blackbody radiation

Heat a poker in a fire and watch what happens. At first it just feels warm. Then it glows a dull red, then bright orange, then yellow-white, then a blinding blue-white. Crucially, the color of its glow tells you its temperature, no matter what the poker is made of. This temperature-driven glow that every warm object gives off is called blackbody radiation, or thermal radiation, and it is one of the most useful clues in all of astronomy.

A 'blackbody' is an idealized object that perfectly absorbs all light falling on it and, when warm, re-emits light in a smooth, characteristic pattern that depends only on its temperature. Stars, planets, and even the whole early universe behave very nearly like blackbodies. The light is spread across many wavelengths, peaking at one particular wavelength set by the temperature, with a long tail to one side. Two simple laws summarize the pattern: hotter objects peak at bluer (shorter) wavelengths (Wien's law), and hotter objects radiate far more total power per square meter (the Stefan-Boltzmann law).

Blackbody radiation matters because it turns a star's color into a thermometer reading from light-years away. Measure the wavelength at which a star is brightest, and you have its surface temperature; measure how much total energy it pours out per square meter and how big it is, and you have its power. The same physics gives the cosmic microwave background its almost perfect blackbody spectrum at about 2.7 degrees above absolute zero — direct evidence of a hot early universe that has since cooled as space expanded.