Title: Digital color codes of stars
Authors: Jan-Vincent Harre, René Heller
First Author’s Institution: Institüt fur Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Status: Accepted for publication in Astronomische Nachtrichten
What is a colour anyway?
A bunch of photons – particles of light – leave the surface of the Sun. They traverse the distance between the Sun and the Earth in about 8 minutes. After a bumpy ride through the atmosphere they finally hit your eyeball. In the back of your eye, they stimulate the photoreceptive cone cells. The wavelength of the photons will be interpreted by your brain as a colour, and you will see the Sun as being white-ish (probably followed by a spike of pain telling you to look away).
Colour is a “life” thing. The eyes of human beings can perceive particular wavelengths of light, which we perceive as the visible spectrum, and we associate colours with those wavelengths to tell them apart. The reason we, and most other life on earth, can interpret this visible spectrum in particular (and not say, UV or radio waves) is because the Sun emits most strongly in these wavelengths; we have evolved to see Sunlight.
Imagine a theoretical object that can perfectly emit light across all wavelengths. This so-called “black-body” emits light in a spectrum that is determined by its temperature alone. At hotter temperatures a more intense spectrum with more ‘blues’. At lower temperatures, a more shallow spectrum with more ‘reds’ (the human body emits infrared radiation, for example). The emission spectrum of stars is often approximated as a black-body, and as so the colour of stars is often assumed to be that of the peak of a black-body of the temperature of the star.
However, a black-body spectrum does not perfectly describe how a star would appear to us, let alone incorporate effects of absorption from elements in stars, reducing the intensity at specific wavelengths. Today’s authors ask the question: “how can we find out exactly what stars look like to the human eye”?
Spectra versus eyes
The authors generated stellar model spectra (using the PHOENIX library) for stars ranging in temperatures from 2300 K to 15,000 K, at metallicities of [Fe/H] = (0, -1, -2), to account for how absorption at different metal abundances changed the spectra. They also generated spectra up to 55,000 K, using a different model (TLUSTY) that accounted for the ‘looser’ outer layers of such massive stars. To mimic the reception of the human eye, they used established colour matching functions (CMFs), which model the reception of the human eye in Red, Green and Blue (RGB) wavelengths, and map this to hexadecimal colour codes. Figure 1 above shows these CMFs over the spectrum of a 2500 K stars — a red dwarf.
Different than expected
The authors calculated the colours for both the synthetic spectra and black-bodies at the same temperature, and found that overall they differed significantly. This is because the absorption occurring in the star’s atmosphere contributes significantly to the final colour, absorbing more of certain wavelengths than of others. Figure 2 above shows the trend of stellar type (ranging from massive O stars to small M dwarfs), and how these differ from black-bodies at the same temperature.
This has interesting consequences for stars that we typically associate with particular colours. They found that stars at high temperatures (~8000 K) were slightly bluer than their black-bodies (because of less absorption of red wavelengths). Conversely cooler ‘red’ dwarfs (~3000 K) were actually more like ‘orange’ dwarfs, with more absorption of red wavelengths occurring than in the black-body. Especially ‘white’ dwarfs, which we tend to think of as white, go through a whole temperature, and thus colour, range as they cool. White dwarfs have relatively small atmospheres and very little absorption, and the cooler stars will actually look orange, not white!
How subjective are these colours?
Just to make sure these colours were as objective as possible, the authors checked some cool phenomena. This included measuring the colour in the case of broadened absorption lines in the cases of extremely fast-spinning stars, and the absorption in the Earth’s atmosphere. In all cases, they found these effects to only cause a 1% change in the RGB colour codes, which would be imperceptible to most humans. Metallicity on the other hand does provide a perceptible difference (You can compare them yourself! Input the Hex codes from Tables 2 and 4 in the paper here).
The results of today’s paper, while seemingly small, have a large significance. For the first time we can look at an image on our screens and know with certainty that it represents a star exactly as we would see it (see Figure 3), connecting us directly to distant stars in the sky. The exact way we perceive colour, and therefore stars, is unique to life raised on Earth by the light of the Sun. Knowing the colour of stars therefore tells us — a little — about our own place in the Universe.