If you glance at the Sun in the sky, you might assume it’s yellow even though it’s really white. (Don’t look at the Sun!) Depending on the time of day and the amount of pollution in the air, the Sun looks yellow, orange, or red. But, photos from the International Space Station (ISS) or the Moon show a white Sun. Why does the Sun look yellow when it’s white? Why do some people think the Sun is green?
The Sun is white. It appears yellow from Earth because of scattering from the atmosphere. It’s peak visible light is in the green part of the spectrum.
Prove to Yourself That the Sun Is White
If pictures from the ISS or Moon don’t convince you the Sun is white, then get yourself a prism and examine the spectrum of light. It’s a complete rainbow or white light. A rainbow in the sky or the kind you see in a waterfall or from a garden hose illustrate the same point.
The Sun emits electromagnetic radiation over a wide range of wavelengths, including some our eyes can’t see (like infrared and ultraviolet). Its peak emission within the visible spectrum is green. But, the Sun is not green because it’s the combination of wavelengths that determines color.
Why the Sun Looks Yellow When It’s Really Not
The reason the Sun looks yellow (sometimes) is because the atmosphere scatters the violet and blue wavelengths more than it scatters the other colors. This is Rayleigh scattering and it also accounts for the reason the sky is blue. On a clear day, the Sun looks mostly yellow. At noon, it can appear white. Near sunrise or sunset the Sun looks orange, red, or magenta. The Sun also looks orange or red if there is a lot of smoke or pollution in the air.
Some people think the Sun looks yellow is because it’s set against a blue sky. The problem with that argument is that clouds are also set against a blue sky, yet still appear white. However, the Sun isn’t a hard sphere in the sky. Its fuzzy edges might contribute to the yellow color.
Finally, ancient people considered the Sun as a form of fire in the sky. Fire is yellow, so the Sun is yellow. If we’ve been drawing the Sun as a yellow object since Neolithic times, it’s just the color we expect to see.
Why Is the Sun a “Yellow Star” If It’s White?
When you look at the color classification of stars, the Sun is a G-type or yellow dwarf star. Why is it a yellow star if it’s white? The reason is that the classification system uses the appearance of stars from the Earth’s surface on a clear day/night.
The black-body radiation from the Sun comes out at a temperature of around 5800 Kelvin, which is yellowish white or “natural white”. In comparison, the tungsten filament in an incandescent bulb is much more yellow at 2854 K (ironically, a “warmer” color even though it’s a cooler temperature). Black-body radiation is just a model used for comparing different stars. The true colors of stars, including the Sun, are a bit different.
Why Is the Sun Yellow or Green in Photos?
Astronomers (and backyard enthusiasts) use filters for viewing and photographing the Sun. The color of the resulting image depends on the filter.
The most inexpensive filter is a neutral density filter. This is a white light solar filter that attenuates so much light that the remaining disk is yellow or orange. These filters are great for viewing sunspots and solar eclipses.
A hydrogen-alpha (H-alpha) filter is a type of bandpass filter that only allows the deep red light (656.3 nm) emitted by hydrogen atoms through. These filters produce a red image of the Sun and reveal solar prominences, flares, spicules, and other types of solar activity.
A solar continuum filter, on the other hand, produces a green image. This is a 540-nm (green) filter that improves contrast while reducing atmospheric effects.
A CaK or calcium-potassium filter is another bandpass filter. This filter produces a deep blue image that is not readily visible to the naked eye, but is easy for digital cameras to capture. A CaK filter shows a lot of surface detail, but less sun spot definition.
Finally, photographers often change the Sun’s color during image processing. Sometimes this improves contrast. Other times, the color is changed to yellow to make the final result easier to recognize as the Sun.
- Cloud Break Optics. Best Telescopes & Gear For: Solar Viewing.
- King, Bob (2015). “Observer’s Guide to the H-Alpha Sun“. Sky and Telescope.
- NASA (2013). “Sun Primer: Why NASA Scientists Observe the Sun in Different Wavelengths“. SDO Solar Mission.
- Wilk, S.R. (2009). “The Yellow Sun Paradox”. Optics & Photonics News: 12–13.