006 / LIGHT AND HEAT
What colour is temperature?
Heat a perfect blackbody from room temperature to a blue giant. The spectrum shifts and the colour changes — now look at it through Earth’s atmosphere.
Find a green star
Set about 5000 °C: the spectral peak lands on green, yet the disk stays white. Then lower the Sun towards the horizon and watch it redden like a sunset.
What is inside the model?
The spectrum is Planck’s law for an ideal blackbody. Colour integrates it with the CIE 1931 colour-matching functions (2°, 360–830 nm, 1 nm), then XYZ → sRGB for a D65 display without chromatic adaptation.
Bλ = 2hc² / λ⁵ · 1 / (exp(hc / λkT) − 1) λmax = b / T, b ≈ 2.898 mm·K Tatm(λ, m) = T₁.₅(λ)^(m / 1.5) m = 1 / (sin h + 0.50572 (h + 6.07995°)^−1.6364)
Atmospheric transmission is the ASTM G173-03 reference spectrum (SMARTS 2.9.2): direct ground-level sunlight divided by extraterrestrial at air mass 1.5. It includes Rayleigh and aerosol scattering and O₂, H₂O, CO₂, O₃ absorption. Other altitudes use Beer–Lambert scaling and the Kasten–Young air mass; for narrow saturated lines this is an approximation.
Data cover 280–4000 nm. Below 280 nm the atmosphere is opaque (ozone). Beyond 4 µm there is no data and no atmosphere is applied — that region is hatched. Disk brightness is an illustrative log scale spanning 12 decades below the Sun, not physical screen luminance.