Let’s say an alien astrophysicist observes our mother star, the Sun, and then sends his data to us,the humans of Earth. What we see is definitely going to leave us astounded, because what he will be seeing will be something we do not expect. It is the Sun but colored green instead of the usual white/yellow. And so begins the formation of a question: What is going on? Which object or phenomenon causes the unexpected change? (And no the answer isn’t “the alien is colorblind”) Well let’s examine this slowly step by step.

The surface of the Sun has a temperature of 5772 Kelvin or 5499 degrees Celsius. On top of that, a celestial object, that is not fully cooled(at 0 Kelvin) , constantly kemits thermic radiation, according to Plank’s law. If we drew a diagram of the intensity of the radiation in relation to its wavelength, we will get a shape resembling a nonsymetrical bell. The top of this bell represents the most common wavelength in the object’s emission spectrum and it is a not so hard number for us to approximate. For that to happen, Wien’s law of displacement is needed(the number can be found via Plank’s law, but Wien’s law is easier to use).

𝜆𝑚𝑎𝑥 · 𝑇 = 0.29𝑐𝑚 · Κ

For the star in question, the bell curve approximation is as follows:

As can be seen, by plugging the surface temperature of the Sun in the equation the resulting maximum wavelength sits at 502 nanometers. If the fact that, the visible portion of the light spectrum is between 380nm(purple) and 750nm(red) and that 502nm is a shade of green, is taken in consideration, then the Sun being green seems quite logical.But fellow human being and reader, hold thine horn,please don’t complain just yet. The difference in perspectives can be explained, due to things being more complicated than they seem. 502 is the maximum wavelength, the tip of this radiation emmiting iceberg. There are more wavelengths generated from the Sun, just in slightly lesser quantities. Still though the quantities are big enough and wide enough in the diagram, that they cover the whole visible spectrum! So in the end you could say we as human can’t distinguish the difference between neighboring wavelenghts and see the light as white.

Well that assumption would still be wrong! You probably forgot the place the alien views the star,space. As you probably know we are not in space, we have the atmosphere between the two of us(us and space). The very same atmosphere acts as a filter.

The incoming radiation,meaning the electromagnetic waves collide with air particles and consequently scatter. The scattering of light in different wave lengths happens well… differently. At the smaller wavelengths the length is comparable to the size of the particles so it impacts and scatter, whereas the bigger wavelenght will “dodge” this fate.

In the same line of thought the purple and blue wavelengths are easier to be redirected, thus we don’t see them as the color of the sum as often. Not so suprisingly, because the atmosphere deflects them, the atmosphere, or as it is otherwise called, the sky, is what we see as blue. In continuation, because those colors are scattered, they are subtracted from the group of wavelengths, leaving behind just the big ones, who in turn make the sum look yellow. Still though we haven’t kept everything in mind. What about the sun’s positioning in the sky? Well it matters. When it is directly above ground the light travels a minimum distance. On the hour’s, though, like sunrise or sundown, the distance travelled is greater, and so more colors, like green and yellow are subtracted, leaving only orange and red, who make those oh so very instagram worthy landscapes. In the end, our mother star takes on different personalities and facets by changing it’s color from yellow to orange to red. But never do we see it green.

So.. are there green stars out there?

How is our biology affected by our next door terrestrial neighbors(even by the star’s color)?

How does the atmosphere interact with the planet?

These questions are what will trouble us in future articles…

(Translation: Dimitrios Mataragkas)

(Proof-reading: Michelle Evrydiki Ioannides)

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