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The color of light emitted by a star is a direct clue to its surface temperature. Hotter stars produce photons with higher energies, which shift the peak of their emission toward the blue end of the spectrum. Cooler stars peak in the red or infrared. Because our eyes perceive a mixture of all colors as white, most stars appear whitish to the naked eye, even though their spectra contain distinct peaks.
In physics, a blackbody is an idealized object that absorbs all incident radiation and re‑emits energy solely according to its temperature, following Planck’s law. Stars are not perfect blackbodies, but they are close enough that their spectral energy distributions can be matched to a blackbody curve, allowing astronomers to infer a precise effective temperature.
Because the peak emission wavelength determines temperature, astronomers isolate specific color bands using optical filters. By measuring the intensity in, for example, the blue (B) and red (R) filters and comparing their ratios, they can locate the peak of the star’s spectrum and calculate its temperature with high accuracy.
Determining a planet’s temperature is more complex. A planet’s atmosphere, surface albedo, and greenhouse effect can significantly alter the energy balance. Astronomers model these factors—stellar flux, orbital distance, reflective properties, atmospheric composition, and rotation—to estimate the planet’s equilibrium temperature and, where possible, its actual surface temperature.
Knowing the temperatures of stars and planets informs us about their composition, evolution, and potential habitability. It also allows us to test fundamental physics under extreme conditions that are impossible to recreate on Earth.
