For the first time, astronomers have observed a fast-rotating star emitting polarised light, a phenomenon that was first predicted over 50 years ago but has eluded our instruments until now.

And based on these findings, they have finally confirmed the crazy spinning rate of Regulus - one of the brightest stars we can see from our planet.

To understand what the big deal here is, we need to backtrack just a little bit to understand what polarised light is.

Normally, light waves travel in whatever direction they please, hitting and bouncing off objects all around us (which is why we can see stuff with our eyes). But light waves can also be polarised, which means they all get rotated in a particular direction.

In 1968 a pair of astronomers, J. Patrick Harrington and George W. Collins II, predicted that a rapidly rotating star would emit polarised light because it spins so crazy fast that its shape gets distorted into a squished oblate shape.

They based their idea on calculations done in 1946 by Nobel-winning physicist Subrahmanyan Chandrasekhar, who first proposed that some stars could be emitting polarised light.

But while these ideas over 50 years ago did prompt the development of a range of instruments aimed at detecting polarisation out there in interstellar space, until now astronomers hadn't actually caught polarisation from a fast-spinning star.

Now an international team from Australia, the US, and the UK has done just that, thanks to a highly sensitive polarimeter developed at the University of New South Wales (UNSW) in Sydney.

"The High Precision Polarimetric Instrument, HIPPI, is the world's most sensitive astronomical polarimeter," says one of the team, astronomer Daniel Cotton from UNSW.

The team pointed HIPPI at Regulus, a blueish first-magnitude star 79 light-years away. It's located in the constellation Leo, and ranked as the 22nd brightest star in the night sky.

Previously, astronomers had extrapolated the star's rotation rate based on models calculated for other stars of its type. But they couldn't confirm this interpretation without more observations directly from Regulus.

Now, thanks to this first-ever detection of polarised light from a rapidly rotating star, we know that Regulus is indeed spinning like mad at 320 kilometres per second (199 miles per second).

That's so fast, the star is basically teetering on the edge of breaking itself up.

"We found Regulus is rotating so quickly it is close to flying apart, with a spin rate of 96.5 percent of the angular velocity for break-up," says Cotton.

This new measurement is useful not just for understanding Regulus itself - it's opening up new ways for us to reveal more details of some of the largest and hottest stars out there, allowing us to find out more about their life cycles.

"Previously, the field has been largely restricted to studying material external to stars or those with extreme magnetic fields," the team writes in the study.

"Now we are able to probe fundamental parameters of the stellar atmosphere itself."

The findings were published in Nature Astronomy.

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