Stars in the night sky vary in color, which is a direct measure of their surface temperature. Observing a star’s color—whether blue, yellow, or red—allows astronomers to immediately determine the temperature of its outer layers. This fundamental relationship between light and heat helps scientists classify and understand the physical properties of these celestial bodies.
The Coolest Stars Are Red
The coolest stars in the cosmos display a distinct reddish hue, sometimes appearing deep orange. These stars belong to the spectral class M, which encompasses stars with surface temperatures generally below 3,700 Kelvin (K), sometimes dropping as low as 2,400 K.
Our own Sun has a surface temperature of about 5,800 K, making it a hotter, yellow star. The population of cool, red stars includes two primary types that differ vastly in size and stage of life.
The most common are red dwarfs, which are small, faint main-sequence stars like Proxima Centauri. These stellar bodies burn their nuclear fuel very slowly, granting them incredibly long lifespans. Red giants and supergiants, such as Betelgeuse, are stars nearing the end of their lives. They have swelled to enormous sizes, causing their surface temperature to drop despite their immense luminosity.
How Star Temperature Determines Color
A star’s temperature and color are connected by the physics of thermal radiation, often simplified by treating stars as “blackbodies.” Every object that has a temperature above absolute zero emits electromagnetic radiation across a spectrum of wavelengths. The hotter an object becomes, the more energy it radiates, and the shorter the wavelength at which its light emission peaks.
This inverse relationship is described by Wien’s Displacement Law. Cooler stars emit the majority of their light at longer wavelengths, which corresponds to the red and infrared parts of the spectrum. Conversely, an extremely hot star emits most of its energy at shorter wavelengths, causing its peak emission to shift toward the blue and ultraviolet end of the spectrum.
The surface temperature of the coolest M-class stars is low enough that their peak energy output resides squarely in the red portion of the visible spectrum, making them appear red to our eyes.
Placing Cool Stars on the Stellar Spectrum
Astronomers classify stars based on this temperature-color connection using the spectral classification system, traditionally organized by the letters O, B, A, F, G, K, and M. This sequence runs from the hottest, blue stars (O-class) to the coolest, red stars (M-class).
The hottest O- and B-class stars are blue or blue-white, with surface temperatures exceeding 10,000 K. A-class stars appear white, and F-class stars are yellow-white. Our Sun is a G-class star, appearing yellow, followed by K-class stars, which are orange.
M-class stars anchor the end of the temperature scale, representing the coolest surface temperatures and the longest visible wavelengths of light. The progression of color—from blue to white to yellow to orange and finally to red—illustrates the decreasing surface temperature across the stellar population.