Although Wien’s displacement law indicates that a star with a temperature above 5,800 kelvins should have a green color, the truth is that scientists struggle with being able to see one. This happens largely because of the cone cells in the human eyes, which are tuned to interpret a range of lights as white. Dr. Alastair Gunn from the University of Manchester explains that it has to do with the variety of cells (red, green, or blue cones), which perceive light as white, and for our eyes to see it as green, the star would have to emit only green light, which is not possible.
Not being able to identify and see a star as green does not depend on the star itself, but on the biological ability that human have to discern the different light spectrums.
Color spectrum of the stars
If, just like flashes of red, yellow, and blue, are emitted by stars in our universe, why are we not able to see the green color they emit? For years, scientists have had trouble identifying a star shining with green light, and this has to do with physical and biological reasons. According to researchers, all stars have a continuous blackbody emission that cannot be pure green. Their light is emitted through thermal radiation across all parts of the electromagnetic spectrum.
Few people know that our sun actually has a peak emission at 550 nm, but we perceive it as white light, not green. According to Wien’s displacement law, a star with a surface temperature of 5800 K should appear green. However, it also emits the same amount of red and blue light due to the laws of thermal radiation, which prevents a star from being entirely green.
Why are we not able to see that green light?
According to Dr. Alastair Gunn from the University of Manchester, “If a star emits maximum light at a certain wavelength, let’s say ‘green,’ it actually emits almost as much ‘red’ light, and our eyes perceive this mix as ‘white’ rather than ‘green.'” This happens for two reasons: because of blackbody emission and because of the three types of cone cells, which respond maximally to light of different wavelengths. These three types are:
- Red cones – sensitive to longer wavelengths (around 560-700 nm).
- Green cones – sensitive to medium wavelengths (around 530-560 nm).
- Blue cones – respond to shorter wavelengths (around 420-490 nm).
Thermal Color Rules of the Universe
Stellar colors progress predictably according to surface temperature:
- Cool red stars (3,000K) – peak in infrared, appear red.
- Orange stars (4,000K) – balanced red-yellow emission.
- Yellow stars (5,000K) – broad spectrum, appear yellow-white.
- White stars (6,000K) – peak in green, appear white.
- Blue stars (10,000K+) – peak in blue, appear blue-white.
Gunn explains that “But for our eyes to see it as green, a star would have to emit only green light, which is not possible.” Regardless of their actual color, our eyes can only perceive stars as white and are unable to see any other kind of green. This is due to the physical principles of thermal radiation and human evolution itself, which has adapted to the sun that, despite shining green, emits light that appears white because of its broad color spectrum.