Would the sky be blue if ....

[bearack]

I was surfing through Yahoo answers and saw this question "What color would the sky be under a red star?".

My first assumption would be that with Rayleigh scattering, regardless of the intensity of the brightness of the star, as long as the composition of the planets atmosphere was the same as Earths, then the sky would be blue but many of the answers suggest that it wouldn't be.

What color would it be and why if using the same Rayleigh scattering principle?

 

[yevaud]

Well, Rayleigh Scatter is due to the various species comprising the atmosphere, plus the optical depth, plus the wavelengths incoming from solar insolation, so it's a trifle hard to answer this with any certainty.

If the composition of the atmosphere was still the same as ours, though, you'd probably NOT obtain a blue sky, as the wavelengths emitted by a Red Star have little of the blue component. I'd say on a very clear day, the sky would be mostly black, and/or with a faint reddish tint.

 

[darkmatter4brains]

hmmm, interesting question.

I'm not sure the sky would be black - at least not like it is at night?? I think the sky is blue now, not because NO red (or any other color) light reaches the surface of the earth, but because blue light is scattered in the atmosphere so much more efficiently than other colors according to:

s ~ 1/lamda^4

in other words how much light is scattered is inversely proportional to the 4th (yes 4th!!) power of the wavelength, IIRC. Blue light is so much more efficiently scattered, having a smaller wavelength, it totally washes out the other colors, similar to daylight washing out the star light.

So, although the sky would not be as bright as it is now, I think it would me mostly reddish ... maybe somewhat like Mars, albeit for different reasons.

But, I'm totally guessing here ....

hmmm, on second thought, although red stars do have mximum luminosity in the red wavelengths, don't they still emit some blue light .... since stars do somewhat approximate a black body spectrum? So, would the sky still be somewhat blueish, IF we had the same atmosphere. In other words, a red star still emits blue light, it's just washed out by the preponderance of reddish light ... but ,our atmosphere acts like such a good filter of red light, that the little bit of blue light the star does emit would still be the strongest wavelength of light getting through to our eyes .....

okay, obviously I have no idea on this one , but it was interesting to think about :lol:

Anybody else have insight into this.

 

[bearack]

hmmm, on second thought, although red stars do have mximum luminosity in the red wavelengths, don't they still emit some blue light .... since stars do somewhat approximate a black body spectrum? So, would the sky still be somewhat blueish, IF we had the same atmosphere.

.

This is exactly what I was thinking. I was thinking regardless of intensity that it still would emit some blue which would still be dispersed in a such similar earth like atmosphere while the rest of the light wavelengths would, for the most part pass through.

 

[yevaud]

Anybody else have insight into this.

No, I believe I am correct. Red Dwarfs have little blue component output. Scattering notwithstanding, there isn't really any blue there to scatter.

Btw, Red bands also scatter fairly efficiently too, as long as the optical depth is sufficient. This is one reason why you see red/orange when the sun in on the horizon - the optical depth is deeper.

 

[darkmatter4brains]

Well, I was thinking of a red giant myself. I dont thin the OP specified red dwarf .... just a reddish star? Or did something change in the post?

 

[yevaud]

True, it didn't specify Dwarfs. Yet, Red Dwarfs are on of the most prevalent stars in the universe. So that's what I specified. As well, I am entirely not certain (but tend towards "Not") if Red Giants can even have planetary bodies. If they do, we aren't talking about anything with a "reasonable" surface condition that would allow a robust atmosphere. Obviously, without one, this question is moot.

 

[bearack]

Thanks yevaud.

I didn't specify nor did I think it relevant, thinking that as long as it emitted light that it had to have blue waves but that only shows my naivety.

Thanks again!

 

[darkmatter4brains]

True, it didn't specify Dwarfs. Yet, Red Dwarfs are on of the most prevalent stars in the universe. So that's what I specified. As well, I am entirely not certain (but tend towards "Not") if Red Giants can even have planetary bodies. If they do, we aren't talking about anything with a "reasonable" surface condition that would allow a robust atmosphere. Obviously, without one, this question is moot.

Well, what a planet would be like around a Red Dwarf and whether it can host life is also still under debate I believe.

As far as Red Ginats, they can definitely have planetary bodies - our Sun will at some point in its life. The question is, what shape will they be in. What I seem to remember from an Astronomy magazine article is that some Giants could potentially have a habitable zone that could house an earth like planet. Obviously, for our Sun, that would not be where the Earth is now. The question remained however, since the Red Giant phase is relatively short, would there be a long enough period of time for life to evolve significantly.

Bottom line is, it's sounding like what the sky looks like depends on the specific (reddish) star and the atmosphere of the planet. There really probably isn't a single answer to this question.

 

[darkmatter4brains]

This whole thread got me interested into looking into all this again. From what Ive seen so far, it looks like Red Dwarfs are actually less likely of having a habitable zone than Red Giants. Apparently, Red Giants may even have enough time to develop life.

Red Dwarfs:
http://www.sciencedaily.com/releases/20 ... 124831.htm
http://en.wikipedia.org/wiki/Red_dwarf

Red Giants:
http://www.dailygalaxy.com/my_weblog/20 ... arfs-.html
http://www.nasa.gov/centers/goddard/new ... rlds2.html

There are a ton more articles out there on both.

Since life is typically thought to require a "robust" atmosphere, it makes Red Giants a potentially good candidate for the topic of this thread too. Although the question remains, would the sky be red or blue :lol: Im pretty sure, Red Giants have significant output in blue light, just not as much as red light - would the atmosphere "filter" out enough of the red to keep the sky somewhat blue?

 

[yevaud]

As far as Red Ginats, they can definitely have planetary bodies

That's rather problematic, as very few Red Giants have as yet had bodies in any useful orbits detected. Maybe in the future. Besides which, how many of them will have a nitrogen/oxygen atmosphere? Barring that, then asking if the sky will appear blue is a non-question, as the gasseous species will be quite different.

This whole thread got me interested into looking into all this again. From what Ive seen so far, it looks like Red Dwarfs are actually less likely of having a habitable zone than Red Giants. Apparently, Red Giants may even have enough time to develop life.

Sorry, but those articles say "may," and in no way have determined that planets are "more likely" around Red Giants than Red Dwarfs. This is certainly a "yet to be determined."

Further, the articles refer to the habitable zone, and don't address the OP of this thread, it essentially hares off onto a new topic - not "will a sky appear blue on a planet orbiting a Red Star." I would say that while a blue component scatters more efficiently than the others in a nitrogen oxygen atmosphere similar to Earth's, that isn't a simple question to answer. Operating under the understanding that most worlds detected have widely different atmospheric constituents, then the answer to the OP will be a resounding "NO."

 

[darkmatter4brains]

I never said any of that was definitive - most things in cutting edge science aren't. I just pointed out that a planet in orbit around Red Giant according to current thinking could have a robust atmosphere. I also neevr said Red Giants are more likely to have planets, I said they MAY be more likely to have planets in a habitable zone according to current thinking. Life on a planet and the type of atmosphere on a planet are interconnected, so that's not really drifting away from that topic either.

I mean, Yevaud, what the heck are you trying to argue here, or did we slip into the ever present I was more right than you were argument that always shows up on these forums.

Let's just simplify this and make it a thought experiment. If you could take the earth and transfer it into the habitable zone of a Red Giant, what color would the sky appear? I don't think this question has been answered in a satisfactory way at all yet?

 

[yevaud]

I mean, Yevaud, what the heck are you trying to argue here, or did we slip into the ever present I was more right than you were argument that always shows up on these forums.

Not at all. However, if the first question is "will a sky appear blue on a world around a red star," but then the linked evidence speaks of the habitable zone, then it isn't really useful data with which to answer the question.

Let's just simplify this and make it a thought experiment. If you could take the earth and transfer it into the habitable zone of a Red Giant, what color would the sky appear? I don't think this question has been answered in a satisfactory way at all yet?

Again, this is not something that can be answered with the (lack of) crucial data. What are a) the atmospheric constituents of the planet? b) What is the Optical Depth?

Now if you're postulating it is an oxygen/nitrogen atmosphere similar to Earth's, then the answer is "barely," as Red Stars do not have much of a blue component output. If the atmospheric constituents are not similar to Earth's, then the answer will be "No."

Remember, scattering of blue light is that blue light preferentially scatters, but that depends. It doesn't mean that it is the only thing that scatters, and under certain conditions (such as the sunset example I gave earlier), then you'll note suddenly the red/orange component scatters more efficiently than the blue. Which is an example of a deeper optical depth than, say, at high noon. For that matter, there's also Mie Scattering, which may/may not be in operation here.

Gotta define these questions better, else you end up asking vague "maybe" kinds of postulates with many possible answers.

 

[SpaceTas]

The terms red giant red dwarf are a bit of an exaggeration, more like orange slightly red. But there is a fairly simple measure of color; the color index. This is the difference in brightness of the star when measured through a filter that only lets green-yellow light (visual) and blue light (blue) as expressed in magnitudes. ( a 1 magnitude difference is a factor of 2.5 difference in flux).

For the Sun (G5) the B-V colour index is near 0.7 and for a M0 (red star) it is near 0.9 or for an M5 (only 3000 K and nearly as red as you can get before going to brown dwarfs).

Comparing an M5 star (B-V=1.7) with a G5 (B-V=0.7) there is a 1 magnitude difference in colour index This translates into there being a factor of 2.5 times less blue light compared to yellow light round an M5 star.

Again using colour index but now green - red (g'-r') (couldn't find a V-R index versus spectral type, but R and g' are similar) A G5 star has g'-r' about 0.5 while M0 about 1.3 and other M stars 1.4. So an M7 star is 0.8 mag (factor 2) relativly brighter in the red than the Sun.

The filters here have peak transmissions B 420 nm, V 520nm (g similar), R 600 nm.
So an M5 star is a has a factor 2.5 less blue, but a factor 2 more red light to scatter. So there is roughly 4.5 times as much red light to be scattered as blue light (compared to Sun).

Given the power of 4 dependence with wavelength Rayliegh scattering, the blue light will be scattered by a factor of (600/420)^4 = 4.2 times more than the red light. The extra red light is a factor of 4.5 so about the same amount of red and blue light being scattered

The ratio for the in-between V band comes out (factor 2.3 in scattering, factor 2.5 in brightness) so about the same amount of scattered light irrespective of wavelength

==> white sky maybe with a touch of red (salmon)

Assuming a really "red" star and a planet with an atmosphere similar to Earth. Of course the real way to do this is take a stars spectrum and input it into an atmospheric scattering model. Make a good final year physics/astronomy/meteorology project

 

[darkmatter4brains]

The terms red giant red dwarf are a bit of an exaggeration, more like orange slightly red. But there is a fairly simple measure of color; the color index. This is the difference in brightness of the star when measured through a filter that only lets green-yellow light (visual) and blue light (blue) as expressed in magnitudes. ( a 1 magnitude difference is a factor of 2.5 difference in flux).

For the Sun (G5) the B-V colour index is near 0.7 and for a M0 (red star) it is near 0.9 or for an M5 (only 3000 K and nearly as red as you can get before going to brown dwarfs).

Comparing an M5 star (B-V=1.7) with a G5 (B-V=0.7) there is a 1 magnitude difference in colour index This translates into there being a factor of 2.5 times less blue light compared to yellow light round an M5 star.

Again using colour index but now green - red (g'-r') (couldn't find a V-R index versus spectral type, but R and g' are similar) A G5 star has g'-r' about 0.5 while M0 about 1.3 and other M stars 1.4. So an M7 star is 0.8 mag (factor 2) relativly brighter in the red than the Sun.

The filters here have peak transmissions B 420 nm, V 520nm (g similar), R 600 nm.
So an M5 star is a has a factor 2.5 less blue, but a factor 2 more red light to scatter. So there is roughly 4.5 times as much red light to be scattered as blue light (compared to Sun).

Given the power of 4 dependence with wavelength Rayliegh scattering, the blue light will be scattered by a factor of (600/420)^4 = 4.2 times more than the red light. The extra red light is a factor of 4.5 so about the same amount of red and blue light being scattered

The ratio for the in-between V band comes out (factor 2.3 in scattering, factor 2.5 in brightness) so about the same amount of scattered light irrespective of wavelength

==> white sky maybe with a touch of red (salmon)

Assuming a really "red" star and a planet with an atmosphere similar to Earth. Of course the real way to do this is take a stars spectrum and input it into an atmospheric scattering model. Make a good final year physics/astronomy/meteorology project

SpaceTas,

Thanks! That's the information I was hoping for and is roughly what I remember from my astrophysics class, so it jives up with me.

 

[MeteorWayne]

Just a side note, from what i recall, the red sunrise/sunset and moonrise is not caused by scattering, but rather the absorbtion of the blue end of the spectrum by the large amount of atmosphere that the light pases through (> 1000 km at the horizon, as opposed to about 100 km for the sun overhead).
MW

 

[silylene]

Actually, the primary reason the sky is blue has to do with statistical fluctuation theory. The stochastic variation in atmospheric density, on a scale of 10 -100 nm or so causes tiny local variations of refractive index on the same scale, and these local variations of refractive index are the primary origin of the blue sky. This is called the Einstein-Smoluchowski theory of fluctuations.

'Rayleigh's Law' isn't really (regardless of how many repeatedly quoted references you have read) what directly causes preferential scattering of blue wavelengths in the upper atmosphere, althought inverse power of 4 lambda correlation he described is quite close to what is observed experimentally (IIRC, the modern experimental exponent is something like -4.1, not -4.0). "To say that the sky is blue because of Rayleigh scattering, as is sometimes done, is to confuse an agent with a law." (first ref below) What Einstein-Smoluchowski showed was that if the scatterers (the individual molecules) were uniformly distributed, there would be hardly any scattering at all. It is only because of density fluctuations in the medium that their individual effects are additive. The atmosphere simply isn't a continuous invariant homogenous medium.

All this said.....
The above discussion is very dependent on the density of the gas, because the mean free path of a photon between molecules is very dependent on the fluctuations of separation between the molecules. Changing the gas density will change the light scattering amplitude, perhaps with a wavelength dependence, IIRC. Point here is that I think for an obvserver on the ground, a thicker planetary atmosphere might yield a different color or color intensity even if the atmospheric composition were the same as earth's. Sorry, it has been about 27 yrs since I last had a course in fluctuation theory and scattering, and I haven't worked on these kind of problems since then. Just realize that how it really happens isn't anything as simple as 'Rayleigh's law'. And let's not even get into Mie scattering!.

some references (the first reference is good but dated in fully describing the modern undertsanding of the physics):
http://www.pro-physik.de/Phy/pdfs/OE004_1.pdf
http://scitation.aip.org/getabs/ser...0001000094000001&idtype=cvips&gifs=yes&ref=no
http://www.iop.org/EJ/abstract/1063-7869/45/1/A04

 

[yevaud]

Just a side note, from what i recall, the red sunrise/sunset and moonrise is not caused by scattering, but rather the absorbtion of the blue end of the spectrum by the large amount of atmosphere that the light pases through (> 1000 km at the horizon, as opposed to about 100 km for the sun overhead).
MW

So why so different here? What exactly does our atmosphere do? Well, the simple answer is that it scatters light. Not all light equally, though. The atmosphere is better at scattering blue light away, which means that blue light gets dispersed all throughout the sky pretty easily. But red light is more likely to pass directly through, which is why things appear redder on the horizon: more of the bluer light gets scattered away, while the red light comes (mostly) through to you.

http://scienceblogs.com/startswithabang ... ut_why.php

Shall I locate a better reference?

 

[MeteorWayne]

Nope, makes sense to me!

 

[yevaud]

Way cool then.

Always a challenge to find a concise, readable, and understandable reference.

 

[yevaud]

Actually, the primary reason the sky is blue has to do with statistical fluctuation theory. The stochastic variation in atmospheric density, on a scale of 10 -100 nm or so causes tiny local variations of refractive index on the same scale, and these local variations of refractive index are the primary origin of the blue sky. This is called the Einstein-Smoluchowski theory of fluctuations.

'Rayleigh's Law' isn't really (regardless of how many repeatedly quoted references you have read) what directly causes preferential scattering of blue wavelengths in the upper atmosphere, althought inverse power of 4 lambda correlation he described is quite close to what is observed experimentally (IIRC, the modern experimental exponent is something like -4.1, not -4.0). "To say that the sky is blue because of Rayleigh scattering, as is sometimes done, is to confuse an agent with a law." (first ref below) What Einstein-Smoluchowski showed was that if the scatterers (the individual molecules) were uniformly distributed, there would be hardly any scattering at all. It is only because of density fluctuations in the medium that their individual effects are additive. The atmosphere simply isn't a continuous invariant homogenous medium.

All this said.....
The above discussion is very dependent on the density of the gas, because the mean free path of a photon between molecules is very dependent on the fluctuations of separation between the molecules. Changing the gas density will change the light scattering amplitude, perhaps with a wavelength dependence, IIRC. Point here is that I think for an obvserver on the ground, a thicker planetary atmosphere might yield a different color or color intensity even if the atmospheric composition were the same as earth's. Sorry, it has been about 27 yrs since I last had a course in fluctuation theory and scattering, and I haven't worked on these kind of problems since then. Just realize that how it really happens isn't anything as simple as 'Rayleigh's law'. And let's not even get into Mie scattering!.

some references (the first reference is good but dated in fully describing the modern undertsanding of the physics):
http://www.pro-physik.de/Phy/pdfs/OE004_1.pdf
http://scitation.aip.org/getabs/ser...0001000094000001&idtype=cvips&gifs=yes&ref=no
http://www.iop.org/EJ/abstract/1063-7869/45/1/A04

Very interesting, and thanks. Fwiw, in physical geography courses, Rayleigh and Mie were certainly taught in depth, as well as the ABCD Laws (Avogadro, Boyle, Charles, and Dalton), such things as triple-point, adiabatic or wet or dry lapse rates, tephigrams, optical depth, imaginary parcels of air, scale height, yadda yadda. But honestly, not once did I ever hear of Einstein-Smoluchowski, so I actually learned something useful today. Not surprised though, as a Geographer would more than not figure it wasn't relevant to their job or research (being Physics, which sounds counter-intuitive, but you probably know exactly what I mean I think). And it wasn't precisely covered in my Planetary Science courses either. Possibly if I'd gone on to Grad school.

 

[silylene]

Yev, I love geography! (I won state in that subject back in HS.) I never knew that was your major.

 

[yevaud]

One of my two majors. Specifically, my emphasis was Atmospheric Physics/Remote Sensing. Pretty cool stuff, and it went along with the other major (Planetary and Space Science) really well. Hell, maybe I will go back to school someday for a Masters. Hope so.

Side story: remember the "Million Man March," and how it was debunked as to the actual numbers? That was Farouk El-Baz, who runs the Center for Remote Sensing in my old Geography Dept. at B.U. I was attending there at the time, remember the event well.

El-Baz is so respected in his field, Star Trek/Next Generation had a shuttle-pod named the "El-Baz," and in the lobby of his Center, there is a picture of him, Gene Roddenbury, and the shuttle. Really cool stuff, I say!

 

[jgrtmp]

If your stipulating a Red Giant - Betleguese seems to be what everyone uses as the average Red Giant. Thus The outer convective edge of the star is ~5AU. Roughly about the orbit of Jupiter. The Oceans would boil away & the hydrogen would pass off in to the Giant's covection zone. The atmosphere wouldn't exist.
If it were a Red Dwarf we would have passed that phase & as the Star's size went below 1AU Earth would retain hydrogen. I too can't even begin to guess the atmospheric tint would be.

 

[MeteorWayne]

Sorry, that's not right. A red Dwarf and a Red Giant are two different animals. A red dwarf is a low mass star that will outlast the sun even if it was created at the very beginning of the universe. It has lifetimes measured in dozens of billions of years.

A Red Giant, after it expels it's outer atmosphere becomes a very hot white dwarf, which slowly cools as it radiates it's heat away.