Our sun is really green, say astronomers

Ask anyone what colour the sun is and they will say yellow or orange or possibly red. They won’t say green. But in reality, the sun is blue/green. The sun emits more green light than any other color.

The sun emits energy at all wavelengths from radio to gamma ray. But, NASA studies show it emits most of its energy around 500 nm, which is close to blue-green light. So one might say that the sun is blue-green! 

So why don’t we see the sun as green? Well, while the dominant wavelength is green, it doesn’t outweigh the total amount of the other visible light wavelengths, which is why it doesn’t look green to the naked eye.

The sun’s radiation frequency is governed by the sun’s surface temperature, which is around 5,500 Celsius. A higher surface temperature would result in a shorter maximum wavelength and so the sun might peak in the blue or violet part of the spectrum.

As we know but sometimes forget, our sun is a star. And every star/sun is different. Each one being a different temperature and emitting different wavelengths of light. 

NASA say the temperatures of stars go all the way from around 3000 K to 50,000K. Blue stars are hotter and red stars are cooler. So it is quite possible that on other planets their sun could be purple or blue. If that was the case, then if that planet had evolved plants they might not be green. 

Astrobiologists say plants on Earth-sized planets orbiting stars somewhat brighter than the Sun may look yellow or orange, while those on planets orbiting stars much fainter than the Sun might look black.

Extraterrestrial plants will look different because they have evolved their own pigments based on the colours of light reaching their surfaces. (For plants on alien worlds, it isn’t easy being green. New Scientist, 2018).

It’s because our sun is green that plants are green. 

Plants are full of the light-absorbing chemical known as chlorophyll. They appear green because chlorophyll absorbs the other light wavelengths but reflects back the green light.

This at first sounds counter-intuitive. Surely if the sun emits more green light than any other, it would make sense to absorb all that green light?. But scientists now believe that it’s the abundance of green light that is problematic. 

Nathan Gabor at the University of California, Riverside, thinks green light’s very abundance is what makes it undesirable for plants. The more light you get in a particular wavelength, the more that signal fluctuates, making it harder to absorb it efficiently.

Experimenting with solar cells, his team found that the best way for solar cells to maintain high efficiency is to take in two different colours of light, neither of which is green. Over the course of the day, when the amount of light in different wavelengths varies, the cell takes in more of one colour and less of the other to keep output steady.

So in another solar system the sun might be purple  – meaning if there were plants on a planet there, they could be purple too. Freaky!

Our sun is really green, say astronomers
Graph showing the sun’s light peaking in teh green spectrum.
Courtesy: Stanford University

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