Why are there no green stars?

Interestingly, the answer doesn’t have much to do with stars and is really a consequence of how our eyes and brains work. Watch this video on how stars emit light and how our eyes and brains receive it:

I don’t have anything to add to what the video says about stars. I am more interested in the discussion of our perception and how our eyes work. The way our eyes work is that if it is hit by two different wavelengths of light where one would individually be perceived as red and the other would be individually perceived as green, when both hit our eyes at the same time we see yellow. A pure yellow wavelength is indistinguishable from a matched mix of green and red. Our computer monitors only emit three colors but can rely on your eyes and brain tricking itself into believing it is seeing all the colors. If we had better eyes, designing monitors would be much more of a challenge.

In the real world though, there is range of wavelengths which would be pure yellow. The reason there are no green stars isn’t because there aren’t stars with emissions peaking at green wavelength light, its because those stars are also always emitting red light and our eyes and brains simply will never process that combination so we perceive it as green. Do green stars definitely exist and we just can’t perceive them or since we can never perceive green stars does that mean they just don’t exist? I tend to lean towards the former because I don’t think the existence of things is determined by our perceptions of them or lack thereof.

The reason I am bringing this up is because all, literally all, religions talk about an unseen world. They use all sorts of interesting names for things which operate in this unseen world. Angels, demons, Devas, ashuras, kappas, and a bunch of other things. What I find really interesting is that while the similarity to ancient teachings is basically never admitted, science has all but confirmed our visual perception is basically garbage. That is, we really don’t perceive things very well which necessitates that there really must be an unseen world. That doesn’t prove any of the other stuff exists, but it is proof at least half the formula for the unseen world is true.

[I just think kappa folklore is funny so any excuse to reference it will do]

Let’s explore how and why our perception is overall fairly poor so we can really nail that down. We have a bunch of rods in our peripheral vision detecting changes in light without much detail. Its basically a motion detector looking for predators. Then we have three types of cones concentrated in the (almost) center of the eye called the fovea which puts in the brunt of the work on the things we focus on. You can see how our three cones receive light below

You will notice that two of the ranges overlap by quite a bit. You are correct in inferring that if you want better perception that is probably not the best way to go about it. Theoretically you want less overlap so there is more contrast between the detections. The way these cells work is a binary. They are on or off. The brain takes that on/off information and generates an image. You can see how yellow light is activating two cones to a high degree so both activated means yellow to the brain. But you can achieve the same result of activating both cones simultaneously with green and red light. Your brain has no way to know how the cones got activated, only that they did.

Well, the reason for what we are looking at is that our ancestors were essentially the equivalent of dedicated nocturnal rats for hundreds of millions of years. The result of all that is that color vision isn’t very useful at night so mammals lost cone genes and thus mostly don’t have good color vision. Rods and two cones. Its wrong to say there is no color there but we call people in that state color blind. Though color blind is, from what I have heard, normal for cats, dogs, lions tigers and bears. It certainly is not a complete lack of color perception though.

Primates in their history switched to become diurnal and moved to having a lot of fruit in their diet. Fruit tends to be colorful so color vision became more useful. At some point one of the cone genes duplicated on the X chromosome and then mutated slightly, improving primate color vision also very slightly by changing the wavelength detection range a little bit. I mention in my book the X chromosome is a good spot for genes you want to rapidly evolve and color vision fits that pattern. This change is recent though and thus not all that definitive or highly differentiated. A recent evolution in progress. In theory it could get much, much better than it currently is.

Its interesting to think about the much better perception of birds and arthropods. Not having obligatory nocturnal ancestors they made out a bit better with their color vision. You can see below birds have one more cone than us and the range of detection overlaps a bit less. Or at least peak detection wavelengths are more distinguished. They also can see things in ultraviolet.

The animal which has the best known color vision is the mantis shrimp. See also.

Mantis shrimp have 12 different cones (or cone equivalents) and thus their color perception is (presumably) extremely extensive is scope. Quite literally unimaginable. What would a rainbow look like to a Mantis Shrimp? I don’t suppose that is anything a human would ever be able to experience.

On top of this, you can add to it that the visible spectrum is quite limited compared to all wavelengths of light.

This graph also demonstrates why the visible spectrum is where it is. Those are the frequencies which can pass through the Earth’s atmosphere. It wouldn’t make much sense to be able to detect light which gets absorbed before it can bounce off any object or hit your eye.

Between the fact there are odd quirks with how our brain interprets the light received as a result the limited color sensitivity by our cone cells, that we can’t perceive the vast majority of light at all, and even if we could a lot of it would never make it through the atmosphere anyway, there is a lot of room for unknown and unseen things to exist and be doing things we just will never know about. That doesn’t mean there are any unseen things, it just means there could be based on what we know about how humans interact with light. It also means things which want to remain hidden only really need to address a pretty small range of frequencies to avoid notice. Contemplating the possibilities and implications of this can be interesting and productive.

I suppose mechanical detectors should have alleviated this deficiency by now. But most of those are probably operated by the government and it wouldn’t surprise me at all if they are hiding and lying about things.