The Color of Heat

Think of metal heating in a fire. First it glows dull red, then bright orange, then yellow, and if it gets hot enough, white. The color always tells you the temperature. Stars follow the very same rule: their color is set by how hot their surface is.

So the coolest stars are red, near 3,000 degrees at the surface, and the hottest are blue-white, many times hotter. Our Sun sits in the middle, a yellow-white star. Betelgeuse and Rigel, both in Orion, show you the two ends with your own eyes.

A star's color reports its surface temperature. Cool stars near 3,000 kelvin glow red; the Sun, near 5,800, glows yellow-white; the hottest stars, above 20,000, glow blue-white. The rule runs one way and never breaks: the hotter the surface, the bluer the light.

Split a star's light through a prism and you get a spectrum — a band of color crossed by dark lines. Each line marks a wavelength absorbed by a particular element in the star's atmosphere, so the spectrum names what the star is made of. Sorted by temperature, stars fall into seven classes: O, B, A, F, G, K, M, hottest to coolest.

The physics is blackbody radiation: any hot, dense surface glows with a spread of colors whose peak shifts toward the blue as temperature climbs — Wien's rule. A star near 3,500 K peaks in the red; the Sun near 5,800 K peaks in the yellow-green; a 12,000 K surface peaks in the ultraviolet, beyond violet, and so looks blue-white.

The dark absorption lines come from the star's cooler outer layers, where atoms remove specific wavelengths. Which lines appear depends mostly on temperature, not composition: hydrogen lines peak in the white A stars near 9,500 K, while cool M stars show broad bands of titanium oxide, a molecule that survives only where it is cold enough. That temperature order is the sequence O, B, A, F, G, K, M.

Heat has a color. A little heat glows red. More heat glows orange, then yellow. The most heat of all glows white and blue-white. Stars do this too.

So a red star is cool. A blue-white star is very hot. A yellow star, like our Sun, is in the middle. The color tells you the heat.

Quantitatively, Wien's displacement law fixes the peak: λ_max × T ≈ 2.9 × 10⁶ nm·K, so a 3,000 K star peaks near 970 nm in the infrared and a 30,000 K star near 100 nm in the ultraviolet. Observers shortcut this with a color index, the magnitude difference B−V from folio 10's bandpasses, which rises for red stars and falls for blue.

The spectral sequence is a temperature sequence, not a compositional one. Line strengths are set by how atoms are excited and ionized, governed by the Saha and Boltzmann relations: hydrogen's Balmer lines peak in A stars because hotter stars ionize the hydrogen and cooler ones leave it unexcited. Cecilia Payne showed in 1925 that nearly all stars share one composition, mostly hydrogen — the classes differ in heat, not in substance.

In an LTE atmosphere the emergent spectrum is a Planck continuum shaped by wavelength-dependent opacity, with line depths measured as equivalent widths and read through the curve of growth. The Harvard sequence — O through M, plus L, T, and Y for the coolest dwarfs — orders effective temperature; the luminosity class, the Morgan-Keenan I through V, is read from pressure-sensitive line widths.

Several temperatures must be kept distinct: the color temperature from the continuum slope, the excitation and ionization temperatures from Saha-Boltzmann line ratios, and the effective temperature defined by the bolometric flux, σT⁴ = F. They agree only for an ideal blackbody; a real star, with its stratified, non-gray atmosphere, forces the distinction — and each diagnostic samples a different depth.