What color is a star?

An interactive blackbody-radiation explorer. Pick a temperature to see the Planck spectrum, the color and brightness a human eye would actually see, where that color sits on the CIE 1931 color map, and the math behind it.

• v2 (the current site, /): a dependency-free rewrite. It has three color models: the original simple one, CIE 1931 colorimetry, and colors of real stars from model spectra (Harre & Heller 2021, with the sRGB gamma correction applied). Brightness follows human vision, so cold objects go black instead of red. Each page ships as a single ~25 KB file.
• v1 (/v1/): the original 2023 Create React App version, kept as it was.

Running it

npm install
npm run build:v2          # v2 only -> build/
npm run build             # v1 into build/v1, then v2
python3 -m http.server 4173 --directory build
npm run test:v2           # physics and slider-scale tests

v1's dev server is npm start.

Layout

• v2/: pages, partials, source, data and tests for the current site. See CLAUDE.md for a map.
• src/, public/: v1.
• docs/v2/PLAN.md: the v2 plan, including the v1 math audit, data sources, verified reference values and open questions.
• docs/WHY.md: why things are the way they are.

Data sources

• CIE 1931 2° standard observer color matching functions (CIE, doi:10.25039/CIE.DS.xvudnb9b).
• Harre & Heller (2021), "Digital color codes of stars", Astronomische Nachrichten, arXiv:2101.06254.
• Hollands et al. (2026), "Any colour you like: the diverse chromaticities of white dwarfs", RAS Techniques and Instruments, doi:10.1093/rasti/rzag069.