There will be those who are too lazy to walk the dog at night (especially in winter) or at 6:30 in the morning, but I love finding it all asleep and being able to calmly raise my head so that, while I walk around the outskirts of my city so that my dogs can do their things, look at the sky to see what I find.
One of my hobbies is, like when I was little, trying to distinguish what I find: The Polar Star, at this time of year also El Carro, the very bright Sirius with a bluish white or orange tone Aldebaranthe red Betelgeuse or the blue supergiant Rigel. Although there is light pollutionFortunately, I live near several municipalities starlight and it doesn’t take much to find authentic shows. I have seen stars of a few colors, but never green.
The reality is that I look at the stars with no other instrument than my eyes and that implies only see those bright enough to emit enough light to activate the conesthe color-sensitive cells that are in the retina. But still, the stars I encounter seem white, blue, red, orange or yellow. Viewing the sky with a telescope things change and there you can see fainter stars or even go up a level and take a look (and try to interpret what space telescopes see). What is hard to see are green stars.
Spoiler: it is as much a matter of the stars as it is of our eyes and their way of perceiving color.
The Sun’s peak emission is green. There are several reasons why we never see it that color.
A star behaves like a black bodythat is, it emits light depending on its temperature. Simply put: they emit light because they are hot. In fact, their color depends on their temperature: the coldest ones emit red light and those that are very hot glow blue.
Different Planck curves for stars of classic colors: blue, yellow, red. POT
However, this is a simplification: they actually emit in a wide range of colors but in different proportions in an asymmetrical bell shape (the Planck curve that you see above these lines) and it is the mixture that gives that final color. The relationship between the temperature of a star and the color where it emits the greatest amount of energy (the tip of the curve) is obtained from the Wien Displacement Law.
If a star has a temperature of about 5,500K (very similar to that of the Sun), its peak emission would be precisely in that green zone. But we have never seen the sun be green. Here our eyes come into play: the color of a star is not an intrinsic property of light, but rather an interpretation of our eyes against that jumble of photons.
The eyes have three types of coneseach of them sensitive to red, green or blue light respectively. That is, if an object emits or reflects red light, only the red cones would send a signal to the brain to perceive it that way. Obviously the brain can interpret more colors: the key is that these three types of cones can send the signal in different proportions and then they are mixed in the brain.
Precisely what happens with the Sun, which although it emits the greatest amount of light in blue and green, simultaneously emits so much red and blue light that the combination ends up averaging in the brain as white (another thing is that is classified like a yellow dwarf star.
Due to the physics of black bodies, there is no stellar temperature that excites only the green cones of the eye without also activating the red and blue cones. And since cameras imitate the vision of the eyes, the stars do not appear green in the photos either.
Yes, but. After all this explanation it turns out that there are a couple of stars that some people claim to see green, but no: in reality They are brain tricks. It is the case of Almacha star system formed by a giant, bright orange star and a triple system of three blue stars so close that from our perspective they cannot be distinguished separately, but rather blend into a point of light. Our brain tries to balance orange with its complementary color, green. The result: we can perceive it as orange. Of course, the cameras do not take issue with this biological processing error. The other is Zubeneschamalia solitary star whose color we confuse either by subjective perception or by effects of the atmosphere.
That doesn’t mean there aren’t green celestial objects in the sky. There are a few nebulae with a very striking green tone thanks to the intense emission of oxygen atoms, we have also seen emerald green comets (the blame lies with diatomic carbon) or planets like Earth for obvious reasons and even Uranus because of the methane in its atmosphere and how it absorbs light.
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Cover | Parastoo Maleki
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