What is the color of the sun?
Category: Space Published: July 3, 2013
The color of the sun is white. The sun emits all colors of the rainbow in approximately equal amounts and we call this combination "white". That is why we can see so many different colors in the natural world under the illumination of sunlight. If sunlight were purely green, then everything outside would look green or would look dark. We can see the redness of a rose and the blueness of a butterfly's wings in sunlight because sunlight contains red and blue light. The same goes for all other colors. When a light bulb engineer designs a bulb that is supposed to mimic the sun, and therefore provide natural illumination, he designs a white bulb, not a yellow bulb. The fact that you see all of the pure colors in a rainbow (which is just sunlight dispersed by water droplets), so that no pure colors are missing in a rainbow, is direct evidence that sunlight is white.
The sun emits all colors of visible light, and, in fact, emits all frequencies of electromagnetic waves except gamma rays. This includes radio waves, microwaves, infrared waves, visible light, ultraviolet waves, and X-rays. The sun emits all of these colors because it is a hot thermal body and emits light through the process of thermal radiation. Just like a hot coal or a toaster element that glows, the sun glows in all colors because of its temperature. That is why incandescent light bulbs emit light that mimics sunlight so well: they contain metal filaments that are heated until they glow in the same way that the sun does.
It may be tempting to examine the color content of sunlight and identify the brightest color (the peak frequency) as the actual color of the sun. The problem with this approach is that the peak frequency does not have a concrete meaning for a broad distribution of colors. The peak frequency is different depending on whether you are plotting brightness as a function of frequency or as a function of wavelength, as shown in the images below.
When plotted as a function of wavelength, sunlight peaks in the violet. When plotted as a function of frequency, sunlight peaks in the infrared. Which is right? They are both right. They are just two different but perfectly valid ways of measuring color content. This shows us why giving special significance to the peak frequency of a broad distribution is rather meaningless. If you insist on saying that the color of the sun is violet (because sunlight's spectrum peaks in the violet when plotted as a function of wavelength) then you logically must also say that the color of the sun is infrared (because sunlight's spectrum peaks in the infrared when plotted as a function of frequency). However, these two statements are contradictory, indicating that giving special significance to the peak of a broad distribution is incorrect. The bottom line is that the sun is white. Period.
To further confuse things, astronomers like to model the sun as a perfect blackbody emitter, which it is not. According to the oversimplified blackbody model, when plotted as a function of wavelength, the sun peaks in the green! When astronomers say that the sun is green, they mean that their oversimplified model of the sun peaks in the green when plotted as a function of wavelength. They don't mean that the sun is actually green! Unfortunately, "The sun is green!" makes for more exciting headlines than, "The sun is literally white. However, the sun would be green if the sun were a perfect blackbody emitter, and if you plot as a function of wavelength, and if the peak of a broad distribution had meaning." Although not as attention-grabbing, the ultimate truth is: the sun is white. In summary, the peak of a broad distribution has little meaning - but if you insist on focusing on the peak, you should remember: the peak of the sun's color spectrum is violet when plotted as a function of wavelength, infrared when plotted as a function of frequency, and green if an oversimplified blackbody model of the sun is used and the spectrum is plotted as a function of wavelength.
Note that the plots above show sunlight as it is measured in space before entering earth's atmosphere (data from the ASTM Terrestrial Reference Spectra). This is the true color content of the sun. The sunlight that we experience on the surface of earth has been filtered by the atmosphere and is slightly different. The atmosphere tends to scatter out blue and violet more than the other colors. As a result, direct sunlight on the surface of the earth is slightly less blue and violet than the original sunlight emitted by the sun, but it is still white. In fact, with a little bit of the blue and violet removed from sunlight by earth's atmosphere, the color of sunlight at earth's surface is even closer to being perfect white than the color of the original sunlight emitted from the sun. Around sunrise and sunset, the scattering of blue and violet can be so strong that the sun can momentarily appear yellow, orange, or even red. But the sun itself is actually white. The sun is white when viewed from space and the sun is white when viewed from earth's surface (except for brief moments at sunrise and sunset). Even when viewed from earth's surface, the sun is almost never yellow.