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:
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 103/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 103 = 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).