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The Color of Nebulae and Interstellar Dust in the Night Sky
by Roger N. Clark
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by Roger N. Clark
The natural colors of nebulae and interstellar dust are quite predictable and very colorful. The natural colors are very saturated and not what is commonly portrayed on the internet in the digital camera era. Hydrogen emission nebulae emit at specific wavelengths and absorption by interstellar dust modifies the color balance in predictable ways through their spectral responses.
The Night Photography Series:
Contents
Introduction
The Color of Interstellar Dust
The Color of Emission Nebulae
Why Don't We See Images Like That Shown Here More Commonly?
Technical
Conclusions
References and Further Reading
Hydrogen is the most common element in the universe and commonly makes up the visible component and color of many nebulae in the night sky. When the hydrogen gas is illuminated by stars emitting ultraviolet light, the hydrogen atoms absorb the UV and emit the energy at other wavelengths. Several of those emission lines are located at visible colors. The emission lines are very narrow and form intense colors much like a neon sign emits highly saturated colors.
Dust is also pervasive in the Milky Way galaxy and the interstellar absorption from dust has a well-known spectrum equal to the inverse of the wavelength raised the power 1.5 (1/wavelength1.5). This means greater absorption at blue wavelengths, making the dust reddish-orange. Dust is also common in other galaxies.
The panorama image shown in Figure 1 of the Horsehead nebula and the Great Nebula (M42) in Orion's sword illustrates hydrogen emission in various colors, orange dust and blue scattered light. The pervasive view on the internet, circa 2000 - 2015, is that hydrogen emission nebulae should be red and one needs an astro-modified camera to show the red hydrogen emission. In this article, I'll show that dominantly red hydrogen emission nebulae are not the natural color except in certain cases where dust is strongly absorbing. The natural colors of hydrogen emission nebulae range from blue and magenta, to red. If oxygen is present, it emits a very saturated green wavelength that changes the color of the nebulae making them teal, cyan, green, or even bluish-white. High oxygen content is common in Planetary Nebulae, making them often appear greenish.
Interstellar dust is made of small particles, particles smaller than the wavelength of visible light. Such small particles scatter blue light away, transmitting red. This makes different colors possible, just like smoke from forest fires we observe here on Earth. When there is a lot of smoke in the air, sunsets and sunrises are typically very red. That is transmitted light, including direct to the sun and light scattered near the sun. Here is an example of a smoky sunrise on the Serengeti. But that same smoke viewed with the sun to the side or behind you will look blue if the particles are small enough. It is similar with interstellar dust, only the dust particles are much smaller than typical terrestrial smoke particles so the color effects are greater. Those small particles produces strong absorption when starlight is transmitted through the dust, making it very red-orange, like a smoky sunrise. But when illuminated from the side, the efficiency for scattering increases in the blue, making the dust appear blue. This is what causes most of the blue colors in Figure 1. The specific mechanism is called Rayleigh scattering and is the same reason why our daytime sky is blue here on Earth and sunsets are red. In the Earth case, the particles causing the blue are mainly the nitrogen and oxygen molecules in the atmosphere and the sunlight scattering off those particles.
The average spectrum of interstellar dust absorption increases with decreasing wavelength. This means that green and blue colors are absorbed more than orange and red colors. The spectra (intensity versus wavelength or color) is well known. I computed the color from the spectra as a function of dust thickness in Figure 2 (increasing dust from left to right across the top of the Figure).
Sometimes the interstellar dust also includes hydrogen, so the dust can be a combination of hydrogen emission and dust transmission. The dust absorbs the blue wavelengths of the hydrogen emission, so we see a small enhancement in the red color of the dust. Increasing hydrogen emission is shown in Figure 2 down the figure.
The computed colors in Figure 2 are well expressed in the colors of the dust in Figure 1. The colors shown in Figure 1 were derived using a color managed workflow with the same methodology as I use to produce daytime color images of terrestrial landscapes and wildlife. The image in Figure 1 was produced in early 2015. The diagram in Figure 2 was computed in December, 2015. The dust color computation verifies the color managed workflow produced natural colors.
Hydrogen emission nebulae emit multiple visible colors, including H-alpha (red), H-beta (blue-green), and H-gamma (blue). Sometimes oxygen and emission lines from other elements contribute. The true color of hydrogen emission nebulae is blue, purple, pink to magenta, and red depending on the abundance and reddening by dust, not the blood red we see from H-alpha modified cameras. Modified cameras with enhanced hydrogen alpha sensitivity result in hydrogen alpha dominating the images of nebulae, hiding the multiple colors and processes present. An unmodified camera shows these colors and their varying intensities better so you can discern chemical and physical processes and composition. Tables 1 and 2 show some of the colors in the deep sky, and Figure 3 shows the typical colors of hydrogen emission nebulae with embedded dust.
Table 1
Table 2. Emission Lines in Nebulae in the Visible Spectral Range
When I first made the calculations of the colors from hydrogen emission nebulae, I did not include the hydrogen-gamma line because it is half the strength of H-beta and the calculations did not produce the colors in Figure 1. The H-beta line is in between the color response of the eye's blue and green receptors, with reduced sensitivity by both (see Technical section below). The H-gamma line is near the peak sensitivity to eye's blue receptors, and the blue peak in digital camera color filters (which are designed to match the human eye). Including the H-gamma line in the calculations produced the colors observed in Figure 1, especially the magenta/pink (Figure 3). Adding small amounts of oxygen (Figure 4) produces additional colors observed in Figure 1, including the teal, blues and purples. Note, all the saturated colors in Figures 3 and 4 may not display well unless you view the figures on color calibrated monitors with wide color gamut. For example, the teal does not show on my laptop as well as on my Adobe RGB color calibrated desktop monitor.
Oxygen is also common in hydrogen emission nebulae, and the contribution of oxygen emission is seen in the Trapezium of the Great Orion Nebula, M42, in Figure 1. The color effects of increasing oxygen emission are shown in Figure 4. Messier 8, the Lagoon Nebula, and M20, the Trifid nebula, shown in Figure 5 illustrate the role of interstellar dust, hydrogen and oxygen emission. Look at the core of M8, where we see bluish-white in the centers of M8 and M20. Those colors indicate relatively low dust and and mix of oxygen + hydrogen found on the blue-white zone in Figure 4. Moving away from the centers of M8 and M20, we see the pink-magenta-red colors of lower oxygen content and increasing dust absorption. Note the color-saturated pink-magenta-red matches the color saturation in Figure 4. Moving further from the center of M8, especially to the left, we see a zone of reddish dust, indicating a strong dust content with hydrogen emission, similar to the colors in the lower left area of Figure 2. In the center of the Figure 5 image is orange-brown dust, with colors similar to those in the upper center of Figure 2, indicating low hydrogen emission in the dust.
Planetary nebulae usually have relatively high oxygen emission, often making planetary nebulae appear blue-green in natural color. In wide field nightscapes, the tiny planetary nebulae often stand out in the Milky way as small green spots among the sea of yellow and red stars and brown-orange dust.
There are two main reasons why the natural color images that I show here and in my astrophotography and nightscapes galleries are not common.
1) The common view of astrophotographers, circa 2000 - 2015, is that one needs an astro-modified digital camera in order to record hydrogen emission nebulae. The reason for this is a largely mistaken view, that view is shaped by processing methodology that is discussed in part 3c of this series, and is discussed in the second reason below.
2) Post processing methodology that includes various forms of histogram equalization. This includes all forms of auto white balance often resulting in unnatural colors. For example, if we take the computed image data from Figure 2 and perform a histogram equalization, we get the result in Figure 7. The orange and red colors are destroyed to create more colors. As commonly implemented, we see interstellar dust in night sky images showing as yellow instead of orange and red. For more on this issue, see Figures 9, 10a, 10b, and 10c of part 3c of this series.
The relative color spectral response of the eye is compared to interstellar dust, hydrogen and oxygen emission in Figure 8. This graphically shows what the colors show in Figures 2, 3, and 4. Here, we see that hydrogen alpha emission is only weakly seen by the eye's red channel. Similarly, hydrogen beta falls between the eye's blue and green receptors and hydrogen gamma falls near the peak of the blue response. Thus, hydrogen emission nebulae will appear bluish unless dust absorbs the blue (both hydrogen gamma and hydrogen beta). Dust is common in hydrogen emission nebulae, so the colors shown in Figures 3 and 4 are common. Dust absorbs more blue than green or red, so appears orange when hydrogen emission is not present. Increasing dust reddens the spectral response, making the dust red-orange. In hydrogen emission nebulae, oxygen emission is much weaker than shown here, but in planetary nebulae, the oxygen emission can be relatively stronger than shown here. The colors of these emission lines are shown on the visual color diagram in Figure 9.
Emission nebulae emit light in narrow emission lines, like neon signs. As a result, the colors are highly saturated, making emission nebulae very colorful. That color is brought out well with stock digital cameras but not modified cameras. The various colors can also be seen visually with the human eye, including pinks, blues and greens, given good sky conditions and an adequately large telescope.
Interstellar dust is a very saturated orange to brownish-red, and with small amounts of hydrogen emission, becomes a saturated red color.
The advantage of a stock digital camera in astrophotography is that the color balance is close to that of the human eye, and shows compositional differences better. Modified digital cameras are too sensitive to hydrogen alpha emission, making scenes containing hydrogen too red, swamping colors from other compositions. Often this shows in amateur astrophotos as dominantly red. The choice of course is personal. I prefer images with more colors to show more processes and chemistry. I believe such images are more interesting, so I only use stock digital cameras for my astrophotography.
References and Further Reading
Clarkvision.com Astrophoto Gallery.
Clarkvision.com Nightscapes Gallery.
The Night Photography Series:
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http://www.clarkvision.com/articles/astrophotography.image.processing
First Published December 20, 2015 Last updated January 16, 2021