Suppose a binary star system has relatively a dim red star (such as a red giant or red dwarf) and a very bright blue star. From standing on a planet much closer to the red star, would it be possible to look up into the sky and see something like this:

enter image description here

where the lighter-coloured part is reflected light from the blue star.

There are really two questions here: (1) would the red star reflect light from the blue one at all, given that its surface is made of slightly ionised hydrogen gas (in contrast to planets like Jupiter, which are mostly not ionised); and (2) would the amount of light reflected be enough not to be overwhelmed by the light from the red star?

I'm also interested in whether the colour difference between the emitted red light and reflected bluish light would be enough to be detected by the human eye. (We can assume the human is wearing very dark glasses.)

A bonus question: if star-reflected starlight of this kind does exist, are there cases where it can be detected from Earth, spectroscopically?

  • $\begingroup$ @probably_someone hmm, that's kind of a good point. My reasoning was that a star is mostly made of hydrogen and thus should reflect similarly to Jupiter, but as you point out that reasoning is flawed - I will amend the question. $\endgroup$ – Nathaniel Feb 15 '18 at 6:47
  • $\begingroup$ You're correct that stars are mostly made of hydrogen. The issue is that the hydrogen in stars is ionized, which gives a material with completely different optical properties. $\endgroup$ – probably_someone Feb 15 '18 at 6:48
  • $\begingroup$ @probably_someone yes, I realised that as soon as I read your comment. $\endgroup$ – Nathaniel Feb 15 '18 at 6:49
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    $\begingroup$ It might be testable for two stars of completely different composition and emission temperatures (e.g., a pure hydrogen/young star and a heavy metal/old star) by looking at absorption and emission line differences. I think this is still a stretch though as it would be difficult to distinguish the source of light in each case. In the heavy metal star, the heavy ions will be partially ionized and will have corresponding absorption and emission lines... $\endgroup$ – honeste_vivere Feb 15 '18 at 16:35
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    $\begingroup$ If one had a "nice" scenario where the spectrum from the heavy metal star did not overlap well with some of these absorption lines but we clearly observed them, one might be able to infer that they result from the light from the second star. I am not sure whether we can then take it a step further to determine reflectivity though. $\endgroup$ – honeste_vivere Feb 15 '18 at 16:36

Stars are almost perfect black bodies so their emissivity is approximately one. As a consequence their absorbance is also approximately one and their reflectivity is approximately zero.

In the situation you describe the cooler star would absorb the light from the hot star and heat up. The increase in temperature would change its spectrum in accordance with Planck's law. If the cooler star rotated slowly enough, and the heat flow within it was slow enough, then the side nearer the hotter star could become detectably hotter and therefore different in colour. I would guess this is only likely in close binaries that are tidally locked.

A disclaimer: presumably if we cool the star enough for the optical surface to be mostly neutral gas rather than plasma the reflectivity could become significant and the argument above no longer applies. Whether such an object still counts as a star is debatable.

  • $\begingroup$ The surface of a star is mostly neutral gas - see astronomy.stackexchange.com/questions/7883/… for example. For many purposes Jupiter is also approximately a black body, but it very much does reflect light in the visible spectrum. I'm not saying this is wrong necessarily, but I don't think it's obvious, and I would need a citation to be convinced. $\endgroup$ – Nathaniel Feb 15 '18 at 7:42
  • $\begingroup$ @Nathaniel I think you have misinterpreted that question and answer. The optical surface of star is the radius in the photosphere at with the degree of ionisation is great enough that it absorbs light. For a star like the Sun the optical surface is definitely ionised gas. $\endgroup$ – John Rennie Feb 15 '18 at 8:15
  • $\begingroup$ Ok, I'll take that, but given that there evidently is more neutral gas outside of that, it isn't clear to me that some incident light wouldn't be reflected from that gas, before it reaches the optical surface (so defined). $\endgroup$ – Nathaniel Feb 15 '18 at 8:24
  • $\begingroup$ Sorry, neutral gas. Typo. $\endgroup$ – Nathaniel Feb 15 '18 at 8:27
  • $\begingroup$ Jupiter is not transparent. It depends how much there is. $\endgroup$ – Nathaniel Feb 15 '18 at 8:28

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