Why aren't there any green stars?

The color of a star is a combination of two phenomena. The first is the star's temperature. This determines the wavelength (frequency) where the peak of its electromagnetic radiation will emerge in the spectrum. A cool object, like an iron rod heated to 3000 degrees, will emit most of its light at wavelengths near 9000 Angstroms ( the far-red part of the visible spectrum) in wavelength. A very hot object at a temperature of 30,000 degrees will emit its light near a wavelength of 900 Angstroms (the far-ultraviolet part of the visible spectrum). The amount of energy emitted at other wavelengths is precisely determined by the bodies temperature, and by Planck's radiation law of 'black bodies'. Below is an example of a black body spectrum courtesy of Prof. Michael Bolte' at the University of California Observatories/ Lick Observatory (www.ucolic.org):

It shows that as the temperature of the object increases, the peak shifts further to short wavelengths. But the phenomenon we call 'color' is another matter. Color does not exist as an objective property of nature.

Color is a perception we humans have because of the kinds of pigments used in our retinae. Our eyes do not sense light evenly across the visible spectrum but have a greater sensitivity for green light, and somewhat less so for red and blue light as the response spectrum below illustrates:

In effect, what you have to do is 'multiply' the spectrum of light you receive from a heated body, by the response of the eye to the various wavelengths of light in the spectrum. When this happens, a very unusual thing happens.

If I were to figure out how hot a star would have to be so that the peak of its emission was in the 'green' area near 4000 Angstroms, I would estimate that the temperature of the star would have to be about 10,000 degrees. There are many such stars in the sky. The two brightest of these 'A-type' stars are Vega in the constellation Lyra, and Sirius in Canes Major. But if you were to look at them in the sky, they would appear WHITE not green! Stars are ranked according to increasing temperature by the sequence of letters:

Type..... Color...............Temperature

O........ blue................30,000
B........ blue-white..........20,000
A........ white...............10,000
F........ yellow-white........ 8,000
G........ yellow.............. 5,000
K........ orange.............. 4,000
M........ red..................3,000
This is NOT the same sequence of colors you see in a rainbow (red, orange, yellow, green, blue, indigo, violet) because the distribution of energy in the light source is different, and in the case of the rainbow, optical refraction in a raindrop is added.

Another factor working against us is that we see stars in the sky using our black/white rods not our color-sensitive cones. This means that only the very brightest stars have much of a color, usually red, orange, yellow and blue. By chance there are no stars nearby that would have produced green colors had their spectral shapee been just right.

So, there are no genuinely green stars because stars with the expected temperature emit their light in a way that our eye combines into the perception of 'whiteness'.

For more information on star colors, have a look at the article by Philip Steffey in the September, 1992 issue of Sky and Telescope (p. 266), which gives a thorough discussion of stellar colors and how we perceive them.


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