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The Color of the Night Sky
by Roger N. Clark
What color is the night sky? Contrary to prevailing views, the moonlessnight sky is rarely, if ever, black or blue. It is actually much more colorful.In this article, I'll describe some of the colors and the physicalreasons for those colors.
The Night Photography Series:
Contents
Introduction
Color Definitions
Airglow
Colors of AirglowAurora
The Colors of the Night Sky Beyond Our Atmosphere
The Zodiacal Light
The Night Sky Beyond the Solar System
Film Era: Blue Night Skies?
Color Balance of Night Sky Images
The Color of the Night Sky Despite Prevailing View
Twilight Blue
Rayleigh Scattered Starlight?
Human Vision and Color Perception of the Night Sky
Discussion
Conclusions
References and Further Reading
- Recommended Cameras and My Gear List forPhotography
Introduction
What color is the night sky? Contrary to prevailing views, the moonlessnight sky is rarely, if ever, black or blue. It is actually much morecolorful, e.g. Figures 1a, 1b.. In this article, I'll describe some ofthe colors and the physical reasons for those colors.
Figure 1a. The Milky Way rises above the La Sal Mountains and Arches National Park, Utah/This scene is all natural light and natural color. The light on the land is that from thenight sky. The brown in the sky is interstellar dust. The pinks are from hydrogen emissionnebulae. The blue patch is Rayleigh scattered starlight off of tiny dust grains and molecules(similar to scattering in the Earth's daytime sky reflecting blue light).Full image description and larger image at:Star Clouds of the Milky Way Above Balanced Rock, Arches National Park.
The daytime Earth's sky is blue due to Rayleigh scattering. Rayleigh scatteringincreases in efficiency as the wavelength of light becomes shorter(blue light has a shorter wavelength than green, yellow, or red light).Rayleigh scattering results when light encounters a particle much smaller thanthe wavelength of light. In our atmosphere, those particles are the moleculesthat make up our atmosphere, including nitrogen and oxygen.
So if the daytime sky is blue due to scattered light, the moonless night skyshould have a component of scattered starlight. It does, but just as the sunis many times brighter than the blue sky, scattered starlight is many timesfainter than starlight. And stars are already pretty faint. But other processesoccur in both the daytime and night sky, including aurora and airglow.Aurora and airglow occur during the day but are usually so much weaker thanthe blue Rayleigh scattered light from the sun that we do not notice it.But at night, the light from airglow, and if at higher north or south latitudes,aurora, are usually brighter than the blue Rayleigh scattered light from stars.
Airglow is caused by luminescence of molecules in the upper atmosphere.The luminescence is the result of excitation of atoms by cosmic raysand ultraviolet light from the sun. Both of these processes are globalin extent, and cosmic rays strike the Earth both day and night. Thus,airglow can be strong at any latitude, including equatorial regions.For example, I have observed strong red airglow in equatorial east Africaon moonless nights and far from any city lights.
Aurora are caused by charged particles from the sun, mainly electrons andprotons, colliding with atoms in the upper atmosphere. The collisionsexcite the atoms and they release the energy as light, at the same wavelengthsas the airglow. Aurora generally occur toward the poles as the magneticfield of the Earth deflects many of the incoming charged particles and the magnetic field traps the particles and funnels them into the polaratmosphere where they collide with the atoms in the upper atmosphere.
If the moon is up and more than a thin crescent, the light from the moonis much brighter than stars and airglow, so Rayleigh scattered lightfrom the moon will color the night sky blue, but that will also resultin too much light to see fainter stars.
If you make images of the night sky, what do you want to show? Do youwant to show only what the human eye can see? The night sky has severalcomponents, which I'll list from near to far and their natural colors:
- Light pollution reflecting on aerosols and clouds in the atmosphere(generally orange, but changing as more LED lights are used),
- Aerosols reflecting light from stars and the Milky Way (more neutral in color, e.g. hazy).
- Airglow, emission line light emitted by atoms in the upperatmosphere. Emission line light is narrow band, like neon signs, so verycolorful and very saturated, usually greens and reds but can be orange,yellow, pink and blue. These are the same colors as polar aurora,but show all over the world. Red (emission from oxygen above 100 km)is more common at lower latitudes. See Part 4c for more info.
- Scattered light in the solar system (e.g. Zodiacal light), generally blue-gray.
- Planets, e.g. red Mars, Yellow Jupiter, Saturn, yellow-white Venus, reddish Mercury.
- Stars: multiple colors (except green). Most stars are red, orange,yellow, and white. Less than 1% of stars are blue. Part 2b give more details.
- Emission nebulae are again like neon signs so very saturated colors,including blue, magenta, intense pink, and green. Some reflection nebulaeare deep blue like our daytime sky from a high mountain altitude (andthe same process--Rayleigh scattering from tiny articles). Parts 2c, 2f give more details.
- Interstellar dust: burnt orange, like reddish rust, or very red dirt.Some interstellar dust includes oxygen, hydrogen and other atoms withnarrow emission lines adding other (saturated colors) to the interstellardust color. Part 2c gives more details.
The true colors of the night sky are actually quite saturated, justhidden by the effects in our atmosphere. If you want to show the colorsof the night sky beyond the earth, you need to SUBTRACT the scatteredlight from the earth's atmosphere, including light pollution and airglow.Parts 2d, 2e, 3a, 3b, 3c give examples of processing.
Photography can be about showing what we see with our eyes, and also what existsbut we can't see. For example, high speed photography can show processes too fastfor us to perceive. Long exposure photography blurs action, for example making awaterfall appear smooth. Or long exposure photography to show us things too faintfor us to see, e.g. the beautiful nebulae and galaxies in the night sky.Photography can also show us light we can't see, for example, ultraviolet of infrared photography, or narrow band photography. All are legitimate forms of photography.
Color Definitions
Here I discuss the natural colors in the night sky. We know the true colors by thespectra of the objects in the night sky and the known spectral response of the human eye.Indeed, with different telescopes and sky conditions, many of the thesecolors can be directly verified visually. Many objects in the night sky can showcolor, including hydrogen emission nebulae, planetary nebulae and stars. The maininhibitor to detecting color visually is low contrast due to airglow and light pollution,and lack of good dark adaptation (see Part 6 for lighting and dark adaptation).
True Color. Color and contrast as close as possible to the humanvisual system. The wavelengths recorded match that of the human eye.
Natural Color. What most film and digital camera daytime imagesare--color spectral response that is close to the human eye response,but may be different in contrast and saturation. The 3 colors can alsobe converted to black and white in various proportions to change contrast.The wavelengths recorded reasonably match that of the human eye.
Enhanced Color. "Extreme" or strong pushing of contrast and/orsaturation. There is a continuum between natural color and enhancedcolor. A daytime landscape image is typically natural color that hasbeen enhanced some. A portrait of a person is typically less enhanced.Fujichrome Velvia film might be considered enhanced color.
False Color. Includes color outside of the visual passband. Forexample False-color IR photography includes near infrared. Mid-infraredor ultraviolet imaging are also false color. It can also be black andwhite (e.g. image one wavelength outside the visual range). Most HubbleTelescope images and most images from professional observatories areFalse Color or Narrow Band Color. Most of my professional scientificwork is false color and narrow band (most commonly narrow bands inthe infrared).
Narrow Band Color. Use of narrow passbands to isolate particularproperties, typically for imaging a specific composition. Narrow bandcan be entirely inside the visual range, outside the range, or both.Narrow band can also be black and white (e.g. an image at one wavelength).
All the above are legitimate imaging options. True color is thehardest to achieve, and is not actually possible with current technology withsome unusual spectral content. It probably comes closest in portraitphotography as people generally want accurate skin tones.
All forms of the above can make beautiful and stunning images.
Airglow
Airglow is more intense around solar maximum (e.g. 2013-2014) and appears brighternear the horizon because we are looking through more atmosphere. This near-horizoneffect is apparent in Figure 1b.
Figure 1b. Maroon Bells Nightscape. This scene is all natural light and natural color. The light on the land is that from thenight sky: light from stars, the Milky Way galaxy, and airglow: light frommolecules in Earth's upper atmosphere excited by solar ultraviolet lightduring the day and from cosmic rays. The molecules emit light throughoutthe night. The green is from oxygen typically 90-100 km high. The redis typically from hydroxyl (OH) 80 to 90 km high. The airglow light isemission line sources, like that from a neon sign. That narrow-band lightcreates enhanced colors on the landscape, in particular greens and redsin the trees.
Full image description and larger image at:Maroon Bells Nightscape Vertical Panorama.
Image Color Balance. The image above (Figure 1b) has gotten a lot ofcomments, some quite off the mark. For example, "consider changing thetint to more accurately depict what was seen that night (more black/bluethan green)." The Earth's moonless night sky is rarely black or blue,especially at solar maximum when this image was obtained in September,2013. With the red and green airglow emission, as noted in the Figure caption, thedominant light on the landscape from the night sky was yellowish greenwhen this image was obtained. The lake reflection loses some of theyellow and enhances blue due to the index of refraction of water beinghigher in the blue than in the red, so the color of the reflection willalways be different. Also, note the reflection toward the bottom of theimage is more brown. That is particularly evident on the left side in thereflection of the pine trees. The brown color is due to light reflectedoff the bottom of the lake, from the brown mud, and that reflectionreduces contrast in the reflected light of the land and sky. In the sky,the Milky way is low in the sky so is reddened much like the sun appearsmore red when low in the sky. I process my images on a color calibratedmonitor using color calibrated workflow. The colors shown representthe correct color tint. Of course I adjust contrast and saturation tomake a beautiful image, but if this image could be made on fine grainedcolor slide film, the colors would be similar but more intense and withhigher contrast. The image was made with a "sunny" daylight color balance on thecamera. More on color balance below.
Colors of Airglow
The dominant colors produced and the processes that create them in airglow and aurora include:
- 557.7 nm, Oxygen: The usually dominant emission line, lookschartreuse when bright enough to see. Originates from 90 to 100 km high.
- 630.0 and 636.4 nm, Oxygen: Strong red color. Peak emission from 230 to 270 km high nut extends from below 150 to 300 km.The 636 nm line is weaker of the two. Often this red appears above the the green, as in Figure 2.
- 650 to 700 nm, Hydroxyl, OH: around 90 km high. If there is no green, butthere is red, it is likely from hydroxyl. Figure 3 shows this case.
- 589 nm Sodium: Yellow, around 92 km high.
There are other lines, like some weak blue molecular oxygen emission lines, O2,at the same altitude as atomic oxygen 558 nm emission, but are too weak toaffect color compared to the 558 nm green oxygen emission.
Aurora
Aurora show similar colors as airglow, because the same atoms can be excited.Aurora can often be many times more intense than airglow and readily show colorto our unaided ayes. Figure 2 shows a strong aurora near Latitude 40 degrees northduring the solar maximum in 2003. Note the green color is similar to that in theMaroon bells image with airglow in Figure 1b. The red in the aurora is much strongerthan the airglow in Figure 1b, but the light is being emitted at the same wavelengths.The colors were so bright, they were evident to my eye.
SeeAurora Photographyfor more details.
Figure 2. Strong auroral ray imaged from the Denver metro areaduring a strong geomagnetic storm.
The Colors of the Night Sky Beyond Our Atmosphere
There are many processes that give the night sky beyond our atmosphere beautifulcolor, including the Zodiacal light, the color of stars, nebulae, bothemission (e.g. red and green emission) and reflection (usually blue) and combinationsof these colors.
The Zodiacal Light
We see bluish-gray light from sunlight scattered by dust in our SolarSystem, called the Zodiacal light. The dust occurs manly in the planein which the planets rotate around the sun, called the ecliptic, wherethe Zodiac is located. The dust is relatively large grains, but has somefine components. The scattering efficiencies are slightly higher in theblue for these grain sizes, but not nearly as much a finer particleswith Rayleigh scatter. Thus, the Zodiacal light is bluish gray, asshown in Figure 3.
Figure 3. Night at Bosque del Apache, New Mexico in natural color. The bluish-grayband extending to the upper left corner is the Zodiacal light.The Milky Way extends toward the upper right. Near the horizon isred airglow also illustrating that airglow is usually moreintense near the horizon.
Figure 4. Green and red airglow over Pyramid Lake, Nevada, natural color. The view is looking northtoward the Black Rock Desert. The Big Dipper is just above the horizon in the green zone.Green and red emission mix to give yellows and orange colors. Sodium emission mayalso contribute to the orange color.Images with a 24mm f/1.4 lens at f/2, 30 second exposures at ISO 1600. This is a 3-frame mosaic.
The Night Sky Beyond the Solar System
The night sky beyond our Solar System is filled with many colors.Stars range from solar type (like our sun) to cooler orange and red stars,to hotter than our sun, with blue-white colors. There are no green stars.In the plane of the Milky Way galaxy is a lot of dust and gas. Dust inour atmosphere makes the setting sun red. This is transmitted light.Dust in the Milky Way is similarly red: a brownish red. If light isreflected, and the particles are very small, the dust appears blue, likesmoke particles reflecting light in our atmosphere, or the daytime sky.The blue is from a process called Rayleigh scattering that scattersblue light more efficiently than red light. Gas in the galaxy is mainlycomposed of hydrogen, which emits light like a neon sign or fluorescentlight when the atoms are excited by the radiation in space, but at thewavelengths of hydrogen are in the red (Hydrogen-alpha at 656.28 nm)with a weaker line in the blue green (hydrogen beta at 486.1 nm).Oxygen is also a common component and it emits greenish-blue light.(The green emission in nebulae is from doubly-ionized oxygen, called O III, at 500.7 nm, and is a bluer green than the chartreuse green airglowlike in our atmosphere which is from singly-ionized oxygen.) Thus, nebula(clouds of gas) can appear pinkish, magenta, green or blue. Examples are shownin Figures 5, 6, and 7.
Many astrophotographers use modified cameras to let in more of the redlight from hydrogen, making nebulae appear very red and are not truecolor images. But a true color image, that is images made with thespectral response close to that of the human eye, show a more pinkishcolor to nebulae. Unmodified digital cameras are very good true colorcameras. True color should not be confused with color balance. An imagefrom true color camera can still be pushed to have unnatural colors dueto the relative color balance between red, green and blue channels.
Figure 5. M8, the Lagoon Nebula and M20, the Trifid Nebula in Sagittarius near thecenter of the Milky Way galaxy. The effects of airglow from the Earth'satmosphere have been removed from this image. The colors include: brownish-red fromdust in the Milky Way galaxy, which is particularly abundant around the galactic center,and the constellation Sagittarius and nearby constellations. The red nebula shows the glow from hydrogen gas, and the blue is light scattered off of fine dust particles,where the blue is created by Rayleigh scattering, the same process that makesour daytime sky blue.
Figure 6. The constellation Orion. Orion is nearly opposite the center of the Milky Way,so we are looking at an arm of our galaxy that contains much less dust. The plane of theMilky Way is just off the left edge of this image. The image contains faint reddish brownsignatures of dust, along with pink emission nebulae containing hydrogen and bluishnebulae from Rayleigh scattered starlight.
Figure 7. The Sword and Belt of Orion shows more detail than in Figure 6.The image contains faint reddish brownsignatures of dust, along with pink emission nebulae containing hydrogen and bluishnebulae from Rayleigh scattered starlight.Note the many blue stars in the image. Galaxies have more redder stars neartheir centers and bluer stars in the outer spiral arms. Compare the numbers of blue stars here to those in Figure 5.
Film Era: Blue Night Skies?
If been asked to look at the film area when film recorded skies as blue. I've beenimaging the night sky since junior high school, long before the digital camera era.I have thousands of color night sky images. The color of the sky in my imagesis all over the place: magenta, green, yellow, red, blue, and black. The problem withlong exposures on film is reciprocity failure, and the different colors in color filmhave different failure rates. Thus the colors drift depending on the film andhow it is handled. Colors were simply not reliable at the long exposuretimes required. Figure 8 shows a nice star trail image with a blue sky.But I started the exposure in deep twilight, so it naturally recordedsome blue. Is the green real or from reciprocity failure? No, film didnot reliably record accurate colors in long exposure night photography,unless one accurately calibrated the reciprocity failure of the differentcolors and used the appropriate color correction filters.
Figure 8. A star trail image made with film on 4x5 sheet film. This image was made nearsolar minimum when airglow was very low. The sky shows as blue and green.The blue is from starting the exposure in deep twilight. The green may be dueto airglow or reciprocity failure.
Color Balance of Night Sky Images
I acknowledge that the prevailing view by today's digital camera night sky landscapephotographers is to make the sky dark blue or black. Some advocate settingcolor balance to fluorescent or tungsten in order to make the sky blue or black,or stretch their images to make their images look blue or black. Often this resultsin strange colors, like purple, for the Milky Way.
Figure 9. An example nightscape image in natural color.The Milky Way rises over the San Juan Mountains of Colorado in June.Key elements to this image include good star colors, the background sky showscolor from airglow, and the landscape is not just a silhouette, but isbeautifully lit by the natural light from the night sky (stars, the galaxy, and airglow),and the colors in the image are natural (too many nightscapes are artificially turned bluein post processing).See the Gallery Image for more information about this image.
The Color of the Night Sky Despite Prevailing View
Many night sky digital photographers have the idea that the night skyis blue and torque the color balance to make everything in the night skya beautiful deep blue. One way they do this is to usea fluorescent light color balance. Figure 10a gives one such exampleprocessed with a fluorescent color balance. Figure 10b showsthe more natural color balance. The star clouds of the MilkyWay are reddish-brown. Stars have more subtle colors, ranging from red to orange to white, and bluish-white. The color of the skyis green from oxygen emission (the green bands are called banded airglow),and red from oxygen and hydroxyl.
Figure 10a. Night sky digital photo of the summer Milky Way usingfluorescent color balance. These colors are not the natural colors.
Figure 10b. Night sky digital photo of the summer Milky Way usingdaylight color balance. These colors show the natural colors we would perceiveif our color vision were sensitive enough.
Twilight Blue
The daytime clear sky is blue because of Rayleigh scattering. Rayleighscattering has greater efficiency toward blue wavelengths and is causedby particles much smaller than the wavelength of light. The daytime skycan also appear white and this is due to Mie scattering: the scatteringby particles similar to and larger than the wavelengths of light.Typically white is caused by aerosol scattering. But as the sun sets, and theas the sky becomes dimmer with approaching night, the sky becomes bluer.This is because the Earth (including mountains and clouds) block sunlighton the lower atmosphere, but the sun still illuminates the upper atmosphere.The large scattering particles (aerosols) responsible for the whiteor washed out blue are no longer illuminated by sunlight because they areconcentrated in the lower atmosphere (haze), leaving onlyair molecules high in the atmosphere, predominantly nitrogen andoxygen, illuminated by the sun, to scatter lightso we see the full effect of the blue color of Rayleigh scattering.Figure 11a shows the sky well after sunset but where high clouds arestill faintly illuminated by the brighter atmosphere near the sun.This blue twilight time is called the blueing hour.
Figure 11a. Twilight on the night the image in Figure 3 was obtained. Natural color.The twilight sky becomes very blue.
Compare the color in Figure 3 with that in Figure 11a. On this night,there is no detectable green airglow from oxygen at the 558 nm emissionline. The night sky is dominated by red in Figure 3, an indication ofemission dominated by hydroxyl. Airglow also occurs during the day, andin fact is much stronger than it is at night. We do not see it becausethe scattered sunlight is so much brighter. But on nights when the greenairglow from oxygen is active, we can usually see it in deep twilightas a green band near the horizon (Figure 11b). On the night in Figure11b the green airglow was strong. You can predict how the beginning ofa night my be concerning the intensity of the green airglow by watchingfor this green color band in twilight. Conditions can change in lessthan an hour, so observing the green band is only an indicator.
Figure 11b. The green band in deep twilight is airglow from oxygenemission. Natural color. The color is usually visually quite apparent in twilight on nightswhen the airglow emission is strong.
As twilight fades, the Rayleigh scattered light from the sun fades to a level below airglow.Twilight has a similar intensity to the airglow in the image in Figure 11c.As twilight fades further, all traces of blue from Rayleigh scattering fade unlessthe Moon is out and more than a thin crescent.
Figure 11c. Image about 1.2 to 1.5 hours after sunset, when the blue from twilight (right side of the image) is similar in intensity to thered and green airglow (left side of the image). Natural color. The bright object in thetwilight is the crescent moon. See thegallery image for more details.
Rayleigh Scattered Starlight?
Certainly all light entering the Earth's atmosphere will have some lightscattered by the Rayleigh effect. So why not stars? Yes, starlightis Rayleigh scattered. But it is also Mie scattered. The sky aroundthe sun is not blue because Mie scattering dominates over the Rayleighscattered component. Similarly with the Moon. Close to the moon, thescattered light is not blue because Mie scattering dominates. This iseasily observed in Figure 11c, where the light around the Moon (the brightobject near the left edge) is surrounded by a yellow halo. Stars do thesame thing: bright halo of Mie scattered starlight and fainter Rayleighscattered component. Because stars cover the sky, the Mie scatteredcomponent dominates, so scattered starlight that brightens the nightsky has color closer to the star colors, and most stars are similar tothe color of our sun (yellow) or redder, with fewer blue-white stars.There are extremely few blue stars, though some of the bluish stars arebright blue giants. The bottom line is that scattered starlight is notdominantly blue from Rayleigh scattering, to the contrary it is closerto neutral relative to the star, from Mie scattering, and because thereare more cooler yellow and red stars, scattered starlight on averagewill be yellowish rather than bluish.
We can get a good estimate of the Rayleigh scattered starlightby measuring the intensity of the clear blue daytime sky. The Sunis visual magnitude of -26.7. Most people know that the Sun is toobright to look at, but the blue sky is not. Measurements of the blue skyindicate a surface brightness of about magnitude 4 per square arc-second.A good dark sky location has a visual magnitude of about 22 per squarearc-second, or about 18 magnitudes fainter. One magnitude is thefifth root of 100, 2.51189, so 18 magnitudes is 2.5118918= 15.8 million. Thus, a dark night sky is about 15.8 million timesfainter than the daytime clear blue sky. The clear blue sky persquare arc-second is 4 - (-26.7) = 30.7 magnitudes fainter than the sun.That is 1.9 trillion (1.9x1012) times fainter! This means thata bright magnitude zero star will show Rayleigh scattering across thesky with a surface brightness of 30.7 magnitudes per square arc-second,or about 8.7 magnitudes (3000) times fainter than a very dark night sky.There are more fainter stars, which we can sum upm the contributions.for example, there are about 140,000 magnitude 9 stars (magnitude 8.5to 9.5) and the Rayleigh Scattering from each would be magnitude 30.7 +9 = 39.7 per square arc-second. The 140,000 stars brightens the Rayleighscattering by 12.9 magnitudes, for a total of 39.7-12.9 = 26.8 magnitudesper square arc-second, or 4.8 magnitudes fainter than a dark country sky,thus Rayleigh scattering of magnitude 9 stars is about 1% of the darksky signal. We can do this calculation for all stars and the totalonly comes to a small fraction of the darkest night sky brightness.Plus fainter stars tend to be cooler red stars, or stars at greaterdistances and reddened by interstellar dust, so there is little bluelight. That means what scattering there is in our atmosphere from faintstars is much redder than what we see from daytime scattered sunlight.
Human Vision and Color Perception of the Night Sky
All of the above may be fine from a technical description of color, butthe light at night is faint and many photographers say we do not see any color.Because of this misperception, some photographers say there is nocolor so we can color our night images any way we want. Another onlinephotographer said "The beauty about night photography is there really is nobasis for comparison to reality, because the eye cannot see what the camera does..."
Photographers can certainly color their images any way they please, includingblue clouds at sunset, green snow, or whatever they want to create a mood or effect.But some are claiming blue is the correct and natural color of the night.However, there are plenty of colors that can be seen in the nightsky, even with the unaided eye to tell us what the color balance should be.
We can actually see some colors at night. For example, bright starsshow colors visible with our unaided eye(s). Antares (the bright star near theright edge in the image), shown in Figure 12 is distinctly orange tothe (normal) eye. Altair (upper left corner) is white/bluish-white.If really well dark adapted (no bright red lights) the star clouds of theMilky Way around Sagittarius show as a faint yellow-brownish color. If you use agood telescope, some of the red emission nebulae do appear as a pastel pink(M8, M20 in the image), and blue reflection nebulae do appear light blue(M20 northern component, the Merope nebula in the Pleiades, M45). M8 and M20 are seen in Figure 12 near the center,and a close-up is shown in Figure 5.
The airglow also shows color when it gets bright enough. On the nightof the image in Figure 12, the green band on the horizon did have adistinctly greenish-gray color compared to the sky above (the red wastoo faint to distinguish red color and appeared gray to em). As the airglow brightens, the colorcomes out, and I have seen pinks, greens and reds many times over thelast year or so from Colorado. The scene in Figure 9, which was made of the same mountain range as that in Figure 12, only a few miles away,was made on a night of strong green airglow to the south, but strong pinkaurora to the north. Before I made a quick exposure of the northern sky,I could tell it was a pinkish-red aurora, and the camera confirmed it Figure 13).
The digital camera is bringing those colors out much better of course, but we cantypically verify that the hues are correct based on those colors we can detect,especially the colors of stars. Repeated experience shows that a daylight whitebalance is the correct white balance to show the true colors of the night sky.I see many night sky photos from other photographers where they used atungsten, fluorescent, or similar, white balance to make the night sky and stars blue. Antaresdoes not appear blue to our eyes, and neither does the sky between stars.
Verify the above yourself. Next time you are out under a clear dark,moonless night sky away from city lights, spend at least a half hourviewing the sky with no lights of any kind turned on. Your eyes willbecome very dark adapted and your be able to see colors of many stars.Some parts of the center of the Milky Way, if it is high in the sky,will show as brown. If you are lucky, airglow or aurora will show color.Try viewing the Milky Way region with a good pair of binoculars, like 7x50or 10x80. You'll see more stars with color, and if you come across thebrighter nebulae, like M8, or if Orion is up (in winter in the northernhemisphere), the Great Nebula in Orion, M42 in Orion's belt will likelyshow some color. A good telescope will show even more colors.
If you have a MacBeth color chart, take it with you when you go outobserving or photographing the night sky. Hold it out after you aredark adapted so that it is illuminated by the night sky. What colors canyou see in the chart? With the reference colors of the chart, you canalso compare to the Milky Way and stars. With binoculars or a telescope,you can defocus the stars and see colors better. Or if you wear glasses,take them off and brighter stars will show their colors better.One of the issues with color perception is the night sky is dominantlyone color, yellowish, so without color contrasts, it is harder to perceivecolor. The MacBeth (or other) color chart helps give a reference.
I challenge the photographers who claim the moonless night sky away from cities sky is blue toactually try the above with no lightsviewing the night sky for at least a half hour.Do you really see Antares, Arcturus, Betelgeuse,Mars, Saturn, and Jupiter as blue? Can you really see no othercolors except blue?Learn to SEE!
For more on color perception at night, see Part 6b of this series: Color Vision at Night.
One key factor in seeing color is to be well dark adapted. I rarely turnon a light at night when imaging the night sky. I usually work only by star light, except for the LCD lighton the camera (which blows away my night vision for several minutes).I see many photographers using bright red lights. That destroys nightvision and warps color perception. If you've been exposed to red lightand you then look at something dark, it appears blue.
See my article onprotecting your night vision and lights for more details.
Figure 12. Summer Milky Way Nightscape with the San Juan Mountains of Colorado in natural color.
Figure 13. Pink aurora in natural color in the north on the same night as the image in Figure 9 was obtained.The yellow color on the horizon is light from the city of Montrose, Colorado.Visually, the city light looked yellow and the aurora looked pink. A daylight whitebalance on the digital camera produced colors consistent with the visual record,while a tungsten white balance did not.
Discussion
Why color the night sky blue? People go the polarregions and image aurora but don't change the colors to blue. Thesesame emissions from oxygen (mostly) and other molecules/ions (e.g. hydroxyl) in the upperatmosphere occur all over the world at some level, so I just do notunderstand the idea of warping the color to blue. To me this is likesaying the sky is blue, so when one images a red sunset, change allthe colors to blue. In the night sky, I find the bands of green, red, yellowand orange to be interesting formations making each night image unique, muchlike cloud formations in the daytime, or much like a sunset is unique.I have observed what is called banded airglow for decades, but beforethe digital era with film, I just thought that the light was from cirrus cloudsand stopped imaging. With film, the exposure times were so long,that the movement of the airglow bands smeared out so much that they weren't recorded.Now with a quick few second DSLR exposure then examining the result on theLCD shows the true nature and beauty of this phenomenon.
Also see my night images gallery:http://www.clarkvision.com/galleries/gallery.night/.
Also see Nightscape Photography with Digital Cameras for how to make stunning nightscape images.
The wavelengths of airglow emission lines, while like thosefrom fluorescent lights, is by different atoms. Fluorescent lightstypically use neon or mercury. Thus, the wavelengths of emission are differentand the color balance would be different. It is technically incorrect to usea digital camera fluorescent white balance for imaging the night sky.
Conclusions
Imaging the night sky shows amazing colors caused by a widerange of natural processes. The night sky is not black, and is rarelyblue when the Moon is not out. When the Moon is out and bright enoughto color the sky blue (through Rayleigh scattered moonlight)stars and the beauty of the Milky Way galaxy are largely lost.
- Recommended Cameras and My Gear List forPhotography
References and Further Reading
Clarkvision.com Nightscapes Gallery.
1) Night and Low Light Photography with Digital Cameras
http://www.clarkvision.com/articles/night.and.low.light.photography/
2) Digital Camera Sensor Performance Summary
http://www.clarkvision.com/articles/digital.sensor.performance.summary
3) The f/ratio Myth and Digital Cameras
http://www.clarkvision.com/articles/f-ratio_myth
4) Digital Cameras: Does Pixel Size Matter?
http://www.clarkvision.com/articles/does.pixel.size.matter
5) Digital Cameras: Does Pixel Size Matter?Part 2: Example Images using Different Pixel Sizes
http://www.clarkvision.com/articles/does.pixel.size.matter2
6) Airglow Formation.http://www.atoptics.co.uk/highsky/airglow2.htm
The Night Photography Series:
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http://www.clarkvision.com/articles/color.of.the.night.sky
First Published December 22, 2013
Last updated September 20, 2019
