Hues of Light under Alien Suns

I’ve been much out­doors late­ly, brain­storm­ing my sto­ry­line, mak­ing notes and trav­el­ing. The idea for this post came after I’ve spent the whole day observ­ing day­light and its hues.

If you had ever won­dered what col­ors of light your world has, I might have an easy answer for you. But I think some would want to see the hues for them­selves. Well, I do.

Before mov­ing to the main dish I must say this method only works for earth-type atmos­pheres (unless you know col­or tem­per­a­tures for oth­er atmos­pheres).

Mars with its blue sun­sets is a good exam­ple of the dif­fer­ence the atmos­phere makes.

I also assume our observ­er is human (his/her eyes are adapt­ed to solar spec­trum which peaks at 501 nm, green-yel­low por­tion of it) and we have a sin­gle star.

The dis­tri­b­u­tion of a wide vari­ety of phys­i­cal, bio­log­i­cal, and man-made phe­nom­e­na fol­low a pow­er law, includ­ing the sizes of earth­quakes, craters on the moon and of solar flares, the for­ag­ing pat­tern of var­i­ous species, the sizes of activ­i­ty pat­terns of neu­ronal pop­u­la­tions, the fre­quen­cies of words in most lan­guages, fre­quen­cies of fam­i­ly names, the sizes of pow­er out­ages and wars, and many oth­er quan­ti­ties. And I’m mak­ing this math­e­mat­i­cal rela­tion­ship my instru­ment of choice as well.

So, what we need to do is sim­ply com­pare two stars. (Don’t frown; it’s easy, assum­ing you use a spread­sheet pro­gram).

First of all we’ll need the prop­er­ties of our Sun and the prop­er­ties of the star in ques­tion (for our exam­ples I will use two stars of spec­tral class­es FV and KV, the Sun being the GV star). These prop­er­ties are effec­tive tem­per­a­tures in Kelvins and mass­es rel­a­tive to solar. That’s it. The mass of the star is arguably its most impor­tant prop­er­ty. And I’m using it as the main com­po­nent in my pow­er law equa­tion to get what I want. And what I want is the col­or tem­per­a­ture of illu­mi­na­tion. What we are going to do is not very sci­en­tif­ic, but it works well under our con­di­tion of earth-type atmos­phere.

The Sun’s effec­tive tem­per­a­ture is 5778 K and its mass is 1. The F star will be 7500 K and 1.4 solar mass­es. The K star will be 3700 K and 0.45 solar mass­es.

My next step will be deter­min­ing the pow­er rela­tions between the Sun and each of the two stars. I’m using MS Excel spread­sheet, so my for­mu­las look approx­i­mate­ly like this:

Exp = LOG(7500; (5778×1.4)) = 0.9916 for our F star

Exp = LOG(3700; (5778×0.45)) = 1.0449 for our K star

For oth­er mass­es and effec­tive tem­per­a­tures you’ll need a recalc, of course.

Now then, let’s move onto the next part to get the hues of day­light.

Col­or tem­per­a­tures

Col­or Tem­per­a­ture is a mea­sure­ment in degrees Kelvin that indi­cates the hue of a spe­cif­ic type of light source. Col­or tem­per­a­tures attrib­uted to dif­fer­ent types of light are cor­re­lat­ed to vis­i­ble col­ors match­ing a black body (e.g. a star), and are not the actu­al tem­per­a­ture. High col­or tem­per­a­tures are con­sid­ered “cold” and low are con­sid­ered “warm”.

Col­or tem­per­a­ture would not be such an impor­tant fac­tor in col­or per­cep­tion if peo­ple were not adapt­able. Our eyes will adjust to var­i­ous light sources which have dif­fer­ent col­or tem­per­a­tures. And even though the pow­er spec­trum den­si­ty of the light is dif­fer­ent, we still see the light as white.

Sim­ply put, col­or tem­per­a­ture is the ten­den­cy of dif­fer­ent “white light” sources to change our per­cep­tion of a col­or. The col­or tem­per­a­ture of a light source caus­es the col­ors of every­thing illu­mi­nat­ed to change, but the change is often bare­ly notice­able.

Color Temperatures
Col­or Tem­per­a­tures

The effect of sun­light on the per­ceived col­or of an object changes with tem­per­a­ture because of how much air the sun­light has to pass through. If the sun is direct­ly over­head (warmer), it pass­es through less air than it does when it is low on the hori­zon (cool­er). Col­ors in north­ern cli­mates, espe­cial­ly in the win­ter, look dif­fer­ent than they do in south­ern cli­mates.

The process­es of absorp­tion and scat­ter­ing in the atmos­phere are respon­si­ble for the appar­ent col­ors of the sun and the sky. While sun­light falls only on non-shad­owed areas, the light of the sky falls on every­thing, so shad­ow hues are deter­mined by sky col­or. The hue gra­di­ent is clear­ly notice­able when the sun and the sky obvi­ous­ly dif­fer in col­or, like near the sun­rise and sun­set, and the dif­fer­ence in col­ors of shad­owed and illu­mi­nat­ed parts becomes the clear­est on light sur­faces such as snow.

The col­or tem­per­a­tures for solar illu­mi­na­tion (out­doors) are:

  • Sun­light (sun­rise or sun­set) – 2000 K – 3000 K
  • Sun­light (1 hour after dawn) — 3500 K
  • Sun­light (ear­ly morn­ing and late after­noon) – 4300 K
  • Sun­light (aver­age noon, sum­mer, mid-lat­i­tudes) – 5000 — 5400 K
  • Day­light (sun­lit sky) 5500 – 6500 K
  • Over­cast sky / haze – 6000 K
  • Light sum­mer shade – 7100 K
  • Aver­age sum­mer shade / hazy sky – 8000 K
  • Open shade on clear day – 9000 K
  • Heav­i­ly over­cast sky – 10000 K
  • Sun­less blue skies – 11000 -12000 K
  • Open shade in moun­tains on a real­ly clear day – 20000 K

In order to get our alien hues, we need to do some sim­ple math again. For our exam­ple I will take the col­or tem­per­a­ture for solar sunrise/sunset, 2000 K. The gen­er­al for­mu­la for our stars will be the fol­low­ing:


TCs is the col­or tem­per­a­ture of stel­lar light in the atmos­phere, CTe is the light col­or tem­per­a­ture from the hue list above, Mstar is the mass of the star rel­a­tive to solar, and Exp is what we cal­cu­lat­ed in the pre­vi­ous step.

For our F star the aver­age sunrise/sunset hue will be 2619 K, brighter and sharp­er than the Sun’s; for our K star this hue will be around 1221 K, dim­mer and soft­er than the can­dle light. To see these tem­per­a­tures in col­or, you might con­sid­er this chro­matic­i­ty (hue and sat­u­ra­tion) table.

Also, check out this page of the The Plan­e­tary Hab­it­abil­i­ty Lab­o­ra­to­ry (PHL) » Sun­set of the Hab­it­able Worlds.

Rec­om­mend­ed read: The Physics and Chem­istry of Col­or: The Fif­teen Caus­es of Col­or by Kurt Nas­sau, 2nd Ed, 2001.


Have you ever won­dered why the sky is blue, or a ruby red? This clas­sic vol­ume stud­ies the phys­i­cal and chem­i­cal ori­gins of col­or by explor­ing fif­teen sep­a­rate caus­es of col­or and their var­ied and often sub­tle occur­rences in biol­o­gy, geol­o­gy, min­er­al­o­gy, the atmos­phere, tech­nol­o­gy, and the visu­al arts. It cov­ers all of the fun­da­men­tal con­cepts at work and requires no spe­cial­ized knowl­edge.

This is an excel­lent sci­en­tif­ic overview of human per­cep­tion of col­or.

The book IS expen­sive, but if you have access to a good library, it is a good choice.

Jeno Marz
JENO MARZ is a science fiction writer from Latvia, Northern Europe, with background in electronics engineering and computer science. She is the author of two serial novels, Falaha’s Journey: A Spacegirl’s Account in Three Movements and Falaha’s Journey into Pleasure. Marz is current at work on a new SF trilogy. All her fiction is aimed at an adult audience.


  1. This is an awe­some post (like MUCH EVERYTHING ELSE on your site to be hon­est)! I love the degree of sci­en­tif­ic accu­ra­cy and I will most def­i­nite­ly come back here for world­build­ing info and ref­er­ence.
    Thank you so very much, Jele­na. You rock!

    1. Thanks Veron­i­ca,

      to be hon­est, I stopped writ­ing these too geeky arti­cles since not many peo­ple are into this. But now I know I have at least one read­er. *hap­py dance*

  2. Two read­ers. I was direct­ed here by a Scri­bophile mem­ber after ask­ing a ques­tion about the pos­si­bil­i­ty of a green sky on a human-hab­it­able world. 

    Is there a col­or tem­per­a­ture that would cause the sky to appear green? What effect would that light have on veg­e­ta­tion? Would such a world be com­fort­able for humans to live on? 

    I am writ­ing a sto­ry about humans crashed on an alien world. They have to be able to breathe the air, and they have to for­age to sur­vive, so the veg­e­ta­tion must be edi­ble. Do you know if such a world is pos­si­ble under a green sky? The green sky isn’t piv­otal to my sto­ry, but I think it would be fun.

    Thanks for your time.

  3. Kathy, there are sev­er­al options for your world. 

    For humans to breathe on an alien world, the atmos­pher­ic com­position would have be pret­ty sim­i­lar to what the Earth has(because we have lim­i­ta­tions for what to breathe — I wrote about lim­i­ta­tions here: Unless, of course, you have genet­i­cal­ly mod­i­fied humans to suit dif­fer­ent con­di­tions. Any­way, regard­less of what star you might have, the col­ors will be in the sim­i­lar col­or spec­trum as ours. The dim­mer the star, the more pal­lid the blue sky tinge will be (clos­er to grey­ish, per­haps, because dim­mer stars, such as M starts, have less blue light in their spec­trum.)

    What the veg­e­ta­tion be adapt­ed to depends on the light of the star.

    The sky col­or is the result of scat­ter­ing in the atmos­phere. Our sky is blue because the blue part of sun­light is scat­tered by par­ti­cles in our air. 

    But here’s where the fun part starts.

    On Earth green skies are cre­at­ed when sun­light refracts through hail­stones high in the atmos­phere. For the hail­stones to reach that alti­tude, the updraft of warm air must be power­ful. Some­times, this pow­er­ful updraft can cre­ate severe weath­er, like large hail­storms or pos­si­bly tor­na­does. So, in a way, green skies are poten­tial indi­ca­tors of severe weath­er, but not nec­es­sar­i­ly tor­na­does.

    The tremen­dous amount of water vapor might also have such effect. So, if your plan­et has a large amount of water vapor in the atmos­phere, the sky might look green­ish. The green is a result of the scat­ter­ing out of blue to the point that the green tint of light can be dom­i­nant. But then again, it would prob­a­bly be hard to breathe in such atmos­phere for a human. 

    And one more thing. Since we are talk­ing fic­tion here, per­haps the green sky effect might be caused by a lay­er of harm­less (or harm­ful – you decide) micro­scop­ic plant cells sus­pend­ed in the mid­dle atmos­phere. After the rain the parts of the sky might appear blue.

    As for edi­ble veg­e­ta­tion — unless it is bio­chem­i­cal­ly relat­ed to these humans, it won’t be edi­ble. These peo­ple would not be able to digest such plants. Unfor­tu­nate­ly.

    I hope this helps. if you have fur­ther ques­tions, feel free to ask!


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