Brown Dwarfs are Violet: A New Calculation of Human-eye Colors of Main-sequence Stars and Substellar Objects

There has always been interest in the perceived colors of stars. They were key to the development of the Hertzsprung–Russell diagram, and they are also used widely in educational and public-outreach imagery. Thus, it is useful to develop tools to compute these colors from spectral energy distributions. This paper presents a collection of objective (CIE coordinate) and subjective (RGB triple) colors for main-sequence stars and brown dwarfs, as well as links to related codes and tables. Using the proposed conversion from CIE to RGB colors, O and B stars are bluer than equivalent blackbodies because of Paschen continuum absorption, and M dwarfs tend to be less red and more beige. Although brown dwarfs over a wide range of effective temperatures (400–2000 K) emit most of their flux in the infrared, their visible spectra are dominated by short wavelengths. Thus, they may appear violet to human eyes.