Stars in the night sky exhibit a captivating array of colors, from fiery reds to brilliant blues and soft yellows. This diversity in stellar appearance holds profound clues about their fundamental properties. Understanding why stars display such varied hues unveils basic principles governing their existence.
The Spectrum of Star Colors
Stars display a wide range of colors, including red, orange, yellow, white, and blue. Betelgeuse in Orion shines with a distinct red hue. Arcturus often appears golden or yellowish, and our Sun is seen as yellow from Earth.
Stars like Spica and Sirius appear blue-white, while Rigel is a notable blue star. Green stars are not observed, as light with a peak emission in green wavelengths combines with other colors, producing a white appearance to our eyes.
Why Stars Have Different Colors
A star’s color directly indicates its surface temperature. Hotter stars emit light at shorter wavelengths, appearing blue, while cooler stars emit more light at longer wavelengths, appearing red. This relationship is rooted in blackbody radiation, where an object’s emitted light spectrum is determined by its temperature. Stars approximate these ideal radiators.
As a star’s surface temperature increases, its peak emitted light shifts towards the bluer end of the spectrum. The coolest stars, around 3,000 degrees Celsius, appear red. The hottest stars, up to 40,000 degrees Celsius, emit intensely in blue and ultraviolet wavelengths. Our Sun, at about 5,500 degrees Celsius, emits most strongly in the yellow-green part of the spectrum, appearing yellowish from Earth.
Astronomers use the OBAFGKM classification system to categorize stars by temperature and color. O-type stars are the hottest and appear blue, while B-type stars are blue-white. A-type stars are white, F-type stars are yellow-white, and G-type stars, like our Sun, are yellow. K-type stars exhibit an orange hue, and M-type stars are the coolest, displaying a red color.
Beyond Intrinsic Star Color
While a star’s surface temperature primarily determines its true color, external factors influence how we perceive it from Earth. Earth’s atmosphere plays a significant role through atmospheric scattering. This phenomenon is responsible for why stars appear to twinkle and why they sometimes seem to shift colors near the horizon, as the atmosphere scatters different wavelengths of light to varying degrees. It also explains why our sky appears blue during the day, as blue light is scattered more effectively than red light.
Interstellar dust also alters the perceived color of distant stars. These particles absorb and scatter light, with shorter blue wavelengths more affected than longer red wavelengths. This effect, known as interstellar reddening, makes distant stars appear redder than their actual color. Astronomers account for this reddening to determine a star’s true color and properties.
The human eye’s limitations also affect our perception of star colors. At low light levels, rod cells are more active than cone cells, making color vision less sensitive. Many stars, especially dimmer ones, may appear white or faint to the naked eye. Only the brightest stars show their true colors distinctly without optical aid.