particles of light - photons - coming from the star.

As Derek explains in the video above, from the perspective of dust, these photons appear to be coming from slightly in front, just like raindrops appear to fly into your windshield on a highway.

And just like those raindrops slow your car down ever so slightly, photons can also slow down these dust particles - photons might not have mass, but they do have momentum.

This braking effect is known as the Poynting-Robertson effect. But how does it explain the slowing surface of the Sun?

As Derek explains, photons are actually stealing angular momentum from the dust. And it looks like the same thing is happening to the outer layers of the Sun.

Photons bounce their way from deep within the Sun over tens of thousands of years, and when they make that final bounce off one of the Sun's particles and escape into space, they're taking a tiny piece of the Sun's angular momentum with them.

The team calculated the theoretical braking torque this effect would have on the Sun, and showed that it could accurately explain how much the Sun's outer layer has slowed down over its lifetime.

In fact, this is the only hypothesis so far that matches what we're seeing happening in our Sun.

The idea that something as tiny as a photon could change the course of a star is pretty mind-blowing. But what's crazier still is how this effect could influence our Sun - and the lifespan of other stars in the Universe. Will the Sun ever stop spinning altogether?

We'll let Derek answer that in the video above, but one thing's for sure, we won't be underestimating the power of a photon again.