Planck curve

If you measure exactly how much light a warm object gives off at every wavelength and plot it on a graph, you get a smooth, lopsided hump: rising gently from long wavelengths, climbing to a peak, then falling off steeply toward short wavelengths. That characteristic hump is the Planck curve, named after the physicist Max Planck, and it is the precise shape of the glow from any blackbody.

The crucial feature is that the entire curve is fixed by a single number, the object's temperature. Change the temperature and the whole curve shifts and rescales in a completely predictable way: hotter means the peak moves to shorter (bluer) wavelengths and the entire curve rises, so a hotter object is brighter at every wavelength, not just near its peak. The curve never quite reaches zero on the long-wavelength side, which is why even a cool object still gives off some radio and infrared light. Planck derived this shape in 1900 by making a radical assumption — that light energy comes in discrete packets — which launched quantum physics.

For astronomers, the Planck curve is a fitting template. Take the measured brightness of a star at several wavelengths, find which Planck curve matches, and you read off the temperature directly. Comparing the height of the curve also tells you how much total power is involved. The cosmic microwave background fits a Planck curve so perfectly that it is the most precise blackbody ever measured anywhere — a stunning confirmation that the early universe was in thermal equilibrium.