Would the sky be blue if ....

[jgrtmp]

I'm sorry, I've failed to see what part of my post is incorrect. I broke it down into 2 sections. The first is for the Red Giant. True it becomes a White Dwarf in the end, because it had enough initial mass to develope a core.
The second section does say Red Dwarf in the beginning. These stars never develope a core & are totally convective through their (as noted) long lifetime. They however do flare like our sun & therefore out gas. Their outer radius does shrink over time & a planet like ours would capture & retain hydrogen. Since the star doesn't core there should be a lack of elements created from the CNO cycle. A Red Dwarf's fusion mechanism is Proton-Proton. Recent discoveries have found planets like ours in orbit around Red Dwarfs. The latest is 6.5 Earth masses & is a waterworld like ours. Thus the hydrogren has combined with a significant amount of Oxygen to form water. Oxygen is not a byproduct of the Proton-Proton process, but the end line of the CNO cycle. Since this is a waterworld it will have an atmosphere. The tint is still the question of this thread & I don't know the answer. The previos replies from the first page of this thread are logical. Remember our atmosphere also contains other ingredients that would have influence.
this is worth the read... http://www.sciencedaily.com/releases/20 ... 131738.htm

 

[SpaceTas]

Actually for this discussion is doesn't really matter much if the host is a red giant or a red dwarf. The color is about the same.

 

[GraemeH]

LOLZ, this question got me thinking too! :lol:

Let's first state that the Earth's sky is blue during the day because of the Tyndall Effect (preferrential scattering of blue light by O2 and N2 molecules) - the blue light is scattered in all directions, from all over the sky, to our retinas. The reason why the sky tends towards red in the evening and morning is that the blue light is even more highly scattered (due to increased optical thickness of the atmosphere) BUT out of our line of sight because of the oblique incident angle.

You can do a nice little experiment with a broad spectrum white light source, a glass of water with a little bit of milk. Look through the glass towards the light source and the glass contents looks red. Look at the glass perpendicular to the light source and it appears blue. If you can get your hands on a very fine particle size (abt 50nm) polymer emulsion the effect is amazing.

Okay, onto M dwarfs .......

Let's assume that we are still talking about a terrestrial atmosphere with the same composition as Earth and that the semi major axis is such that the M dwarf disc has the same apparent size in the sky :

Hey presto! The sky at mid day still appears blue because the preferential scattering of the blue light from all directions of the sky. HOWEVER, the disk of our M dwarf sun would be a distinct shade of red!

Yes, an M dwarf has less intensity in the visible blue component than our G class star and this would manifest itself in the fact that the sky would get redder earlier in the evening (or stay redder later in the morning) since the direct line of sight red light dominates the blue being scattered from the rest of the sky, at an earlier time.

BTW, B band filter (peak transmission at about 435nm) magnitudes give a good guide for intensity comparison in the blue part of the visible spectrum.

GJ1214 has a spectral class of M4.5 and a B magnitute of 13.8
Sol has a spectral class of G2V and a B magnitude of 5.48

Remembering that high magnitudes are less bright, then GJ1214 has 5.48/13.8 x 100 = 40% the intensity of blue light versus the sun.

We could even guestimate how much earlier the evening would become redder if we orbited an M4.5 class star. Let's assume that the length of day is the same and that on Earth the sky reddens 1 hour before Sol sets below the horizon. Therefore, if we orbited our M4.5 star we could expect the sky to redden at 1/0.4 = 2.5 hours before the sun has completely set.

Hope this helps!

 

[eburacum45]

My colleague at Orion's Arm and myself wrote this essay some years ago about this question, mostly concerned with the effects of increased atmospheric pressure but it also mentions red dwarf stars towards the end.
http://www.orionsarm.com/xcms.php?r=oa- ... lienworlds

My impression is that a planet orbiting a red dwarf star would have a dark, purplish sky at low atmospheric pressures, and a whitish sky with a hgher atmospheric pressure (which agrees with Spacetas's estimate). But it is important to remember as well that some red dwarfs are actually quite yellowish, at least as yellow as an incandescent light bulb, so there will be a difference between a cool red dwarf and a hotter one.

 

[SpaceTas]

GraemeH your ratio calculation using magnitude scale is wrong. The magnitude scale is logarithmic with:

m1 - m2 = -2.5log(L1/L2)

m1 and m2 are the magnitudes and L1 and L2 are the luminosity or brightness. So a difference of 5 magnitudes corresponds to a factor (ratio L1/L2) of 100.

invert above equation
L1/L2 = 10^( -(m1-m2)/2.5 )

^ symbol means raised to the power of
Ill set it up so ratio of fainter to brighter as done by GraemeH :
L1/L2 =10^( -(13.8-5.48)/2.5)
= 10^(-3.338)
= 0.000459
== 0.0459 %

 

[SpaceTas]

Now I try to confirm GraemeH 's B band values for the Sun and GJ 1214.

The Sun here is a reference to the absolute magnitude of the Sun in various bands http://www.ucolick.org/~cnaw/sun.html
This confirms your B value (Johnson or KPNO under Vega zero systems) but there are about 0.03 mag differences when I use other references or use absolute V 4.83 (Wikipedia on absolute magnitude combined with B-V (wikipedia on color index). A 0.03 mag uncertainty is in the range expected for most magnitude measurements.

Now going 1214 the wikipedia entry http://en.wikipedia.org/wiki/GJ_1214 gives apparent magnitudes taken from the Simbad database http://simbad.u-strasbg.fr/simbad/s...&Radius=2&Radius.unit=arcmin&submit=submit+id.

GJ1214 does have a R (Johnson) band apparent magnitude of 13.8

Now using the apparent blue magnitude of 16.40 and the distance of 13pc it s possible to calculate the absolute magnitude of GJ1214 in the blue using the distance modulus formula (derived from the definition of the magnitude scale and the inverse square law http://en.wikipedia.org/wiki/Absolute_magnitude

ie m-M = -5+5log(d)

m = apparent magnitude
M = absolute magnitude == apparent magnitude at a distance of 10 parsec (10pc)
d = distance in parsec (distance at which a star has a parallax of 1 arcsec) roughly 3.

so M = m +5 -5log(d)
= 16.4 + 5 -5*log(13)
= 15.8

So redoing my previous calculation:
L1/L2 =10^( -(15.8-5.48)/2.5 )
= 0.0000745
== 0.0074 %
even less blue light compared to Sun.
Note that this is actually the wrong comparison. The M stars are fainter in the blue not only because they are red (less blue) but more importantly they are overall much fainter. So the comparing blue magnitudes includes both effects.

These stars are extremely faint in the blue, so there is little blue light to be scattered. So despite the greater efficiency with which blue light is scattered with respect to red light (yes it follows a roughly wavelength to the 4 th power relationship whatever precise mechanism you use to describe scattering) the sky is not necessarily blue.

 

[GraemeH]

GraemeH your ratio calculation using magnitude scale is wrong. The magnitude scale is logarithmic with:

m1 - m2 = -2.5log(L1/L2)

m1 and m2 are the magnitudes and L1 and L2 are the luminosity or brightness. So a difference of 5 magnitudes corresponds to a factor (ratio L1/L2) of 100.

invert above equation
L1/L2 = 10^( -(m1-m2)/2.5 )

^ symbol means raised to the power of
Ill set it up so ratio of fainter to brighter as done by GraemeH :
L1/L2 =10^( -(13.8-5.48)/2.5)
= 10^(-3.338)
= 0.000459
== 0.0459 %

Hi SpaceTas,

Oooops! You are absolutely right and thank you for pointing out my silly error - I had the magnitude data to hand and in my haste to answer the question with an example, I made the very basic error. Apologies to all

The use of B magnitudes to give an indication of what color the sky will be for an N2/O2 atmosphere still holds true. though.

 

[darkmatter4brains]

Saw this ...

Hey presto! The sky at mid day still appears blue because the preferential scattering of the blue light from all directions of the sky. HOWEVER, the disk of our M dwarf sun would be a distinct shade of red!

...........

We could even guestimate how much earlier the evening would become redder if we orbited an M4.5 class star. Let's assume that the length of day is the same and that on Earth the sky reddens 1 hour before Sol sets below the horizon. Therefore, if we orbited our M4.5 star we could expect the sky to redden at 1/0.4 = 2.5 hours before the sun has completely set.

then this ...

GraemeH your ratio calculation using magnitude scale is wrong. The magnitude scale is logarithmic with: .....

Hi SpaceTas,

Oooops! You are absolutely right and thank you for pointing out my silly error - I had the magnitude data to hand and in my haste to answer the question with an example, I made the very basic error. Apologies to all

The use of B magnitudes to give an indication of what color the sky will be for an N2/O2 atmosphere still holds true. though.

which left me confused again .... so, what would the color of the sky be again? :?

Blue, but reddening earlier, is that the general consensus?

 

[SpaceTas]

In my earlier post I estimate the colour. For the extreme case of an M7 star (nearly as red as you can get) I estimate a white colour. My final uncertainty is how the eye would mix the colours. Maybe it would be a very faint salmon colour. For a hotter M star the balance would tip toward the blueish (as pointed out eburacum45.

When I get some more time I'll redo my estimate for an M0 star.(less red)

You can get an idea of the disk colour by looking at Betelgeuse through a telescope. Don't stare at it as the eye/brain will tend to enhance its redness. There is an excellent book called the "Colours of Stars" David Malin, which has a colour chart.

 

[Space_Goose]

Question concerning Star light and Color

This is going to be a very hard question to ask as I am not quit sure how to ask it.

I realize that my question deals with two factors, one of the color of the light emitted from a star. Second, the composition of a planets atmosphere. I guess what I am trying to figure out is, as we humans go to different worlds out side our solar system, and I believe eventually we will, are likely to visit stars that are a different color than our own sun. In fact assuming we do at some point find a planet orbiting it, I think Proxima Centauri has got the be the likeliest first choice for a manned mission to another star. As I am sure most people on these forums know, Proxima Centauri is a red dwarf.

So I guess for the sake of my writing, I am trying to come up with some scientifically viable sky colors that we might encounter in other star systems. I have been looking at Titan, with its Orange Sky, and Mars with its White/reddish colored skies and am just curouios about the different probable colors. Is there a scenario where we might encounter a Green Sky or Purple Sky, or Yellow Sky, something crazy like that?

Also, on another post I made a few months ago, neilsox replied and made a comment that the Orange Sun of Alpha Centuari B would likely affect our ability to see colors. I am just curious to what extent this would be noticeable.

Thanks

 

[MeteorWayne]

Re: Question concerning Star light and Color

There's another thread that has thrashed out some of this, when I find it I'll merge this one in and let you know which one it is.

Wayne.