The brilliant points of light scattered across the night sky are massive spheres of plasma held together by the immense force of their own gravity. These stellar furnaces shine with an intense light that is not uniform in appearance. Stars display a fascinating variety of colors, from deep red to stark blue. This color is not a random feature but a precise indicator of the star’s physical properties.
The Relationship Between Temperature and Color
The color a star exhibits is a direct result of its surface temperature, a principle explained by the physics of thermal emission. Any heated object radiates light across a continuous spectrum of wavelengths. As temperature increases, the total energy emitted rises, and the peak of that light shifts toward shorter, higher-energy wavelengths.
A cool object primarily emits low-energy, long-wavelength light, appearing red if visible. As the temperature climbs, the emission peak shifts from red, through orange and yellow, and eventually into the blue and ultraviolet parts of the spectrum.
This means that the hotter a star is, the more its light output is concentrated at the blue end of the visible spectrum. The color we see is essentially the star’s way of announcing its surface temperature. Astronomers can determine a star’s temperature simply by observing its hue, similar to how a blacksmith estimates the heat of metal by its glowing color.
The Full Spectrum of Star Colors
The observed colors of stars correspond directly to their surface temperatures. The coolest stars, with surface temperatures around 2,000 to 3,500 Kelvin, appear red because their light output peaks at the longest visible wavelengths. Betelgeuse, an enormous supergiant in the constellation Orion, is a familiar example of a red star.
Orange stars, such as Aldebaran, have temperatures between 3,500 and 5,000 Kelvin. Stars in the 5,000 to 6,000 Kelvin range, like our Sun, appear yellow or yellow-white. Although the Sun appears yellow from Earth due to atmospheric scattering, if observed from space, it would look distinctly white.
Stars between 7,500 and 10,000 Kelvin are classified as white, exemplified by Vega. These stars emit nearly equal amounts of light across the entire visible spectrum, which the human eye perceives as white. The hottest stars, exceeding 10,000 Kelvin, appear blue or blue-white, such as Spica.
A star whose peak emission wavelength falls exactly in the green part of the spectrum does not appear green to us. This is because a star peaking in green also emits substantial amounts of red and blue light across the continuous thermal spectrum. When these colors mix, our eyes combine them to see a color that is white or blue-white, not pure green.
Organizing Stars by Color and Heat
To bring order to the vast number of stars, astronomers utilize the formal Spectral Classification System, a standardized temperature and color scale. This system categorizes stars using a sequence of letters: O, B, A, F, G, K, and M. The sequence is arranged from the hottest stars to the coolest stars.
O-type stars are the hottest and appear blue, while M-type stars are the coolest and appear red. The full sequence of colors is:
- O and B stars are blue.
- A stars are white.
- F stars are yellow-white.
- G stars are yellow.
- K stars are orange.
- M stars are red.
Our Sun, for instance, is a G2 star, fitting into the yellow star category with a surface temperature of approximately 5,800 Kelvin. Each main class is further subdivided numerically from 0 to 9, allowing for more precise temperature designation within the broader color category.