blackbody radiation
Heat any object and it glows. A poker in a fire shifts from dull red to orange to brilliant white as it gets hotter, and even an object too cool to see by eye still pours out invisible infrared. A 'blackbody' is an idealised object that absorbs every wavelength falling on it and, when warm, re-emits a glow whose colour depends only on its temperature, not on what it is made of.
Physicists study this with a small hole in a heated cavity, which behaves almost perfectly as a blackbody. Measure how much energy comes out at each wavelength and you get a smooth, humped curve. As the temperature rises the whole curve grows taller and its peak slides toward shorter, bluer wavelengths — which is why hotter things look whiter and a star's colour reveals its temperature.
The exact shape of this curve was measured with great care in the 1890s, and it became a quiet scandal. Classical physics, combining electromagnetism with the statistics of heat, simply could not reproduce it. The problem of the blackbody spectrum was the loose thread that, when pulled, began to unravel classical physics — and Planck's attempt to fit the curve in 1900 lit the first spark of the quantum age.
Hotter bodies glow brighter and bluer — the shape of the curve is set by temperature alone.
No real object is a perfect blackbody, but the Sun, a kiln, and the cosmic microwave background are all close enough that their spectra match the ideal curve remarkably well.