Stars don’t just produce light – they reflect it too, and no one noticed until now

We tend to think of stars as the objects that make most of the light in the Universe, while planets, moons, rocks, dust and gas reflect this starlight. But it turns out that stars reflect light, too.

We haven’t really noticed this phenomenon before, because the amount of reflected light is minuscule compared to the colossal amounts stars generate in the first place.

The source of this revelation is a new study on binary stars – twins locked in a spiralling mutual orbit, each reflecting a tiny amount of the other’s light.

Spica is a binary star located around 250 light-years away in the constellation of Virgo, the two stars so close to each other that one orbit takes just four days. They are so hot and close together, the pair cannot be visually resolved individually – instead, changes in the light spectra reveal each individual star.

Now a team of astronomers has found that the polarisation of the light, or the orientation of its wave, varies as the two stars orbit.

Light that travels directly is unpolarised, oscillating along multiple planes at once. When it’s reflected off a nonmetallic surface, the light becomes polarised, and oscillates along just one plane.

The team’s observation of polarised light emanating from Spica indicates that reflection is occurring, so they deployed some computer models to figure out what was going on.

“We were able to determine that the amount of polarisation we observed was exactly that predicted for a reflected light model,” said physicist Jeremy Bailey of the University of New South Wales in Australia.

“Our modelling showed that stars are actually quite poor reflectors of light. The Sun, for example, reflects less than 0.1 percent of the light falling on it.

“However, for hotter stars, such as the components of Spica, with temperatures of 20,000 to 25,000 Kelvin, the amount of reflection increases to a few per cent. The total amount of reflected light coming from the Spica system is, however, still very small.”

That total of reflected light is only a few percent of the incident light, but it can be easily discerned because it is so highly polarised, the researchers said. They developed their own high-precision polarimeters able to easily pick it up.

This research also adds a new tool to our kit for sniffing out binary stars. Say you have a binary system with a really close, face-on orbit, so that its spectra remain constant and you can’t tell them apart easily, nor can they be resolved optically. But they’d still be reflecting each other’s light, so polarisation could unmask them.

It can also reveal details about binary star systems. For instance, the polarisation of Spica confirmed that the system’s orbit is clockwise – consistent with previous findings. And it could, Bailey noted, be used to determine the masses of the stars in a binary.

It won’t actually help much with single stars, because they don’t tend to be close enough to another light source. Any light they’re reflecting is from very far away, and there’s just not enough of it to be detectable or useful.

But most stars have binary companions – as many as 85 percent. The Sun likely had a binary twin once upon a time, too, although disruptions of binary pairs are common. Astronomers believe that most, if not all, stars are born in pairs, but some lose their twins.

Which means that the study of binary pairs is actually pretty fundamental to the study of stars in general.

The team will be turning their polarimeters to other binary stars to further their research into the light the stars reflect off each other, to test the techniques on other systems, and to develop a new way of studying them.

“We expect it to work even better for hotter stars,” Bailey said, “and it could be used to find binary systems that are not detectable by other methods, and to study binary star orbits and properties.”

The research has been published in Nature Astronomy.

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