The color of a star is a direct indicator of its surface temperature. This relationship often seems counterintuitive, as the hottest stars shine with brilliant blue light, while the coolest ones glow red. This principle is similar to how a piece of metal behaves when heated. As the metal warms, it first emits a faint, dull red glow. As the temperature dramatically increases, its light shifts toward orange, yellow, and eventually a dazzling white or blue-white. Stars operate on this same physical principle.
Red Stars: The Coolest Color
The coolest stars are red, placing them at the M-class designation in astronomy. These stars possess surface temperatures falling below 3,700 Kelvin, with some reaching down to about 2,400 Kelvin. This low temperature dictates that the peak of their radiation is pushed toward the longer wavelengths of the electromagnetic spectrum. Longer wavelengths correspond to the red and infrared light that our eyes perceive as a deep, characteristic crimson glow.
Cooler objects primarily release their energy at a lower frequency, resulting in a light output dominated by red light rather than the shorter, higher-energy blue light. The most common examples of these cool, red stars are red dwarfs, which are small, low-mass stars that burn their nuclear fuel at an incredibly slow rate. Their slow-burning nature gives them extraordinarily long lifespans, potentially lasting trillions of years.
Red dwarfs are the most numerous type of star in the Milky Way. They are generally too dim to be seen without a telescope. Their low surface temperature means they emit very little visible light, with much of their energy released as infrared radiation. This makes them faint and inconspicuous despite their overwhelming abundance.
Mapping Temperature to Color: The Stellar Classification Scale
Astronomers use a standard system to classify stars by their temperature and intrinsic color, known by the sequence of letters O, B, A, F, G, K, and M. This sequence arranges stars from the hottest blue stars down to the coolest red stars. The surface temperature of a star is the single factor determining its placement within this system, as it dictates the peak wavelength of its light emission.
The stellar classes are defined by temperature:
- O-class stars shine intensely blue, exceeding 30,000 Kelvin.
- B-class stars are blue-white.
- A-class stars appear white, with temperatures in the 7,500 to 10,000 Kelvin range.
- F-class stars are yellow-white.
- G-class stars are distinctly yellow, like our own Sun, with surface temperatures around 5,200 to 6,000 Kelvin.
K-class stars have an orange hue, and their temperatures range between 3,700 and 5,200 Kelvin. M-class stars are the coolest, displaying a red color with temperatures below 3,700 Kelvin. This progression from blue to red represents a continuous decrease in surface energy, confirming that a star’s color is an accurate thermal thermometer. The intrinsic color of a star is a measure of its heat.
Why Our Eyes Sometimes Deceive Us
While a star’s physical color is determined by its temperature, our visual perception of that color can sometimes be misleading. A notable example is the absence of a truly green star, even though some stars have a peak light emission in the green part of the spectrum. Stars do not emit light in a single, pure color but rather across a broad spectrum of wavelengths.
For a star to appear green, it would need to emit only green light, which is physically impossible for a thermal radiator. Stars with temperatures that peak in the green range, such as G-class stars, emit significant amounts of red and blue light alongside the green. The human eye blends these overlapping colors together, and the result is perceived as white or yellow-white, not pure green.
Another factor altering the visual color of stars is the Earth’s atmosphere, which can scatter light. This atmospheric interference causes stars near the horizon to appear redder than they actually are. When light travels through more of the atmosphere, the shorter blue wavelengths are scattered away, leaving the longer, redder wavelengths to reach our eyes. A star’s true intrinsic color can be temporarily masked by the conditions of observation.