The Sun is depicted as yellow in children’s drawings, in weather icons, on flags, and in the standard astronomical shorthand. Astronomers themselves classify it as a “yellow dwarf,” 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’s 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’s atmosphere adds on the light’s way through.
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.
What the Sun’s light actually contains
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’s emission spectrum sits at roughly 500 nanometres, in the green part of the spectrum, corresponding to the Sun’s surface temperature of about 5,800 kelvin. But the spectrum is broad enough, and the eye’s 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’s “color temperature,” in the language of physics and photography, is essentially the same as the white-balance standard most cameras use.
The classification “yellow dwarf” is a piece of astronomical convention that does not, in fact, describe the Sun’s appearance. It refers to the Sun’s 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 “yellow” in the label is comparative rather than literal. To a human eye, the Sun is white.
Why it looks yellow on the way through the atmosphere
The Earth’s 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’s explainer on sky colour, 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.
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’s introductory account, 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.
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’s 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.
The angle of the Sun matters
The effect is not constant. The amount of scattering depends on how much atmosphere the light has to pass through, and that in turn depends on the Sun’s position in the sky. When the Sun is directly overhead at noon on a clear day, sunlight is taking the shortest possible path through the atmosphere. The scattering is minimal, and the Sun is close to white. The Royal Observatory explicitly notes this: at noon, the Sun looks essentially white from the ground. Only when the Sun is lower in the sky does the path length through the atmosphere increase, the scattering intensify, and the apparent colour shift more strongly toward yellow, orange, and ultimately the deep red of sunset.
At sunset and sunrise, the light has to traverse the longest possible path through the atmosphere, and most of the blue, green, and even yellow wavelengths have been scattered out by the time the direct beam reaches the horizon. What remains is largely red and orange. The red sunset is the same Rayleigh scattering as the blue sky, simply seen from the other end of the process. The blue light that has been removed from the direct beam is, at that moment, lighting up the sky in some direction other than where the observer is looking.
What the Sun looks like from elsewhere
Photographs from the International Space Station, and earlier from the Apollo missions on the Moon’s surface, show the Sun as a white disc against a black sky. The Moon has no atmosphere to scatter blue wavelengths away, and astronauts standing on its surface during the daytime saw a sun that was white, not yellow. The same is true of any photograph taken from above the atmosphere, including the long sequence of solar observations from spacecraft such as NASA’s Solar Dynamics Observatory, which images the Sun in true and false colour depending on the wavelength filter being used.
The disconnect between the Sun’s actual colour and the colour it appears to us is one of the more durable examples of an everyday observation that turns out to be an atmospheric artefact. The Sun is doing what the laws of stellar physics predict for a 5,800-kelvin blackbody, emitting a broad white-light spectrum that the human eye reads as white. Our atmosphere then performs a small, wavelength-dependent edit on the way through, removing some of the blue and lighting up the sky with it. The Sun ends up looking slightly yellow. The sky ends up looking distinctly blue. These are the same edit, seen twice.
