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.

Tau.Neutrino said:

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.

This article is worth quoting in more detail.

> 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.

Don’t use the word “spiralling”. It’s wrong.

> 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.

Spectroscopic binary, very common.

> 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.

Nice

> 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.

Because the polarisation is affected by the star’s magnetic field, it is only unpolarised if that magnetic field is chaotic, such as that of the Sun and most stars. But not necessarily all stars. Reflected light doesn’t just oscillate along one plane. Only part of it, up to a maximum of about 50% IIRC is polarised.

> 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.

Nice work.

> “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.

That’d be about right. The Sun is not a perfect blackbody, It’s only with the advent of space telescopes that can see in UV and IR that we’ve been able to pin down the blackbody temperature of the photosphere to 5777 degrees. Which makes me wonder if the polarisation method might be more effective in other wavelengths.

> “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.

Interesting.

> This research also adds a new tool to our kit for sniffing out binary stars.

Yes. I wonder. Spectroscopic methods don’t work when we’re looking down the spin axis. This new method wouldn’t give a variation with time around the orbit, but it’s the ideal angle for maximum polarisation.

> But most stars have binary companions – as many as 85 percent.

Not that many. It’s more like 50%, and no more than about 65%.

> The Sun likely had a binary twin once upon a time.

Where’d they get that from? No.

mollwollfumble said:

Tau.Neutrino said:

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.

This article is worth quoting in more detail.

> 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.

Don’t use the word “spiralling”. It’s wrong.

> 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.

Spectroscopic binary, very common.

> 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.

Nice

> 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.

Because the polarisation is affected by the star’s magnetic field, it is only unpolarised if that magnetic field is chaotic, such as that of the Sun and most stars. But not necessarily all stars. Reflected light doesn’t just oscillate along one plane. Only part of it, up to a maximum of about 50% IIRC is polarised.

> 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.

Nice work.

> “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.

That’d be about right. The Sun is not a perfect blackbody, It’s only with the advent of space telescopes that can see in UV and IR that we’ve been able to pin down the blackbody temperature of the photosphere to 5777 degrees. Which makes me wonder if the polarisation method might be more effective in other wavelengths.

> “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.

Interesting.

> This research also adds a new tool to our kit for sniffing out binary stars.

Yes. I wonder. Spectroscopic methods don’t work when we’re looking down the spin axis. This new method wouldn’t give a variation with time around the orbit, but it’s the ideal angle for maximum polarisation.

> But most stars have binary companions – as many as 85 percent.

Not that many. It’s more like 50%, and no more than about 65%.

> The Sun likely had a binary twin once upon a time.

Where’d they get that from? No.

>> This research also adds a new tool to our kit for sniffing out binary stars.

> Yes. I wonder. Spectroscopic methods don’t work when we’re looking down the spin axis. This new method wouldn’t give a variation with time around the orbit, but it’s the ideal angle for maximum polarisation.

Oops, the magnitude of the polarisation doesn’t change around the orbit but the direction does, which is even more useful. So there is a time-variation around the orbit. What about hot Jupiters? They reflect light too, and that light would be polarised. Looking along the spin axis, it might be difficult to distinguish between a binary star and a hot Jupiter.