For the budding SF writer, part 2: Local color

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Now that we’ve fixed where the planet is going to be, we need to think about what life is like on the planet – quite literally.  There are milions of details to consider – let’s just pick one to start out with.  Let’s think about the star and how that’s going to affect what you see when you look around.

Everything starts with the star.  Luckily for the writer, stars are all pretty simple in some respects.   The star’s luminosity determines its color, which will determine how things look on the planet — at least, to any human visitors to it.  Remember, if we have an Earth-like planet at some distance D from the star, how bright the star is relative to our sun (L) will be about D*D, although there will be some adjustments for atmospheric composition and albedo (see my previous post.)

Here’s a table summarizing the information  on stellar coloration as a function of luminosity:  this is only data for what are called “main sequence” stars, that is essentially middle-aged stars which “burn” hydrogen into helium in their cores.  If you want to use a star in its old age, or a newly forming protostar in a story, you have to dig a little deeper.

L Color
.01 Red
.1 Orange
1 Yellow
10 White
100 Blue-white
1,000 and higher Blue

As stars get increasingly dim, a larger fraction of their light will be in the infrared spectrum, that is in long wavelengths which we can’t see.  As they become increasingly bright, more of their light will be in the ultraviolet – still invisible to they eye, but at shorter wavelengths than we can see.  (This can also lead to obvious complications for humans visiting such planets.)

There’s a lot of room for play when you start thinking about the color of different things on the planet.  Let’s start with the sky.  Earth’s sky is blue because light from the sun is scattered by air molecules.  They scatter short wavelengths (that is, bluer light) more readily than longer wavelengths (i.e., red light), which is what gives the sky its characteristic blue tint.  When you look at the sky, you’re seeing light which has been scattered.Because of this, the coloration of various features of sky light should more or less mimic the properties of the star: skies on planets with reddish stars should be blue, but less blue than our own sky – maybe greenish blue or green?  Detailed computer simulations could show if this were true.   Particles in the atmosphere due to smog, pollen or volcanic eruptions will change the sky color; for example, fire from smoke or volcano can make the sun or moon appear green or blue when seen through it (hence the phrase, “once in a blue moon”, which doesn’t mean the second full moon in a month.)  Atmospheric refraction warps the appearance of the rising sun and moon: thicker atmospheres will warp them more, thinner ones less.  It’s again something to have fun with when thinking of story details.  The green flash is due to atmospheric refraction and layering of the atmosphere when the sun sets over a flat landscape like the ocean: an ocean planet with the right atmosphere could have an extended green flash every night…Now that we’ve gotten the sky colors, let’s go on to the landscape, and think about what plant life and ground cover will look like, in the next post on this subject.

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