We can see the beauty of white, yellow, red, orange, and blue stars, but not green. Why is this?
In: Physics
I supposed that if you were travelling at the right velocity, you could experience a blueshift causing a star to appear green.
There are actually many colors emitted by stars, including green. What you call white stars are actually many colors together. These colors come out of the different chemical elements in the star, usually mostly hydrogen, some helium, and the rest (~1%) made of several other elements. Hydrogen itself, though dominated by red light when excited, also emits violet, blue and green. These colors get scattered in our atmosphere and that’s why you see mostly yellow-orange from the sun and a blue sky.
But what about viewing it outside our atmosphere? Well, you still won’t see green stars, but that has two reasons — any star burning hot enough to produce most of its light as green will also be producing blue and red light, and human eyes will mix these and perceive it as white (or yellow if it’s equally red and green without blue).
All objects emit electromagnetic EM radiation (“light”) based on their temperature. They emit pretty much all frequencies, peaking at a certain point and tapering off in either direction. An object that is “red hot” peaks in the infra-red range, and as we look at the visible spectrum, the light is more intense at the red end and tapering off towards the blue, so the object glows red. A much hotter object will peak in the ultra-violet range; the visible portion of its light will be dimmer* in the red end and brighter in the blue, making the object appear “blue-hot.”
*(relatively. Overall output is much higher for the hotter object, but the “red” side of its radiation curve is relatively dimmer than the “blue” side.)
So what about something in between, something that peaks right in the middle of the green part of the visible spectrum? Well the curve doesn’t have a sharp point in the middle, it’s actually relatively flat on top (and the visible spectrum is comparatively extremely narrow), so anything that peaks near the visible spectrum will have the intensity spread pretty evenly across all frequencies that we can see, making the object appear “white hot” instead.
The green color sits right in the middle of the color spectrum, and the blackbody radiation curve is a broad bell curve, much wider than a specific color. If the peak is in infrared, it also captures a bit of red, and we have a red star. If the peak is in red, it also captures a lot of green and some blue, and we get a yellow star. If the peak is in green, it also captures a lot of red and blue, and we get a white star. If the peak is in blue, it also captures a lot of green and some red, and we get a light bluish cyan star. Finally, if the peak is in UV, it also captures some blue and we have a blue star.
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