Archive for arXiv:2101.06254

On the Colours of Stars

Posted in The Universe and Stuff with tags , , , on January 20, 2021 by telescoper

There’s an interesting paper on the arXiv by Harre & Heller with the title Digital Color Codes of Stars. Here’s the abstract:

Publications in astrophysics are nowadays mainly published and read in digitized formats. Astrophysical publications in both research and in popular outreach often use colorful representations of stars to indicate various stellar types, that is, different spectral types or effective temperatures. Computer generated and computer displayed imagery has become an integral part of stellar astrophysics communication. There is, however, no astrophysically motivated standard color palette for illustrative representations of stars and some stars are actually represented in misleading colors. We use pre-computed PHOENIX and TLUSTY stellar model spectra and convolve them with the three standard color matching functions for human color perception between 360nm and 830nm. The color matching functions represent the three sets of receptors in the eye that respond to red, green, and blue light. For a grid of main sequence stars with effective temperatures between 2300K and 55,000K of different metallicities we present the red-blue-green and hexadecimal color codes that can be used for digitized color representations of stars as if seen from space. We find significant deviations between the color codes of stars computed from stellar spectra and from a black body radiator of the same effective temperature. We illustrate the main sequence in the color wheel and demonstrate that there are no yellow, green, cyan, or purple stars. Red dwarf stars (spectral types M0V – M9V) actually look orange to the human eye. Old white dwarfs such as WD1856+534, host to a newly discovered transiting giant planet candidate, occur pale orange to the human eye, not white. Our freely available software can be used to generate color codes for any input spectrum such as those from planets, galaxies, quasars etc.

This reminded me of a post I wrote in 2011 about why you never see any green stars. They say a picture paints a thousand words so here’s an illustration from the above paper:

This shows that although there are stars in the Main Sequence whose spectra peak at wavelengths corresponding to green light, none of them look green. The paper also claims that there are no yellow stars either. The Sun can look yellow when viewed from Earth but that is to do with scattering in the atmosphere: from space, the Sun looks white.

For another discussion of the use of colour representations in cosmology, see here.