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Tom Mazanec
2012-Oct-20, 10:47 AM
Let's say we hit the jackpot and get a Ia supernova at a few hundred light years...not close enough to cause ecological damage but close enough to outshine the moon. Would there be any weird optical effects of a subarcsecond light that bright? For example, I understand eclipses of the sun have "light ripples" on the ground a split second before/after totality (I have never been fortunate enough to confirm). What color would the sky be? Any other comments?

Hornblower
2012-Oct-20, 02:11 PM
Let's say we hit the jackpot and get a Ia supernova at a few hundred light years...not close enough to cause ecological damage but close enough to outshine the moon. Would there be any weird optical effects of a subarcsecond light that bright? For example, I understand eclipses of the sun have "light ripples" on the ground a split second before/after totality (I have never been fortunate enough to confirm). What color would the sky be? Any other comments?

My educated guess is that there would be a fluctuating mottled ripple effect on the ground similar to the pattern on the bottom of a swimming pool when the surface is choppy. That would be consistent with the twinkling we see with bright stars in typical atmospheric turbulence.

I saw vivid shadow bands on the beach during the few seconds before and after totality during the 1970 solar eclipse at Virginia Beach. They were roughly linear because the light source was a sliver instead of a point.

The sky color depends only on the color of the light source, not on its angular size.

chornedsnorkack
2012-Oct-20, 03:21 PM
The sky color depends only on the color of the light source, not on its angular size.

Colour AND brightness.

While moonlight is only slightly yellower than sunlight and Moon is bright enough to brighten the sky a lot (I found an estimate of 25 times or 3,5 magnitudes over background skyglow), this does not suffice to make sky blue or any colour but gray.

How much would a white light source have to be brighter than Moon in order to make sky blue rather than gray?

Tom Mazanec
2012-Oct-20, 03:45 PM
Remember, the light would be bluer than sunlight or moonlight.

Romanus
2012-Oct-20, 05:11 PM
We have a vague idea of what this would look like; at least one description of SN 1006 from Europe describes how brilliantly it twinkled, as from the observer's location it was near the southern horizon. That was a very bright SN, about -7.5; one as bright as the full Moon would be like nothing else. I'm guessing that--at high altitudes--it would look like any other star, just extremely bright. At low altitudes, though, I would probably be a kind of "Super Sirius": brilliantly flashing many different colors in a way that would make the whole landscape look psychedelic were the environment dark enough.

Ara Pacis
2012-Oct-20, 10:19 PM
It would also be visible during the day, same as the moon sometimes is.

chornedsnorkack
2012-Oct-21, 07:14 AM
It would also be visible during the day, same as the moon sometimes is.

Yes, but sometimes Venus also is.
So, about -4, -5 the point source would be bright enough to see in blue day sky.
Above -6, the direct light of the point source would surpass all other light sources (stars and skyglow) combined, and cast shadows.
Above -9, the scattered light of the point source would exceed skyglow and brighten the sky over background levels.

But this assumes merely Rayleigh scattering of visible blue light.

Would the ultraviolet and soft X-ray emission of the supernova on being absorbed in upper air cause the atmosphere to luminesce?

George
2012-Oct-21, 11:30 PM
Colour AND brightness.

While moonlight is only slightly yellower than sunlight... Do you mean spectrally or visually? Both objects have arguably no hint of yellow visually. The yellow designation appears to have emerged from the early days of spectral classifications (Vega = white) and the fact that we only gaze upon the Sun when on the horizon, which appears yellow due only to atmospheric extinctions. [My avatar is an unfiltered image from Kitt Peak's solar telescope.] Forgive me for my pedantry on this, but as a world "re-nouned" heliochromologist, I feel obliged to make a somewhat colorful fool of myself with this trivial point; it's a pertinacity I seem to have acquired. :)


... and Moon is bright enough to brighten the sky a lot (I found an estimate of 25 times or 3,5 magnitudes over background skyglow), this does not suffice to make sky blue or any colour but gray.

How much would a white light source have to be brighter than Moon in order to make sky blue rather than gray?
If we use a value of 1 cd/m^2 for a safe lower limit of photopic vision, then we would need a minimum of about 40 full Moons. The blue sky, near the Sun, has a luminance of about 10,000 cd m^-2, or 10 mag. difference. The Sun reduced by 10 mag. is -16.73, or 39x brighter than a full moon (-12.74 mag. mean).

However, Hornblower, in an older related thread (http://cosmoquest.org/forum/showthread.php/96692-Luminosity-question-out-of-the-blue-sky?) has measurments suggesting a lower limit might be 1/50,000 reduction in solar luminance, which yields only an 8 Full Moon requirement. [This may perhaps require blocking the direct light for dark adaptation.]

If the light source happens to be bluer than the Sun, then this will affect the result as well. Using the 1/50,000 reduction, and a 5 to 1 blue to red ratio greater than that of the Sun, requires that about 5 O-type stars at 3 lyrs. is all that is needed. Of course, only 1 larger O-type star would likely do the trick, too.

chornedsnorkack
2012-Oct-22, 06:57 AM
Do you mean spectrally or visually? Both objects have arguably no hint of yellow visually.
Checked the numbers.
The B-V colour index of Sun is +0,66.
Moon is +1,22. Actually closer to Mars (+1,45) than Sun.


If the light source happens to be bluer than the Sun, then this will affect the result as well. Using the 1/50,000 reduction, and a 5 to 1 blue to red ratio greater than that of the Sun,
The B-V colour index is -0,30 for B0 and only -0,33 for O5 - getting close to the infinite temperature Rayleigh-Jeans tail already!

Som about 2,5 times greater than that of the Sun. But 4 times greater than Moon.

George
2012-Oct-22, 12:50 PM
Checked the numbers.
The B-V colour index of Sun is +0,66.
Moon is +1,22. Actually closer to Mars (+1,45) than Sun.
Right, it is more yellow... spectrally. But is an overhead Moon seen as yellow? Worse, is it orange (per the B-V color index value of 1.22)?

The disconnect between the remarkably effective color index system and visual color is obvious and, surprisingly, ubiquitous. Magazines, textbooks (though I think these are getting better), H-R diagrams, color indexes, etc. often ignore the broader white band that exists, which includes the Sun -- a visually white star. Even the color extremes are misleading since most people, I think, see "red" stars as orange, and blue stars are never a saturated blue since the other colors of their spectrum are relatively strong, making "blue stars" appear bluish-white. There is certainly some subjectivity in all this due to the variation of people's vision, but it isn't all that subjective.

[We might want to start another thread if you find this interesting enough to discuss. I may be out of pocket for a while, however.]

ngc3314
2012-Oct-22, 02:12 PM
That B-V for the full Moon must be too red. Detailed studies (this one includes earlier references (http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1973AJ.....78..267L&data_type=PDF_H IGH&whole_paper=YES&type=PRINTER&filetype=.pdf)) give B-V=0.85 near full Moon, reddening slightly for large off-Sun angles. Redder than the Sun, but not by all that much.

George
2012-Oct-23, 04:18 AM
That B-V for the full Moon must be too red. Detailed studies (this one includes earlier references (http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1973AJ.....78..267L&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf)) give B-V=0.85 near full Moon, reddening slightly for large off-Sun angles. Redder than the Sun, but not by all that much.Thanks, that makes more sense. [My Google attempt was fruitless to get a B-V for the Moon to verify the value.] But 0.85 is still a K type star, so I wonder how a V-R value for the Moon might compare? I would think these stars might be a bit yellow or yellowish-orange, and the Moon is not.

In my new attempt to get some values I stumbled across some history that states the B-V color index comes from the values of the photographic imaging of the past, which favors blue light (B mag. value), with visual observation (V). Vega happens to be as bright photographically as visually, so its B-V = 0. This is from an early 1960s book, where it mentions the little 3" round photographic plates that fit into the Schmidt scopes from the 1930s. Cool stuff!