Color of the Night Sky

What color is the night sky? Contrary to prevailing views, the moonless
night 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 physical
reasons for those colors.

The Night Photography Series:

Contents

Introduction

Color Definitions

Airglow

Colors of Airglow
Aurora

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

Introduction

What color is the night sky? Contrary to prevailing views, the moonless
night sky is rarely, if ever, black or blue. It is actually much more
colorful, e.g. Figures 1a, 1b.. In this article, I’ll describe some of
the 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 the
night sky. The brown in the sky is interstellar dust. The pinks are from hydrogen emission
nebulae. 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 scattering
increases 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 than
the wavelength of light. In our atmosphere, those particles are the molecules
that make up our atmosphere, including nitrogen and oxygen.

So if the daytime sky is blue due to scattered light, the moonless night sky
should have a component of scattered starlight. It does, but just as the sun
is many times brighter than the blue sky, scattered starlight is many times
fainter than starlight. And stars are already pretty faint. But other processes
occur in both the daytime and night sky, including aurora and airglow.
Aurora and airglow occur during the day but are usually so much weaker than
the 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 rays
and ultraviolet light from the sun. Both of these processes are global
in 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 Africa
on moonless nights and far from any city lights.

Aurora are caused by charged particles from the sun, mainly electrons and
protons, colliding with atoms in the upper atmosphere. The collisions
excite the atoms and they release the energy as light, at the same wavelengths
as the airglow. Aurora generally occur toward the poles as the magnetic
field of the Earth deflects many of the incoming charged particles and
the magnetic field traps the particles and funnels them into the polar
atmosphere 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 moon
is much brighter than stars and airglow, so Rayleigh scattered light
from the moon will color the night sky blue, but that will also result
in too much light to see fainter stars.

If you make images of the night sky, what do you want to show? Do you
want to show only what the human eye can see? The night sky has several
components, 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 upper
  atmosphere. Emission line light is narrow band, like neon signs, so very
  colorful 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 nebulae
  are deep blue like our daytime sky from a high mountain altitude (and
  the 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 with
  narrow emission lines adding other (saturated colors) to the interstellar
  dust color. Part 2c gives more details.

The true colors of the night sky are actually quite saturated, just
hidden by the effects in our atmosphere. If you want to show the colors
of the night sky beyond the earth, you need to SUBTRACT the scattered
light 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 exists
but we can’t see. For example, high speed photography can show processes too fast
for us to perceive. Long exposure photography blurs action, for example making a
waterfall appear smooth. Or long exposure photography to show us things too faint
for 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 the
spectra 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 these
colors can be directly verified visually. Many objects in the night sky can show
color, including hydrogen emission nebulae, planetary nebulae and stars. The main
inhibitor 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 human
visual system. The wavelengths recorded match that of the human eye.

Natural Color. What most film and digital camera daytime images
are–color spectral response that is close to the human eye response,
but may be different in contrast and saturation. The 3 colors can also
be 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/or
saturation. There is a continuum between natural color and enhanced
color. A daytime landscape image is typically natural color that has
been 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. For
example False-color IR photography includes near infrared. Mid-infrared
or ultraviolet imaging are also false color. It can also be black and
white (e.g. image one wavelength outside the visual range). Most Hubble
Telescope images and most images from professional observatories are
False Color or Narrow Band Color. Most of my professional scientific
work is false color and narrow band (most commonly narrow bands in
the infrared).

Narrow Band Color. Use of narrow passbands to isolate particular
properties, typically for imaging a specific composition. Narrow band
can 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 the
hardest to achieve, and is not actually possible with current technology with
some unusual spectral content. It probably comes closest in portrait
photography 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 brighter
near the horizon because we are looking through more atmosphere. This near-horizon
effect 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 the
night sky: light from stars, the Milky Way galaxy, and airglow: light from
molecules in Earth’s upper atmosphere excited by solar ultraviolet light
during the day and from cosmic rays. The molecules emit light throughout
the night. The green is from oxygen typically 90-100 km high. The red
is typically from hydroxyl (OH) 80 to 90 km high. The airglow light is
emission line sources, like that from a neon sign. That narrow-band light
creates enhanced colors on the landscape, in particular greens and reds
in 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 of
comments, some quite off the mark. For example, “consider changing the
tint to more accurately depict what was seen that night (more black/blue
than 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, the
dominant light on the landscape from the night sky was yellowish green
when this image was obtained. The lake reflection loses some of the
yellow and enhances blue due to the index of refraction of water being
higher in the blue than in the red, so the color of the reflection will
always be different. Also, note the reflection toward the bottom of the
image is more brown. That is particularly evident on the left side in the
reflection of the pine trees. The brown color is due to light reflected
off the bottom of the lake, from the brown mud, and that reflection
reduces 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 appears
more red when low in the sky. I process my images on a color calibrated
monitor using color calibrated workflow. The colors shown represent
the correct color tint. Of course I adjust contrast and saturation to
make a beautiful image, but if this image could be made on fine grained
color slide film, the colors would be similar but more intense and with
higher contrast. The image was made with a “sunny” daylight color balance on the
camera. 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, looks
  chartreuse 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, but
  there 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 to
affect 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 color
to our unaided ayes. Figure 2 shows a strong aurora near Latitude 40 degrees north
during the solar maximum in 2003. Note the green color is similar to that in the
Maroon bells image with airglow in Figure 1b. The red in the aurora is much stronger
than 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.

See

Aurora Photography
for more details.

Figure 2. Strong auroral ray imaged from the Denver metro area
during 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 beautiful
color, including the Zodiacal light, the color of stars, nebulae, both
emission (e.g. red and green emission) and reflection (usually blue) and combinations
of these colors.

The Zodiacal Light

We see bluish-gray light from sunlight scattered by dust in our Solar
System, called the Zodiacal light. The dust occurs manly in the plane
in which the planets rotate around the sun, called the ecliptic, where
the Zodiac is located. The dust is relatively large grains, but has some
fine components. The scattering efficiencies are slightly higher in the
blue for these grain sizes, but not nearly as much a finer particles
with Rayleigh scatter. Thus, the Zodiacal light is bluish gray, as
shown in Figure 3.

Figure 3. Night at Bosque del Apache, New Mexico in natural color. The bluish-gray
band extending to the upper left corner is the Zodiacal light.
The Milky Way extends toward the upper right. Near the horizon is
red airglow also illustrating that airglow is usually more
intense near the horizon.

Figure 4. Green and red airglow over Pyramid Lake, Nevada, natural color. The view is looking north
toward 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 may
also 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 in
our atmosphere makes the setting sun red. This is transmitted light.
Dust in the Milky Way is similarly red: a brownish red. If light is
reflected, and the particles are very small, the dust appears blue, like
smoke particles reflecting light in our atmosphere, or the daytime sky.
The blue is from a process called Rayleigh scattering that scatters
blue light more efficiently than red light. Gas in the galaxy is mainly
composed of hydrogen, which emits light like a neon sign or fluorescent
light when the atoms are excited by the radiation in space, but at the
wavelengths 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 airglow
like in our atmosphere which is from singly-ionized oxygen.) Thus, nebula
(clouds of gas) can appear pinkish, magenta, green or blue. Examples are shown
in Figures 5, 6, and 7.

Many astrophotographers use modified cameras to let in more of the red
light from hydrogen, making nebulae appear very red and are not true
color images. But a true color image, that is images made with the
spectral response close to that of the human eye, show a more pinkish
color to nebulae. Unmodified digital cameras are very good true color
cameras. True color should not be confused with color balance. An image
from true color camera can still be pushed to have unnatural colors due
to the relative color balance between red, green and blue channels.

Figure 5. M8, the Lagoon Nebula and M20, the Trifid Nebula in Sagittarius near the
center of the Milky Way galaxy. The effects of airglow from the Earth’s
atmosphere have been removed from this image. The colors include: brownish-red from
dust 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 makes
our 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 the
Milky Way is just off the left edge of this image. The image contains faint reddish brown
signatures of dust, along with pink emission nebulae containing hydrogen and bluish
nebulae from Rayleigh scattered starlight.

Figure 7. The Sword and Belt of Orion shows more detail than in Figure 6.
The image contains faint reddish brown
signatures of dust, along with pink emission nebulae containing hydrogen and bluish
nebulae from Rayleigh scattered starlight.
Note the many blue stars in the image. Galaxies have more redder stars near
their 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 been
imaging 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 images
is all over the place: magenta, green, yellow, red, blue, and black. The problem with
long exposures on film is reciprocity failure, and the different colors in color film
have different failure rates. Thus the colors drift depending on the film and
how it is handled. Colors were simply not reliable at the long exposure
times required. Figure 8 shows a nice star trail image with a blue sky.
But I started the exposure in deep twilight, so it naturally recorded
some blue. Is the green real or from reciprocity failure? No, film did
not reliably record accurate colors in long exposure night photography,
unless one accurately calibrated the reciprocity failure of the different
colors and used the appropriate color correction filters.

Figure 8. A star trail image made with film on 4×5 sheet film. This image was made near
solar 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 due
to airglow or reciprocity failure.

Color Balance of Night Sky Images

I acknowledge that the prevailing view by today’s digital camera night sky landscape
photographers is to make the sky dark blue or black. Some advocate setting
color 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 results
in 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 shows
color from airglow, and the landscape is not just a silhouette, but is
beautifully 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 blue
in 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 sky
is blue and torque the color balance to make everything in the night sky
a beautiful deep blue. One way they do this is to use
a fluorescent light color balance. Figure 10a gives one such example
processed with a fluorescent color balance. Figure 10b shows
the more natural color balance. The star clouds of the Milky
Way are reddish-brown. Stars have more subtle colors, ranging
from red to orange to white, and bluish-white. The color of the sky
is 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 using
fluorescent color balance. These colors are not the natural colors.

Figure 10b. Night sky digital photo of the summer Milky Way using
daylight color balance. These colors show the natural colors we would perceive
if our color vision were sensitive enough.

Twilight Blue

The daytime clear sky is blue because of Rayleigh scattering. Rayleigh
scattering has greater efficiency toward blue wavelengths and is caused
by particles much smaller than the wavelength of light. The daytime sky
can also appear white and this is due to Mie scattering: the scattering
by particles similar to and larger than the wavelengths of light.
Typically white is caused by aerosol scattering. But as the sun sets, and the
as the sky becomes dimmer with approaching night, the sky becomes bluer.
This is because the Earth (including mountains and clouds) block sunlight
on the lower atmosphere, but the sun still illuminates the upper atmosphere.
The large scattering particles (aerosols) responsible for the white
or washed out blue are no longer illuminated by sunlight because they are
concentrated in the lower atmosphere (haze), leaving only
air molecules high in the atmosphere, predominantly nitrogen and
oxygen, illuminated by the sun, to scatter light
so we see the full effect of the blue color of Rayleigh scattering.
Figure 11a shows the sky well after sunset but where high clouds are
still 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 emission
line. The night sky is dominated by red in Figure 3, an indication of
emission dominated by hydroxyl. Airglow also occurs during the day, and
in fact is much stronger than it is at night. We do not see it because
the scattered sunlight is so much brighter. But on nights when the green
airglow from oxygen is active, we can usually see it in deep twilight
as a green band near the horizon (Figure 11b). On the night in Figure
11b the green airglow was strong. You can predict how the beginning of
a night my be concerning the intensity of the green airglow by watching
for this green color band in twilight. Conditions can change in less
than an hour, so observing the green band is only an indicator.

Figure 11b. The green band in deep twilight is airglow from oxygen
emission. Natural color. The color is usually visually quite apparent in twilight on nights
when 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 unless
the 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 the
red and green airglow (left side of the image). Natural color. The bright object in the
twilight is the crescent moon. See the

gallery image for more details.

Rayleigh Scattered Starlight?

Certainly all light entering the Earth’s atmosphere will have some light
scattered by the Rayleigh effect. So why not stars? Yes, starlight
is Rayleigh scattered. But it is also Mie scattered. The sky around
the sun is not blue because Mie scattering dominates over the Rayleigh
scattered component. Similarly with the Moon. Close to the moon, the
scattered light is not blue because Mie scattering dominates. This is
easily observed in Figure 11c, where the light around the Moon (the bright
object near the left edge) is surrounded by a yellow halo. Stars do the
same thing: bright halo of Mie scattered starlight and fainter Rayleigh
scattered component. Because stars cover the sky, the Mie scattered
component dominates, so scattered starlight that brightens the night
sky has color closer to the star colors, and most stars are similar to
the color of our sun (yellow) or redder, with fewer blue-white stars.
There are extremely few blue stars, though some of the bluish stars are
bright blue giants. The bottom line is that scattered starlight is not
dominantly blue from Rayleigh scattering, to the contrary it is closer
to neutral relative to the star, from Mie scattering, and because there
are more cooler yellow and red stars, scattered starlight on average
will be yellowish rather than bluish.

We can get a good estimate of the Rayleigh scattered starlight
by measuring the intensity of the clear blue daytime sky. The Sun
is visual magnitude of -26.7. Most people know that the Sun is too
bright to look at, but the blue sky is not. Measurements of the blue sky
indicate a surface brightness of about magnitude 4 per square arc-second.
A good dark sky location has a visual magnitude of about 22 per square
arc-second, or about 18 magnitudes fainter. One magnitude is the
fifth root of 100, 2.51189, so 18 magnitudes is 2.51189¹⁸
= 15.8 million. Thus, a dark night sky is about 15.8 million times
fainter than the daytime clear blue sky. The clear blue sky per
square arc-second is 4 – (-26.7) = 30.7 magnitudes fainter than the sun.
That is 1.9 trillion (1.9×10¹²) times fainter! This means that
a bright magnitude zero star will show Rayleigh scattering across the
sky 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 8.5
to 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 Rayleigh
scattering by 12.9 magnitudes, for a total of 39.7-12.9 = 26.8 magnitudes
per 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 dark
sky signal. We can do this calculation for all stars and the total
only comes to a small fraction of the darkest night sky brightness.
Plus fainter stars tend to be cooler red stars, or stars at greater
distances and reddened by interstellar dust, so there is little blue
light. That means what scattering there is in our atmosphere from faint
stars 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, but
the light at night is faint and many photographers say we do not see any color.
Because of this misperception, some photographers say there is no
color so we can color our night images any way we want. Another online
photographer said “The beauty about night photography is there really is no
basis for comparison to reality, because the eye cannot see what the camera does…”

Photographers can certainly color their images any way they please, including
blue 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 night
sky, 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 stars
show colors visible with our unaided eye(s). Antares (the bright star near the
right edge in the image), shown in Figure 12 is distinctly orange to
the (normal) eye. Altair (upper left corner) is white/bluish-white.
If really well dark adapted (no bright red lights) the star clouds of the
Milky Way around Sagittarius show as a faint yellow-brownish color. If you use a
good 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 night
of the image in Figure 12, the green band on the horizon did have a
distinctly greenish-gray color compared to the sky above (the red was
too faint to distinguish red color and appeared gray to em). As the airglow brightens, the color
comes out, and I have seen pinks, greens and reds many times over the
last 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 pink
aurora 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 can
typically verify that the hues are correct based on those colors we can detect,
especially the colors of stars. Repeated experience shows that a daylight white
balance 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 a
tungsten, fluorescent, or similar, white balance to make the night sky and stars blue. Antares
does 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 hour
viewing the sky with no lights of any kind turned on. Your eyes will
become 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 7×50
or 10×80. You’ll see more stars with color, and if you come across the
brighter nebulae, like M8, or if Orion is up (in winter in the northern
hemisphere), the Great Nebula in Orion, M42 in Orion’s belt will likely
show some color. A good telescope will show even more colors.

If you have a MacBeth color chart, take it with you when you go out
observing or photographing the night sky. Hold it out after you are
dark adapted so that it is illuminated by the night sky. What colors can
you see in the chart? With the reference colors of the chart, you can
also 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 dominantly
one color, yellowish, so without color contrasts, it is harder to perceive
color. 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 to
actually try the above with no lights
viewing 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 other
colors 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 turn
on a light at night when imaging the night sky. I usually work only by star light, except for the LCD light
on the camera (which blows away my night vision for several minutes).
I see many photographers using bright red lights. That destroys night
vision and warps color perception. If you’ve been exposed to red light
and you then look at something dark, it appears blue.

See my article on

protecting 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 white
balance 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 polar
regions and image aurora but don’t change the colors to blue. These
same emissions from oxygen (mostly) and other molecules/ions (e.g. hydroxyl) in the upper
atmosphere occur all over the world at some level, so I just do not
understand the idea of warping the color to blue. To me this is like
saying the sky is blue, so when one images a red sunset, change all
the colors to blue. In the night sky, I find the bands of green, red, yellow
and orange to be interesting formations making each night image unique, much
like cloud formations in the daytime, or much like a sunset is unique.
I have observed what is called banded airglow for decades, but before
the digital era with film, I just thought that the light was from cirrus clouds
and 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 the
LCD 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 those
from fluorescent lights, is by different atoms. Fluorescent lights
typically use neon or mercury. Thus, the wavelengths of emission are different
and the color balance would be different. It is technically incorrect to use
a digital camera fluorescent white balance for imaging the night sky.

Conclusions

Imaging the night sky shows amazing colors caused by a wide
range of natural processes. The night sky is not black, and is rarely
blue when the Moon is not out. When the Moon is out and bright enough
to color the sky blue (through Rayleigh scattered moonlight)
stars and the beauty of the Milky Way galaxy are largely lost.

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:

http://www.clarkvision.com/articles/color.of.the.night.sky

First Published December 22, 2013


Last updated September 20, 2019