Sky colour

Note that despite the large (relative) size of Earth's moon, and thus the effect it has on our tides, the sun also has a (lesser) effect. Have a look at this. Now imagine there are two or more suns.

I'm assuming that the stars in this proposed star system are closely bound (like Alpha Centauri A and B, or even closer together), not like Proxima Centauri**; otherwise, all but the closest star (the planet's sun) would simply be seen as other stars in the night sky (albeit rather closer than most) and would have a negligible effect on the tides.





** - For the sake of argument, I'm assuming Proxima Centauri is the third star in a three-star system (which it may or may not be).
 
I think you're right about Proxima Centauri, Ursa. But it has been many years since I've navigated a starship through the Centauri system, so I'd better check Wikipedia...

Its individual component stars are named Alpha Centauri A (α Cen A), with 110% of the mass and 151.9% the luminosity of our Sun, and Alpha Centauri B (α Cen B), at 90.7% of the Sun's mass and 50.0% of its luminosity. During the stars' 79.91 year orbit about a common center, the distance between them varies from about that between Pluto and the Sun to that between Saturn and the Sun. They average 1.34 parsecs or 4.37 light years away from the Sun.[10]
A third star, known as Proxima Centauri, Proxima or Alpha Centauri C (α Cen C), is probably gravitationally associated with Alpha Centauri AB. Proxima is now placed at the slightly smaller distance of 1.29 parsecs or 4.24 light years from the Sun, making it the closest star to the Sun, even though it is not visible to the naked eye. The true separation of Proxima from Alpha Centauri AB is about 0.06 parsecs, 0.2 light years or 13,000 astronomical units (AU), equivalent to 400 times the size of Neptune's orbit.

So, you're right. It probably is. Or may not be. :confused:
 
I think I'm going to take the multiple sun approach, that way I can time their passage through the sky so that the rayleigh scattering is like a sunset almost the whole time. It will also help contribute to the dry arid state of the planet. There will have to be a little bit of blue at one point of the day, but I could work that in to great effect

As a physics PhD I'm not hot on multiple suns - not that they are not common (they are) and not they will have planets around them (I'm sure we'll find some) but to get the 'Star wars' effect with multiple suns should make any planet that close to get pretty sunsets completely unliveable and unviable (either fried dry or in an unstable orbit that eventually sees the planet chucked out into space or into the star etc.... or the general dynamic instability causes small objects being constantly tossed too and fro by the suns will cause loads of cataclysmic collisions i.e. comet and asteriod bombardments.)

There are of course some situations that might work - but they are a bit 'boring'. So say you have two suns circling each other very tightly and the planet far off orbiting the centre of mass of the two tightly bound stars. Or if the distance between the two stars is always quite a large distance so that a planetery system could fit in...So that the system from the planet looks very much like ours.

To go back to your original question, as others have pointed out it does take a very unusual star to really generate different light radiation (and you probably don't want to be near a blue giant - nasty things that blow up quite visicously!) Our sun is called 'yellow' but that's a bit of a misnomer - it looks yellow to us because of our atmosphere. It's really pumping out white light.

The blue comes from rayleigh scattering from oxygen molecules - so if you have other molecules in enough quantities in the atmosphere you could change the light scattered. There was a idea, first originated in the 70s to pump the atmosphere with some gas (I forget what it was) to artifically cool the planet but the side effect would be that the sky would be white - I assumed the gas molecule was effective at scattering everything so effectively blocking some light therefore energy hitting the surface. Then there is the sulphur dioxide from volcanoes which can have a dramatic but temporary effects - people were amazed post Krakatoa at the sunsets. It has been suggested that the skies in 'The Scream' might be representations of what it was like. (Or perhaps not, I see this is a contentious point)

The simplest way would be to get your biosphere to produce some gas/molecule in large enough quantities and you could justify probably any colour. Apart from black. Any colour as long as it isn't black. I'd make alien flora and fauna generate it as a possible explanation. Hydrogen gas for example I think scatters red light - but remember Earth is too small mass-wise to keep any hydrogen in the atmosphere
 
Really helpful information, Venusian. Thanks, and I think you just solved it for me. Hydrogen gas would make it red, perfect.

Now, how to get around it escaping the atmosphere, or if the fauna releases enough of it, will it matter if the gas escapes?
 
oops, thinking about it a bit more (well, not very well - it's 4.30am here in London), Hydrogen is red in space I think because its the main emission/absorption wavelength of the molecule - not because of scattering of white light. You'd probably need a lot of it in the atmosphere to see this effect....Drat!


As for the reason Hydrogen escaping Earth's gravity. It is that at room temperature (well to be frank, most temperatures) the average speed of a hydrogen molecule is faster than the esacpe velocity of the planet - so it effectively shoots off into space with no problems whatsoever. You'd need a heavy Earth to hold on to it - don't know how massive it would have to be though, could work it out very approximately on a scribble of paper in the morning...


The other problem is that with a planet with oxygen, lots of Hydrogen would probably burn with it quite nicely!


To get a nice range of scattering you'd probably need quite a large molecule in the air - something considerably bigger than oxygen - which I'm assuming will have to be there for humans to breath. A dry dusty world could kick up a large amout of big particles that just won't be washed out of the atmosphere - like Mars. But big molecules in the atmosphere must be replenished otherwise they will eventually hit the surface again.....


I will ponder this over a quick snooze and get back to you :)
 
Took the family on a holiday to Queensland a few years ago. Just in time for a dust storm. The sky darkened to a dirty grey-brown colour. The sun dimmed until it looked more like a moon -- you could stare straight at it without blinking, if that's your idea of fun.

But as VB notes above, since it wasn't replenished, all that dust eventually blew into the sea, and the beautiful blue skies we paid good money to see were back.

We also met a taipan at the airport, but that's another story that has nothing to do with the colour of the sky...apart from turning the air blue with curses. :eek:
 
Ok, I'm back and fresh!

I'll start with scientific accurate - as much as I can manage :) and then put in speculation to change it, at the end!

Ok, if you have a moon/planet with a translucent atmosphere, and assuming you have a normal main sequence star with no fancy debris between star and planet, then because of Rayleigh scattering (light being scattered by objects - gas molecules in this case - much smaller than the wavelength of the light) then Blue is the predominate colour no matter what you do. In maths terms it's because the scattering is inversely proportional to the wavelength to the power of four. And the blue end of the visible spectrum has shorter wavelengths than the red end. So red light just isn't as affected.

The other factor is the Rayleigh cross-section of the molecules. The bigger the cross-section the bigger the overall scattering in terms of amplitude and frequencies that it impacts. (again for the Maths peeps, the scattering is proportional to the cross-section squared - note not as strong as the wavelength dependency) This cross-section is in simple terms a size - but it is not purely a measurement of 'length' of a molecule - it should really be based on QM calculations of where the electrons are etc...

So the bigger the molecule you can put into the air the more of the red end of the spectrum gets scattered, but it also scatters blue more efficiently as well...

So the range of sky colours you could get from increasing the size of the cross-section of the molecules in the atmosphere are (according to my thinking):

Indigo/Violet*, Blue, Aquamarine, Pale Blue-Green, White**

* Indigo really is Violet which really is dark Blue but Isaac Newton, for mystical reason of his own wanted the visible spectrum to have 7 colours. Lapis Lazuli is a nice word for that end of the spectrum I think.
** Paraphrasing Dulux, with a hint of blue, probably.

Essentially all shades of blue and closely related to blue. White comes about when enough red comes through and balances out some of the blue. Yellow, Orange and Red skies are for sunsets and only for bits of the sky! Not for the normal background.

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Now you could try and infuse the atmosphere with much bigger molecules to try and get more interesting effects. But the problem here is that big molecules tend to be very short-lived in the atmosphere for a variety of reasons - e.g. either they settle back down into the surface or the action of the light will break down the molecule in the atmosphere. And the molecules you'll need ~the same size as the wavelength of the light are really quiet big. Oxygen for example is only about 28 nm (blue light ~450nm).

A further problem here for those of us who want really nice coloured skies is that as larger particles tend to impact all frequencies of light equally so we tend to get more 'cloud'-like light patterns (diffuse white, greys and blacks)

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Case of the top of my head:

-Water-rich planets (i.e. those with open water anywhere in large quantities) will have atmospheres saturated in water vapour. This impacts the sky colour by whitening it. It also means that the planet will have a defacto water cycle that will help to 'wash' out all other impurities.

-Dry planets will have a lot of dust in the air (no water to wash it out). Then the colour of the dust becomes important. Mar's sky is pink because the dust that is constantly whipped up into the air is mainly iron oxide. But, hey what if the surface was mostly copper oxides not iron - then you could have a green sky!

-Coloured polluntants are a possibility, but think of them like a coloured perspex that augments the basic colour of the sky. For example burning soot and petrol gives a brown hue to the air (Years ago I used to work on the top floor of a building in central London and on clear sunny days you could see the boundary layer between the smog - dirty brown and the clean air above it. Yeuk!) On your alien world you could have the plants pump out any sort of colour - but like smog they would probably be restricted to the very low atmosphere.

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Ok finally, if you want a far out SciFi reason for an Orange or Red sky proper...

Then if you have a small gas with a cross-section that is an imaginary number (you know, have some particle like Unobtanium or IsntRealium;)) then I think*** Blue light would get preferentially reflected back and that would leave the red end of the spectrum to flood the sky - hence giving you some pale yellows, oranges and reds. It would need some sort of freaky particle that interacts very strangely with EM fields. Hey it's SF we write, why not...


***Well the equation probably breaks down - but I tried to go for the opposite to let the red light in!
 
Sounds as if it would be easier and cheaper to drug, hypnotise, or otherwise manipulate those looking at the sky.
 
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In one of my short stories the sky is blue because that's the colour of the thread that was used to weave it...

...Saves on understanding all the physics stuff.
 
Is reversing the polarity done before or after they have to decide if Data is alive or not?
 
Data literally uses the phrase "Reverse the Polarity" concerning the magnetic door in Star Trek: Generations. Course, reversing the polarity on a magnetically-controlled door would open it. Go figure

source
 
Venusian Broon said:
Ok finally, if you want a far out SciFi reason for an Orange or Red sky proper...

Then if you have a small gas with a cross-section that is an imaginary number (you know, have some particle like Unobtanium or IsntRealium;)) then I think*** Blue light would get preferentially reflected back and that would leave the red end of the spectrum to flood the sky - hence giving you some pale yellows, oranges and reds. It would need some sort of freaky particle that interacts very strangely with EM fields. Hey it's SF we write, why not...
Click to expand...

This has potential, I shall go away and think. Thanks VB.

Ursa major said:
Sounds as if it would be easier and cheaper to drug, hypnotise, or otherwise manipulate those looking at the sky.
Click to expand...

I'm sorely tempted...
 
Read a story many years ago, don't remember the title or author, but IIRC it was in one of Harlan Ellison's Dangerous Visions anthologies.

The totalitarian regime running the show had almost eliminated colour, seeing it as unhealthy for some reason. Cars were white. Houses were white. Everything was white. At the end of the story, our hapless protagonist is arrested because he was caught staring at the sky. That's now against the law, because the sky isn't white...
 
Dust kicking the sky is an excellent method, then there's the natural color of the sky (if you have a diverse planet that isn't entirely colored in desert), and there's also algae, as well as certain gases with molecules large enough to reflect the blue side of the visible spectrum (of course, you would need something constant, whether it be organic or volcanic maybe?) old topic, I know, but it helped me alot and I felt the need to summarize what I gathered from here and other websites.
 
I remember this thread... zombie thread wants brains... bbbbrrrraaaaiiiinnnnssss...




Btw, I never decided on a solution to this problem for me, and not long after that got distracted with a different project, so I still don't have any ideas to give.
 
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