Wienna

Meet Wienna, a flame that knows the hotter you are, the bluer you shine.

Any hot object, idealised as a blackbody, glows with a spread of wavelengths. Heat it up and two things happen: it gets brighter at every wavelength, and the peak of its spectrum moves to shorter wavelengths. Wilhelm Wien found in 1893 that the peak wavelength times the temperature is a constant: λ_max T = b, with b ≈ 2.898×10⁻³ metre-kelvins.

For the Sun's surface temperature of about 5,772 K, the peak is at about 502 nanometres, in the blue-green. A red-hot poker around 1,000 K peaks in the infrared near 2.9 micrometres, with only its short-wavelength tail visible as red.

Astronomers read star temperatures from their colours: red stars are cool, blue-white stars hot. Thermal cameras use the same idea at room temperature, where the peak is near 10 micrometres.

The peak's location depends on whether you plot per unit wavelength or per unit frequency, so 'the peak' is not unique. Real objects are not perfect blackbodies, and a candle flame's colour also comes from chemistry.