Neutral metals like Na or even some molecules like TiO produce the precise absorption lines in cool red stars. In hot blue stars, Hydrogen is most responsible.
Ionized Oxygen gives planetary nebulae strong green light.
Hydrogen atoms are ionized by nearby ultraviolet radiation to produce emission nebulae's red color.
In the sky, clouds appear white because their droplets are larger than visible wavelengths, and when reflectors are larger than the wavelengths being reflected the efficiency is nearly independent of the wavelength. Since all colors are uniformly reflected, clouds appear white.
Broad emission lines in a quasar appear because of a thin gas that is strongly heated as it falls toward the black hole.
Emission lines in a comet appear as broad bands because molecule fragments are excited by sunlight and emit radiation as they return to lower energy levels, the broad bands being characteristic of molecules.
How do we get sunburned?
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