Double Star Astronomy

Part 5: Double Star Color

A characteristic and problematic aspect of visual double star astronomy is the fascination with star color. Two stars, narrowly separated, of contrasting magnitude and spectral type — this is the ideal of a "showcase pair."

Unfortunately, this enjoyable aspect of astronomy has been muddled by the amateur preference for idiosyncratic color language, made worse by a poor understanding of how star color arises and how it is perceived. These three issues must be understood by the astronomer who wants to describe star color reliably. 

Color Perception Is Idiosyncratic

Although it may seem discouraging to learn, your color perception is part of your individuality. It is very unusual for a group of three or more people to observe the same double star for the first time and agree on the colors of the components. This is a fact acknowledged by William Herschel at the very beginning of double star astronomy:

Here I must remark, that different eyes may perhaps differ a little in their estimations [of star colors]. I have, for instance, found, that the little star which is near α Herculis, by some to whom I have shewn it has been called green, and by others blue. Nor will this appear extraordinary when we recollect that there are blues and greens which are very often, particularly by candle-light, mistaken for each other. (Preface to Catalogue of Double Stars, 1782)

Even astronomers who are aware of this problem often do not appreciate the extent to which star color reports will vary from one observer to the next. To illustrate this variation, we can turn to the testimony of one of the most ardent star color advocates, Sissy Haas, who helpfully quotes historical star color descriptions in her Double Stars for Small Telescopes (Sky Publishing, 2008). Here is a small sample:

double starObserver 1Observer 2Observer 3

36/37 Herpale blue, blueyellowish white, yellowish peach
ρ Herazure white, azure whitebluish white, pale emeraldgreenish white, greenish
μ Herbrilliant orange, ghostly wisppale white, purple
μ Herboth pure goldlight apple green, cherry red
ν Draboth brilliant whiteboth pale grayboth yellow white
ψ Draboth lemony whiteboth pearly whitewhitish yellow, lilac
STF 2248 Drawhite, blueyellowish, ash
STF 2440 Dragreenish white, bluepale yellow, blue
ζ Lyrboth goldish whitetopaz, greenishgreenish white, yellow
36 Ophboth citrus orangeruddy, pale yellowboth golden yellow
61 Ophboth straw yellowboth silvery white
η Oristraw yellow, silvery yellowwhite, purplish
ε Booamber yellow, deep bluepale orange, sea green
39 Booboth whitish goldboth whitewhite, lilac
ξ Boobright white, vivid grayorange, purpleyellow, deep orange
44 Booboth grapefruit orangepale white, lucid gray
ψ Drawhite, blueyellowish, ash

These examples come from experienced and careful observers such as Rev. T.W. Webb, Admiral William Smyth, William Olcott and Haas herself, and could be multiplied to fill this page.

These examples illustrate two problems: a naive imprecision (really, a form of excited handwaving) in the use of color names such as vivid gray, yellowish peach or silvery green; and a difficulty in recognizing star colors accurately, for example in the confusion between yellow and orange, white and lilac, blue and ash, or gold and cherry red.

I will try to clarify this muddle by looking first at the physical basis of star color, the temperature of the star. I will then describe some of the obstacles to accurate color perception, and finally suggest a method to describe color that will minimize the ambiguity added by language to the individuality already present in the perception. 

The Blackbody Limits of Star Color

Temperature is kinetic energy in atoms — motion in a gas, vibration in a solid — but when we say that an object is at a certain temperature, we do not mean that every atom in the object has exactly the same kinetic energy. Instead there is a distribution in the energy of the individual atoms that creates an average temperature.

The Blackbody Curve. An explicit way to record the temperature of an object is to measure the number and energy of photons (quantum units of energy) that the object emits into its less energetic (lower temperature) surroundings. These photons can be very long wavelength, low energy radio waves, moderate energy infrared (heat), high energy light, or very short wavelength, very high energy ultraviolet and xrays. This distribution in the photons emitted by a body at a specific temperature is described in quantum theory as an idealized blackbody curve or blackbody flux profile.

Remarkably, this curve has the same basic shape across all temperatures. There is always a single definite peak energy, but the distribution is skewed or imbalanced so that there is always a much greater number of photons at energies below the peak than above it, and the lower energy distribution extends much farther from the peak. As temperature increases, the emitted flux distribution changes in two ways: (1) the overall flux, including the peak of the curve, shifts from lower into higher energy photons, and (2) the total quantity of flux, expressed as the peak radiance in watts, increases as well (diagram, below).

The peak radiances of the curves in this diagram have been set to the same height, to show the differences in the shape of the curves with temperature. But note the extremely large changes in the peak radiance, from 30,000 watts at 1500K to 30 billion watts at 24,000K! If the curves for 24000K and 1500K were drawn together at their actual peak values, the curve for the 1500K body would be a flat line across the bottom of the graph.

The peak radiance of this generic curve, and therefore its overall shape, is explicitly related to the temperature of the blackbody by Wien's displacement law:

λmax = 2.898 x 10–3/T[K] meters

which shows that as temperature (the denominator) increases, the peak wavelength is displaced to shorter wavelengths (higher energies). The amount of energy emitted depends on the surface area of the object, but for objects of the same size the invariant rule is, as the temperature of a body increases, the peak emittance is at shorter wavelengths and the total emittance is greater.

An important detail is that the median of the blackbody curve is at a lower energy than the peak, which means more than half the emitted photons are at energies below the peak value. Infrared (heat) is always powerfully present in radiating bodies.

This blackbody curve describes the energy radiated by any body whose radiation arises from its temperature, which includes the radiation emitted by the photosphere temperature of stars.

Here a familiar example will make two important points. The Sun, at a surface temperature of 5780K, should theoretically have its peak flux at about 2.898/5780 x 10–3 = 0.000000501 meters or 501 nm (blue green). Of course, we do not see the Sun as blue green, in part because its "ideal" emittance profile is strongly filtered by atomic elements in its own photosphere and by the Earth's atmosphere, and in part because the color perception of the human eye is strongly affected by the apparent brightness of the light source — and the Sun is exceedingly bright! Therefore:

(1) The actual emittance profile of a star will be only approximately described by the blackbody curve predicted from its surface temperature (that is, its spectral/luminosity type)

(2) The apparent color of a light (a star) will depend on how bright its light appears to the eye.

The Color Temperature. The human eye cannot perceive the entire spectrum of photons emitted by a luminous body; the change in the blackbody radiator is only perceptible within the narrow range of our light sensitivity, from about 400 nm to 700 nm. This narrow range is indicated in the diagram (above) by the band of spectral hues. To determine the apparent color of a blackbody object, we have to disregard everything that is outside the visual range.

We can calculate the visual color associated with a blackbody by means of standard sensitivity curves for the human cones. Thus, at a temperature of 3000K the blackbody curve slopes strongly upward into the red wavelengths, indicating it would have a reddish color. At a temperature of 12,000K the curve slopes upward into the blue wavelengths, indicating a blue color. These calculations reduce the flux profile to a specific hue for every color temperature, illustrated in the table (below).

color temperature
for common illuminants and light sources
 color mnemonic correlated illuminant or light source
1000S9 lower limit of visible blackbody curve
1850S5 candle flame
2000S3 sunlight at sunrise/sunset (clear sky)
2750M8 60W incandescent tungsten light bulb
2860M7  CIE A: 120W incandescent light bulb
3400M2 photoflood or reflector flood lamp
3500M2 direct sunlight one hour after sunrise
4100K7 CIE F11: triband fluorescent light
4300K6 morning or afternoon direct sunlight
5000K2 white flame carbon arc lamp (CIE D50: warm daylight illuminant)
5400G8 noon summer sunlight
6400F6 xenon arc lamp
6500F5 average summer daylight (CIE D65: cool daylight illuminant)
7100F1 light summer shade
7500A8 indirect northern skylight
8000A6 deep summer shade
9300A1 white point of a CRT (television screen)
10640B9 clear blue sky
30,000B0 mountain noon sky
Sources: Mitchell Charity, MIT; Kodak USA

Note: These color samples grossly exaggerate the chromatic contrast and drastically reduce relative luminance due to the gamut limits of digital displays; they are useful only to suggest the hue of the blackbody color.

We can learn several interesting facts from this table. First, note that the hue correlated with the blackbody temperature of a 120 watt incandescent light bulb is decidedly orange, although we commonly perceive these bulbs as providing a "warm white" light. In fact, the hue is as shown, and this can be appreciated by observing from outside and at night the drawn white window shades of a room illuminated by incandescent light. This illustrates again the point made above, that the apparent color of a light depends quite a lot on its apparent brightness and on the surroundings in which the light is perceived.

The second important fact illustrated by this table is that the range of blackbody colors is actually quite limited: the hues vary from red orange through yellow to white, then into pale blue. There are no yellow greens, greens, blue greens, or green blues; no blue violets, violets, magentas, bluish reds or reds — the nearest we get to a "lilac" color is the radiant blue of a high mountain sky.

The third important fact is that in a substantial range of colors — from about 4500K to 7500K, corresponding approximately to the surface temperatures of type A5 to K5 stars — the apparent hue is almost indistinguishable from a colorless or "white" light.

The Color Stimulus. These limitations in color perception arise from the shape of the blackbody curve within the wavelength limits of our visual sensitivity. The eye judges color as the relative proportional luminance of the flux profile between about 400 to 700 nm, with most of the sensation arising from wavelengths between about 500 to 600 nm. Within this range, the proportional differences in luminance between the peak and minimum luminance of a blackbody profile are different from what the full emittance curve would lead us to expect. In particular, at high temperatures, the upward slope in blue wavelengths changes relatively little, and these small differences are difficult for our eyes to perceive. This can be illustrated by plotting the radiance of the flux profiles on a logarithmic scale as a proportion of a constant peak radiance, which approximates the overall response of the eye to radiance differences (diagram, below).

Even assuming that the eye is not equally sensitive to all wavelengths within its range, the diagram illustrates that there is a visually large difference between the flux profiles at 1500K and 3000K, which will produce a readily perceptible hue contrast in the "red to yellow" side of the hue variation. In contrast, the flux profile at 6000K is essentially flat and will appear as a pure white.

The difference between the flux profiles at 12000K and 24000K is visually very subtle because the slopes of the profiles are nearly identical, and they will appear much closer to a "white" color than the very low temperatures. An additional complication is that our eyes are evolved to accept a slightly bluish light, equivalent to the daylight mixture of sunlight and skylight (the D65 illuminant in the table above), as a pure white at all luminance levels. As a result, our eyes are predisposed to perceive faint and bluish starlight as indistinguishable from faint white light.

There are also physical reasons why the blackbody curve only approximates a star's visual color. Stellar spectra are selectively filtered by the star's own photosphere, which produces absorption bands that selectively darken wavelengths within the blackbody flux. Wolf-Rayet and carbon stars can be surrounded by clouds of dust or gas that alter the spectrum through both absorption and excitation emittance. Finally (and most commonly), interstellar dust absorbs starlight primarily in the very short wavelengths, making stars appear redder and dimmer than they actually are.

While it might seem that color is also affected by relative motion — bodies rapidly receding from us would appear more red, and bodies approaching us would appear more blue — it turns out that this Doppler effect is perceptible only in the extremely high velocity differences between remote galaxies. The velocity differences among stars cannot produce perceptible shifts in color. 

"I Can See the Spectral Type." Finally, it's worth asking whether the theoretical connection between the blackbody curve and stellar temperature makes it possible to interpret star color directly as a spectral type. This sometimes appears when an amateur describes a color as a spectral type: "a typical type M color". But it is more common in the claim that the observer can determine the spectral type simply by visual inspection.

While there is enough variation across individual visual capabilities to make this claim impossible to deny outright, two facts make it highly doubtful: stars of the same spectral type have widely varying colors, and stars of many different spectral types can have exactly the same color. The simplest way to illustrate this is by looking at the range of B–V color index values for stars with a simple (not complex or compound) spectral type in the Yale Bright Star Catalogue, with the median BVCI indicated by a white line (diagram, below).

The B–V color index essentially measures the slope of the radiance profile within the visual spectrum, as I've done in the previous diagram, so it is a good proxy for visual color. It's obvious that there is very little difference in the color range across the "early" type O, B and A stars, and these substantially overlap with the color range of "solar type" F and G stars — for example, stars from type O through type G can all have a BVCI of 0.40. There does seem to be a reliable difference between the "early" type stars and "late" type K and M stars, and the C or carbon do not overlap with any other spectral type. But the G type stars overlap the entire range of K and M, and the K and M stars are indistinguishable from each other by color alone. So, while there is a likely basis for the ability of most observers to distinguish between "early" and "late" type stars and to recognize carbon stars, there is no physical basis for the claim that unique spectral types can be identified by color alone.

I made a careful study of apparent color in several hundred relatively faint (below magnitude 4) binary stars in the W.F. Struve and other catalogs, first noting any apparent color and only later comparing the color to the spectral type available in catalogs. I was surprised that even spectral type A or B stars could appear to have a distinctly yellow hue, and type G stars a distinctly orange hue, exactly as the chart above indicates. The principal causes of these uncertainties or "misidentifications" were probably variations in the brightness of the stars, and variations in the amount of dust along the line of sight.

Worse, the majority of fainter stars lacked any visible color — the star appearance provided no information at all about spectral type. I conclude that the visual identification of spectral types may be feasible in the nearest, brightest stars free of any discoloration from interstellar dust, but otherwise is only an anecdotal boast.

So, despite the precision and convenience we obtain by equating star color with blackbody color, the actual response of the human eye to stellar lights is much less straightforward. The theoretical blackbody does help to understand the physically feasible range of star colors, the hues we can exclude — yellow green, green, blue green, violet, magenta and red — as having a physical origin in a star's flux profile, and the relative color sensitivity of our eye to different blackbody temperatures. To delve further we have to consider the peculiarities of color perception. 

Color Perception

The human visual system interprets color into the color appearance of stars based on the star's apparent brightness (its contrast with a dark background), its contrast with the brightness and color of nearby stars, and the existing chromatic adaptation or "white point" of the eye. This interpretative behavior of color vision has its own patterns and rules, which can help astronomers evaluate and report star color.

The Colormaking Attributes. All uniform or homogenous colors can be described using just three terms, called the colormaking attributes. These are (1) brightness for the apparent luminance or radiance of the light; (2) hue for the specific "color name" of the light (red, yellow, blue, etc.); and (3) saturation for the hue purity or "color intensity" of the light. Brightness varies from bright to dim, saturation varies from intense [hue] to white, and hue is assigned a categorical color label.

The hue intensity or saturation in lights is perceptually reduced by an increased proportion of "white" light in the color, or how much the emitted photons are spread across the wavelengths of visual sensitivity. Because blackbody profiles span the entire electromagnetic spectrum, star light is well spread across the entire visual range. As a result, all star colors are low in saturation, meaning that they appear strongly diluted with white light — the hues appear as tints. This subtlety in star colors is a principal reason why individual reports of star color can be so different.

Saturation depends on brightness: as lights become dimmer, their saturation can appear to increase. Saturation is also dependent on the visual width of a color area, with larger color areas appearing more saturated. For these reasons star color is often more visible or striking if the stars are viewed slightly out of focus, so that the star image appears both as a disk and with reduced brightness.

Unlike brightness or saturation, which vary as a linear dimension of intensity or strength from a zero point, hue is described by a hue circle in which one hue blends imperceptibly into another. Spectral hues wrap around the circle in spectrum order from red to violet, and the circle is closed by joining red and violet through their mixture, which creates a magenta hue (diagram, below).

This circle is created by two basic dimensions of chromatic sensitivity in the eye: yb or yellow vs. blue and rg or red vs. green (although "red" here means a violet red or crimson, and "green" is actually a cyan or turquoise). The sensation of hue is created by an imbalance in these hues toward one end or the other of their possible range. Thus, an "orange" hue sensation occurs when the yb response is imbalanced toward yellow, and the rg response is imbalanced toward red.

The hue of star color can be described as a dominant wavelength, which is the single wavelength of spectral light whose hue, adjusted for brightness and saturation, exactly matches the apparent hue of the total blackbody emittance. The diagram shows the hues associated with specific spectral wavelengths (in nanometers, as indicated in italic type), and the corresponding blackbody temperatures. The colored circles provide hue samples of the different colors, but these are all much more saturated than the corresponding star colors, especially across the blue blackbody series.

This illustrates that the orange and red hues in the blackbody colors have a substantial amount of yellow content — the most extreme (coolest) color is a red orange — and at the effective temperature of M type stars (~3000K) the hue is actually an orange yellow. In the same way, none of the blackbody "blue" colors come close to purple, and these hues are all on the "green" side of the hue circle. As illustrated in the previous section, effective temperatures of around 6000K, though technically green hues, are so flat across the visible range that they appear indistinguishable from white.

Chromatic Adaptation. This seemingly fixed framework of colormaking attributes is actually incredibly flexible, through processes called adaptation.

Of particular interest to astronomers is the process of chromatic adaptation, by which the eye adapts to the entire light environment to identify what a "white" color should be, primarily by discounting any unchanging hue in the available light. The most common example is the way tinted sunglasses will appear to "disappear" or become unnoticeable after about 10 minutes, or the way a difference in the color of light that is perceptible when we pass from a room illuminated by incandescent lights to a room illuminated by fluorescent lights will disappear within a minute or two.

A simple procedure will demonstrate chromatic adaptation. Observe a distinctly colored K or M star such as Betelgeuse or Antares, and a "white" A or F star such as Procyon or Altair, under three conditions: (1) normal dark adaptation, (2) after staring for several seconds at a book page illuminated by a strong red light, and (3) staring at a page illuminated with a strong green light. The red star will appear whitish under condition (2) but strongly red under condition (3), while the white star will appear faintly green under (2) but faintly red under (3).

The main effect of chromatic adaptation under a color neutral dark adaptation is that the color of a single bright star will tend to shift toward white after it is viewed for an extended period. For this reason, star colors will appear most vivid on first glance, and the color will become somewhat less prominent the longer a star is observed. This is especially significant in stars with an effective temperature above ~6000K, which will shift more readily toward an apparent white color than lights below ~3000K: chromatic adaptation has a much larger effect on star color than it would on a light of "pure" color, and the effect is stronger on the apparent color of type O, B and A stars.

Chromatic Contrast. A major consequence of the opponent dimension organization of hues is that hues located on opposite sides of the hue circle will appear to have enhanced saturation, and an intensely chromatic light will cause a white light next to it to appear tinted with the hue opposite on the hue circle. Thus an orange yellow star at a dominant wavelength of around 600 nm will shift the hue of a white or faint companion toward a blue hue at around 470 nm, and enhance the apparent saturation of an already blue companion in this direction.

Alternately, stars that are of similar or analogous hues can appear more contrasted. Thus, a pair of stars in which one star is orange and the other star is yellow orange can appear more contrasted, so that the pair appears red orange and yellow.

Both types of effect are called chromatic contrast, and they can significantly alter the apparent color of paired stars.

It is important to understand that chromatic contrast can create the appearance of color where none exists. The classic 18th century demonstration of this effect is quite pretty, and easy to view if you have never seen it. Illuminate a white wall or large sheet of white paper with a "white" halogen light and the light of a candle or a yellow tinted light, and adjust their positions so that they cast separate shadows of equal darkness. Then place an object such as your hand or an upright book so that it casts a shadow from both light sources onto the wall. The shadow cast by the white light will appear colorless or slightly yellow, while the shadow cast by the candle will appear to be a beautiful dark blue. A similar arrangement using a white and red light in two small lamps will produce a luminous green shadow (image, right).

I urge you to set up these simple demonstrations for yourself, because it is difficult to appreciate how powerful they can be until you actually see them. The same effects can appear in double stars in which one star is colored but the other star is very pale, white or dim: a red star will push its fainter companion star toward a green hue, and a yellow star will push its companion toward blue, and this effect is more pronounced when the companion star is significantly fainter than the primary. In fact, the most lively color contrast observed in double stars is typically the Albireo palette of yellow and blue.

Star color is extremely bright, although far away, so the proportion of chromaticity to brightness is very small. As a result most stars (excepting carbon stars and late type giant stars) have a very attenuated color, so that the blackbody colors appear as a delicate tint; and this is accentuated by the small visual extent or "pinpoint" appearance of the Airy disk.

Summary. The diagram below summarizes the most important visual attributes of star color:

(1) All star hues, with the exception of cool and strongly filtered emittance from carbon stars (type C), are weakly saturated: the color appears mixed with a significant amount of white light.

(2) The "blue series" of star colors appear much less differentiated than the "red series" colors, largely because the saturation of the blue hues is lower.

(3) Hue differences are reduced when the color area is very small, as illustrated by the upper row of dots below the square color swatches.

(4) Changing the brightness of a light can produce a shift in the hue. The blue series of hues will appear to shift toward white (apparent gray) as their brightness declines; the extremely hot blue hues may appear to shift toward blue violet; the yellow to red hues will appear to shift toward red. These brightness related hue shifts are illustrated in the lower row of dots.

The brightness effects on hue are more significant on the red side of the black body series. Thus, Betelgeuse (alp Ori) and Herschel's Garnet Star (mu Cep) have the same spectral type (M2 Ia), but because Betelgeuse is much brighter (visual magnitude 0.4 vs. 4.1) its color appears orange yellow while the fainter mu Cep appears distinctly orange red.

Because chromatic contrast and brightness changes enhance the saturation of opposing hues and increase the apparent difference between similar hues, project contrast hues onto faint or white stars, and shift hue toward the spectrum extremes, the appearance of star color is very sensitive to context and therefore fundamentally deceptive. 

Viewing and Describing Star Color

When we combine the adaptive behavior of color perception, the deceptive influence of visual context, the complex and variable nature of stellar emittance profiles and the wide variation in color perception that appears across "normal" color observers, the result is that star color is a highly idiosyncratic and unstable aspect of visual astronomy. It is a recreational pleasure, and nothing more.

This is certainly no reason to ignore the gorgeous visual impact of several famous close binaries, and it is equally no reason not to report apparent star color. It is sufficient reason to be wary of one's own color perceptions, to use methods that can make color perception more accurate, and to describe color in language that is consistent and unambiguous.

Optics & Apparent Color. A major influence on perceived star color is the aperture and type of instrument used to make the observations. Unfortunately, observer reports that I have seen on this topic vary widely — as does almost anything to do with star color! — and in particular there seems to be disagreement as to whether larger aperture makes star colors more or less distinctive.

My own experience is that some of the most vivid or distinctive colors are available in small aperture telescopes or large binoculars. These are almost exclusively in the yellow and orange part of the spectrum and most striking in stars located within the Milky Way. This is partly due to the large field of view possible with these instruments: a colored star becomes more distinctive when many "white" stars are visible around it.

Some observers report more vivid color in larger aperture instruments, but I have not personally noticed this effect in double star astronomy. Certainly aperture can improve the color appearance in planetary astronomy by making the image brighter, but the effect in very small star images will be primarily to reduce the apparent color in O, B and A stars, and shift the apparent color of K and M stars toward yellow.

Reflecting telescopes are reputed to transmit more accurate color than refractors, due to the fact that transmittance through glass and the nonreflective coatings on glass surfaces filter the light, especially at the spectral extremes. However I find this effect is noticeable only in visually large areas, such as the surface of the Moon or Jupiter's disk, and only when making a side by side comparison between one telescope with another. Through a single instrument, and with the nearly point images of stars, chromatic adaptation will eliminate any tint produced by the optics to produce a stable perception of neutral color.

Improving Perceptual Accuracy. A few simple methods can be used to make star color perception more accurate. These serve primarily to counteract the effects of brightness, chromatic adaptation and chromatic contrast on color appearance.

• Use a consistent optical presentation – Because aperture, type of objective and magnification can all significantly affect apparent color, it is important to use the same type of telescope and the same eyepiece when surveying star colors. The eyepiece should deliver a magnification for the aperture sufficient to make the Airy disk readily visible.

• Be aware of chromatic adaptation – Accurate color perception requires a consistent adaptation to white. A simple way to retain chromatic adaptation is to glace at and then away from the star, each time in a different direction, so that a different part of the retina exposed.

However a bright, colored star will shift this adaptation, as will prior exposure to red light from observatory lights or flashlights. In situations where chromatic adaptation has been distorted in these ways, it is acceptable to observe a bright, white (type late F or early G) star, or a book page illuminated with a dim white light, or the moon with a high power eyepiece or with the naked eye, as a way to consistently reset the white point of the eye. Some of these techniques may impair dark adaptation, but dark adaptation is not especially useful to observe star color.

• Emphasize first glance color – Chromatic adaptation will cause star color to change across an observing interval, especially if one or both stars are bright or very bright. It is therefore most important to capture the color of stars at first glance, especially if they are bright or very bright. Note also how color appears to change across an observing interval. Then rest or reset the eye, and observe again.

• Observe defocused star images – Especially with bright or very bright stars, the defocused image will produce a colored disk that is larger and fainter than the focused star image. This will somewhat reduce the effects of chromatic adaptation (because the light is fainter) and improve color perception (because the area is larger).

• Consider the effects of chromatic contrast – Refer to the color circle (above) to evaluate the possible effects of chromatic contrast.

• Hide the primary star – If possible, hide the light of the primary star by moving it past the edge of the eyepiece field of view to assess the color of a fainter secondary star. This eliminates chromatic contrast and the suppressing effect of glare from the brighter star.

Reduce Language Ambiguity. A major source of unreliability in star color descriptions is in the use of color language. Not only are imprecise or exaggerated color descriptions difficult for other people to interpret, they encourage the observer to make impulsive and inaccurate visual observations.

Sissy Haas prefers a vocabulary that seems inspired by the farmer's market and commercial paint store — she reports star colors as grapefruit orange, straw yellow, gloss white, amber yellow, peach white, pearly white, citrus orange, yellowish peach, sun yellow, silvery blue, bright orange, bluish turquoise, tangerine orange, whitish powder blue, azure white, silvery sapphire, dusty gold, pumpkin orange, lemony white, ash white, aquamarine, plum red, deep white, vivid gray, banana yellow and so on. What for example is the difference between a citrus orange, bright orange, tangerine orange and a grapefruit orange? And how closely does a pumpkin orange star actually match the daylight color of a pumpkin orange pumpkin? The problem with this kind of verbal clutter is that it is impossible for another person to identify the color behind the poetic exaggeration.

In contrast, William Herschel used a limited and standard set of color terms: "garnet, red, pale red, pale rose-colour, white inclining to red, white, white inclining to blue, blueish white, blue, greenish, green, dusky," and these terms (excepting the rose and greens) match very well the blackbody series. His approach exemplifies the first principle of star color description: make color description as simple and consistent as practicable.

The simplest method of color description, used for all scientific and professional color description, relies on the three colormaking attributes. In describing lights (stars), these are the brightness of the color as a source of light; the hue or "color" of the color; and the saturation of the color, or its mixture with white.

• Brightness is characterized by the contrast between the luminance adjectives bright and faint.

For consistency, avoid using any other terms such as brilliant, dim, shining, glaring, etc. Instead, use the terms in a standard visual scale that does not exceed five or six steps:

Very bright
Very faint

Why not just report the star's magnitude? After all, stars are seen against a common dark background and at approximately the same level of light adaptation, so relative brightness will correlate closely with a star's visual magnitude. The complication is that the same star will appear brighter in a larger aperture telescope, so the perceived brightness is also aperture dependent; also, brightness differences become perceptually exaggerated near the faintest (threshold) magnitude values. However, if you are habituated to a single observing instrument, and can estimate magnitude reliably (this is easy to test, by comparing your estimates to catalog values), magnitude is a useful alternative to brightness.

Be aware that a very bright primary will make a nearby companion star appear fainter than it would appear in isolation — in part by direct contrast, and in part by brightening the background through glare or diffusion.

The remaining two attributes, hue and saturation, are actually two aspects of the single attribute of chromaticity or color content. One cannot appear without the other; hue becomes more ambiguous as saturation goes down, and both are affected by differences in brightness.

• Hue is the "color" of the color. Here it is important to use the standard hue terms red, orange, yellow, white, blue and violet, and to denote hue mixtures by placing the secondary or tinting hue first: yellow orange is an orange mixed with a smaller proportion of yellow, orange yellow is a yellow mixed with a smaller proportion of orange. (Alternately, a hue combination can denote a color that appears midway between the two anchor hues: then yellow orange and orange yellow mean the same thing, and denote a hue that is equal proportions of yellow and orange.) Obviously, whichever approach you choose, you should use it exclusively.

• Saturation is the concentration or purity of the hue, characterized by the adjectives intense to pale. Again the anchor terms can be expanded into a scale:

Very intense
Very pale

where very intense can mean the most intense color visible in a star (e.g., the color of a carbon star), and very pale can mean a color that contains so much white that a distinct hue is barely perceptible. (The term bright should not be used to describe intense saturation ["bright orange"], as this will confuse saturation and brightness.)

Finally, the three attributes should always be listed in the order (1) brightness, (2) saturation and (3) hue (or hue mixture).

If the star does not appear to have a recognizable and stable color, then applying a specific hue description disguises this perceptual fact. Brightness will always be easy to describe, but when saturation is low then hue can become elusive. In these situations the description bright [or dim] and ambiguous is accurate. Don't reach for silvery green, gloss white or whitish powder blue instead.

The result is a color language that is easy to learn and remember, reliable to use and simple to interpret:

Very bright, pale yellow orange
Faint, ambiguous
Bright, medium orange
Faint, very pale blue
Average, medium yellow

The difficulties in perceiving faint or very pale chromaticity lead to the second principle of star color description: the more a star color appears subtle, elusive or difficult to describe, the more likely it is illusory. This is especially true when the star color appears green or violet, colors outside the blackbody range. In these situations it may be useful to note any context features that might affect the color perception, for example a bright primary of a contrasting hue.


Last revised 11/26/13 • ©2014 Bruce MacEvoy