Stars’ distinct colors reveal fundamental properties about these distant celestial bodies. The color of a star directly relates to its surface temperature, serving as a cosmic thermometer for astronomers to classify and understand stellar evolution.
The Link Between Star Color and Temperature
The color of a star is a direct indicator of its surface temperature. The coolest stars appear red, while the hottest stars shine blue.
As a star’s temperature increases, its color progresses through a predictable sequence. Red stars are the coolest, with surface temperatures generally below 4,500 Kelvin (K). Moving up the temperature scale, stars become orange, then yellow, like our Sun, which has a surface temperature around 5,772 K. Further increases in temperature lead to white stars, and finally, the hottest stars appear blue or blue-white, with surface temperatures exceeding 10,000 K, often reaching over 30,000 K.
Why Color Reveals a Star’s Heat
The underlying scientific principle connecting a star’s color to its temperature is blackbody radiation. Stars behave approximately like “blackbodies,” idealized objects that absorb all electromagnetic radiation and then emit thermal radiation in a continuous spectrum based solely on their temperature. As an object gets hotter, the peak wavelength of the emitted light shifts.
This shift in peak wavelength dictates the perceived color. Hotter objects emit more energy at shorter, bluer wavelengths, causing them to appear blue or blue-white. Conversely, cooler objects emit more energy at longer, redder wavelengths, making them appear red. This phenomenon is why a metal coil glows red when heated to a moderate temperature but would turn yellow-white or even blue-white if heated to extreme temperatures.
The Full Spectrum of Stellar Hues
Orange stars, for example, have temperatures between approximately 3,700 K and 5,200 K. Yellow stars, like the Sun, fall within the range of 5,200 K to 6,000 K. White stars generally have surface temperatures between 7,500 K and 10,000 K.
Astronomers utilize this color-temperature relationship for stellar classification, assigning letters (O, B, A, F, G, K, M) to categorize stars based on their temperature and corresponding color. O-type stars are the hottest and blue, while M-type stars are the coolest and red. This system allows scientists to determine a star’s surface temperature simply by observing its color.