The Sun is depicted as yellow in children\u2019s drawings, in weather icons, on flags, and in the standard astronomical shorthand. Astronomers themselves classify it as a \u201cyellow dwarf,\u201d a G2V-type main sequence star. The yellow framing is so consistent across human culture and science that it can be surprising to learn that the Sun is not actually yellow at all. Seen from space, above Earth\u2019s atmosphere, the Sun is white. Astronauts on the International Space Station and on the Apollo missions to the Moon, where there is essentially no atmosphere, photographed and described it that way. The yellow is something the Earth\u2019s atmosphere adds on the light\u2019s way through.<\/p>\n
The physics that makes the Sun look yellow from the ground is the same physics that makes the sky look blue. Both come from a single phenomenon called Rayleigh scattering, named after the British physicist John William Strutt, the third Baron Rayleigh, who worked out the equations in the 1870s and won the Nobel Prize in 1904 for his work in related areas. The two observations, blue sky and yellow Sun, are not separate facts. They are the same fact seen from two angles.<\/p>\n
What the Sun\u2019s light actually contains<\/p>\n
The Sun emits light across the entire visible spectrum, from violet at around 400 nanometres of wavelength to deep red at around 700 nanometres. The intensity is not perfectly uniform across this range. The peak of the Sun\u2019s emission spectrum sits at roughly 500 nanometres, in the green part of the spectrum, corresponding to the Sun\u2019s surface temperature of about 5,800 kelvin. But the spectrum is broad enough, and the eye\u2019s sensitivity across the visible range is balanced enough, that the combination of all these wavelengths together reads to the human visual system as white rather than green or any other single colour. The Sun\u2019s \u201ccolor temperature,\u201d in the language of physics and photography, is essentially the same as the white-balance standard most cameras use.<\/p>\n
The classification \u201cyellow dwarf\u201d is a piece of astronomical convention that does not, in fact, describe the Sun\u2019s appearance. It refers to the Sun\u2019s place in the Hertzsprung-Russell diagram of stellar types, where G-class stars are between the hotter, bluer F-class stars and the cooler, oranger K-class stars. The \u201cyellow\u201d in the label is comparative rather than literal. To a human eye, the Sun is white.<\/p>\n
Why it looks yellow on the way through the atmosphere<\/p>\n
The Earth\u2019s atmosphere is made up mostly of nitrogen and oxygen molecules, both of which are much smaller than the wavelengths of visible light. When sunlight enters the atmosphere, these molecules scatter the incoming light, redirecting some of it in all directions. According to the Royal Observatory Greenwich\u2019s explainer on sky colour<\/a>, the amount of scattering depends strongly on wavelength. Shorter wavelengths, at the blue and violet end of the spectrum, scatter more than longer wavelengths, at the red and orange end. The dependence is steep: scattering goes roughly as the inverse fourth power of wavelength, so blue light at 400 nm scatters about nine times more than red light at 700 nm.<\/p>\n This has two consequences. The first is that the sky, seen from any direction other than directly at the Sun, looks blue. The blue you see when you look up is sunlight that started out as part of the direct beam from the Sun and was scattered sideways by air molecules into your line of sight. According to NASA Space Place\u2019s introductory account<\/a>, the sky would look violet rather than blue if not for two complications: the upper atmosphere absorbs much of the violet end of the spectrum, and the human eye is more sensitive to blue than to violet. The net effect is the familiar blue.<\/p>\n The second consequence is that the direct beam of sunlight, the part that reaches your eye without being scattered, is depleted of the wavelengths that were scattered away. Most of the blue and violet has been redirected elsewhere in the sky. What remains is the yellow, orange and red end of the spectrum, slightly enriched by the absence of its bluer counterparts. The Sun\u2019s apparent colour shifts toward yellow because some of the blue in its light is no longer arriving at the observer along the direct line from the Sun to the eye.<\/p>\n