Why aren’t there any green stars?

An eagle-eyed viewer noticed that there were apparently no green stars in the deep-sky images and she wondered why this should be.

Not a particularly easy question to answer, but it is a combination of the characteristics of black-body radiation and the physiology of the eye (colour cameras of course are built to have a similar colour response as the eye – otherwise recorded images, photos,  wouldn’t look too good!).

The blackbody spectrum from hot bodies at different temperatures is continuous from long to short wavelengths, and peaks at a wavelength that is a function of the temperature.  So, if we heat up a bar of steel with a blowtorch it will appear to get red hot, then yellow/orange hot and finally blue/white hot just as it is about to melt.  But it never appears green hot.  We can treat the stars as black body radiators in a similar fashion to the hot steel bar and again we note that the green part of the spectrum, or green stars, are missing.  The reason for this is that due to the continuous  nature of the blackbody spectrum, although the peak wavelength might be in the green region, there is also plenty of emission occuring at the red and blue wavelengths as well.  The eye/brain system “mixes” these colours together so the result is something close to white light (sunlight actually peaks in the green part of the spectrum, but we know that neither the Sun nor its light appear green!).  At lower temperatures we would see a much bigger contribution from the red/infrared part of the spectrum – and as we can’t “see” infrared radiation, the emitting body would appear red.  At much higher temperatures with emission peaking in the blue/violet/ultraviolet region – the emitting body would appear blue as we cannot see ultraviolet light (unless we’ve had a cataract operation and had our organic lens replaced by a plastic one).

So, it is a complex combination of the physiology of the eye (or spectral response of the camera) plus the characteristics of blackbody radiation that leads us to the extraordinary result that we don’t see green stars.

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2 Responses to Why aren’t there any green stars?

  1. Graeme says:

    Good explanation, Greg. There’s one star that *appears* green which is an interesting sight in a telescope – Antares in Scorpius is a large red supergiant which has a blue B-type companion at 3″ separation (mag 6.5). However, the eye plays tricks on us and it is often described as green through a telescope – I think it’s a contrast effect with the orange/red colour of the brighter Antares A.

    Very weird – and not helped much in the UK by the fact Antares doesn’t get very high in the sky so it’s like looking through a swimming pool when trying to observe it!

  2. Greg Parker says:

    True Graeme 🙂 I didn’t want to go into the detail to explain a counter example (which as you say isn’t strictly a “green” star) – but you are quite right!

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