The color of a star offers astronomers a direct indicator of its surface temperature. This relationship is often counterintuitive to our everyday experience, where red is synonymous with heat. In the cosmos, the opposite is true, and the star’s color shifts across the visible spectrum in response to its thermal energy. The temperature of these surface layers determines the precise wavelength at which this emission peaks. By measuring a star’s light, scientists can accurately estimate its surface temperature.
The Hottest Star Colors
The most intense heat is found in stars that shine with a blue or blue-white hue. These massive, luminous stars have surface temperatures that typically begin around 10,000 Kelvin (K) and can soar past 40,000 K. The immense energy output pushes their light emission predominantly toward the shortest, highest-energy wavelengths.
Moving down the temperature scale, stars appear white or yellow-white, with surface temperatures ranging from approximately 6,000 K to 10,000 K. Our own Sun falls into the yellow class, maintaining a surface temperature near 5,800 K. Orange stars are cooler still, with temperatures between 3,700 K and 5,200 K.
The coolest stars are those that appear red, and their surface temperatures can be as low as 2,000 K to 3,500 K. These red stars, often called red dwarfs, emit most of their light in the longer, lower-energy red and infrared wavelengths. The full color sequence, from hottest to coolest, progresses from blue, through white, yellow, orange, and finally to red.
The Physics Linking Color and Temperature
The precise link between a star’s color and its temperature is governed by blackbody radiation, quantified by Wien’s Law. A star behaves as a near-perfect blackbody, meaning it emits light across a continuous spectrum based solely on its temperature. This thermal emission is not dependent on the star’s chemical composition.
Wien’s Law states that the wavelength at which a body emits the maximum amount of radiation is inversely proportional to its absolute temperature. As the star’s surface temperature increases, the peak wavelength shifts toward the shorter, higher-energy end of the electromagnetic spectrum. This shift means that hotter stars peak in the blue and ultraviolet region.
Conversely, cooler stars have their peak emission shifted toward longer wavelengths, primarily in the red and infrared parts of the spectrum. The color we perceive is determined by where its energy output is most intense. An object peaking in the blue will still emit some red light, but the dominance of the short wavelengths makes the star appear blue.
The Stellar Classification System
Astronomers formalize the temperature-color relationship using the Stellar Classification System, which arranges stars into distinct spectral types. This system uses a sequence of letters: O, B, A, F, G, K, and M, running from the hottest stars to the coolest stars. This classification is more precise than using color names because it is based on the star’s unique spectral lines.
O-type stars are the hottest, shining brightly in blue or blue-white, with temperatures exceeding 30,000 K. B-type stars are also blue and slightly cooler, while A-type stars are white or blue-white, with temperatures around 7,500 K to 10,000 K. The Sun is a G-type star, appearing yellow-white, with its temperature falling between 5,200 K and 6,000 K.
K-type stars are orange, and the M-type stars are the coolest, displaying a red color with temperatures below 3,700 K. The absorption lines within a star’s spectrum reveal the temperature-dependent state of the atoms and molecules in its atmosphere. For example, the hottest O-type stars have simpler spectra due to the high energy ionizing most elements, while the coolest M-type stars show complex lines from molecules like titanium oxide.