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Why are blue stars hotter than red stars?

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Open cluster NGC 290 — Star Stars in the open cluster NGC 290, as seen by the Hubble Space Telescope.

Stars exhibit different colors primarily because they have different surface temperatures. In astronomy, a star's color is a direct indicator of its temperature, which is categorized through stellar classification.1

The relationship between color and temperature is governed by the physics of blackbody radiation. As a star's surface temperature increases, the peak wavelength of the light it emits shifts toward the shorter, higher-energy end of the visible spectrum, which corresponds to blue light. Conversely, cooler stars emit more of their energy at longer, lower-energy wavelengths, appearing red.213

This temperature-color correlation is reflected in the standard spectral sequence, where the hottest stars (classes O and B) are blue or bluish white, with surface temperatures typically exceeding 10,000 K. In contrast, the coolest stars (class M) appear red, with surface temperatures around 3,000 K.21