Astronomers don’t use thermometers to measure distant worlds—they use light. Stars and planets emit or reflect radiation that carries clues to their heat. By breaking that light into its spectrum, scientists can find the peak wavelength of emission, which shifts with temperature, thanks to Wien’s displacement law.

Spectroscopy splitting light into wavelengths unlocks deeper insight. For stars, absorption and emission lines indicate surface temperature, composition, density, gravity, and more. Measuring the overall energy output and knowing the star’s size lets astronomers apply the Stefan Boltzmann law, revealing temperature. 

Planets pose a tougher challenge: they shine faintly, mostly in infrared. By measuring this thermal emission and calculating the equilibrium temperature based on distance from their star, reflectivity (albedo), and stellar luminosity scientists approximate planetary temperatures. Infrared telescopes and bolometers detect the faint glow, and methods like eclipse mapping even reveal day-night heat differences on exoplanets like HD 189733b.

In short, astronomical thermometers are built from physics: light, spectra, and robust math unlocking the heat of worlds across the cosmos.