Explained | What colour is the Sun?

One of the best things about science is how a very simple question can lead you down a rabbit hole. Here’s one for you: What color is the sun?

By Phil Plait for 'Scientific American'

One of my favorite things about science is how a very simple question can lead you down a rabbit hole. Here’s one for you: What color is the sun?

Most people, I’d wager, would say yellow. You probably pictured it as yellow in your head when you thought of it just now.

Not too long ago, a conspiracy theory hit social media when a person tweeted that she remembered the sun being yellow when she was younger but that it was now white. (She also claimed it was oddly shaped in her photograph—which was likely caused by the image being very overexposed.) What color was right? The one seen by her or by her camera? Well, neither was right, really. But her camera was closer to the “truth.”

The sun is white—kind of. It depends on your interpretation of color, the way colors work, the way our eyes see and, just as importantly, the air we see through.

Back in the 1850s the new study of thermodynamics—how, in part, temperature affects the way objects behave—had physicists hot and bothered. Over time they developed the concept of a “blackbody,” an object that perfectly absorbs all radiation that hits it. Such an object, in the absence of any radiation around it, would be perfectly cold and would radiate no heat at all. But in the presence of light, it would begin to warm, and as it did so, it would reradiate that heat in the form of light and emit light all across the spectrum. That light would have a peak brightness at a specific color, depending on the object’s temperature, and it would be dimmer at different wavelengths. This idea produced what would become the blackbody curve, a graph showing how brightly an object emits light at different wavelengths depending on its temperature.

The sun is a ball of extremely hot gas (or, more poetically, as well as more accurately, a miasma of incandescent plasma) and acts very much like a blackbody. The biggest difference between it and a true blackbody is the presence of hydrogen and other elements in its atmosphere that absorb very narrow wavelength ranges of light, which produces gaps in the blackbody curve.

When we measure the sun’s spectrum (how bright it is at each wavelength) using satellites above Earth’s atmosphere, we find that it emits light across the visible part of the spectrum—that is, the kind of light our eyes can see. That’s no coincidence! Humans evolved to see where the sun emits the most light. Our star also gives off ultraviolet and infrared light, though not nearly as much. Surprisingly, perhaps, the sun is brightest in the blue and green parts of the spectrum and dims toward the red. Naively, you might think this means the sun is teal! But it’s clearly not.

That’s because of how we see. Recently I wrote about how cells in our eyes detect light. Cones are specialized cells that detect color. There are three kinds of cones, called L, M and S, which are tuned to see long wavelengths (toward the red end), medium (yellow and green) and short (blue), respectively. The process is complicated—it’s biology, after all—but when light hits these cones, they send signals to the brain corresponding to how intense the light is at different colors. By comparing those signals, the brain interprets them as colors. If the S and M cones are activated strongly but the L isn’t, you might see a greener hue, whereas a strong L signal will tilt things toward red. If the light coming in is equally bright across the visible spectrum, we see white. This is what happens with the sun, so it looks white.

Except that’s not really the case. This answer is true for sunlight in space before it hits our atmosphere. Astronauts, for example, see the sun as white (not that they look directly at it, because vision is generally considered a big plus when working in space). When the sunlight travels through our air, however, some of it gets absorbed or scattered away. Not all colors are affected equally: light toward the blue end gets scattered away much more than red does. That’s why the sky is blue—we see that scattered light coming from all over the sky, which tints it cerulean. The sun doesn’t emit as much purple light as blue, and our eyes aren’t as sensitive to purple, so the sky doesn’t look violet, even though that color scatters even more than blue. This process changes the color of the sun a bit. Removing the bluer light from the sun should make it look a bit yellower.

Also, our brain interprets color in a relative way. We compare the color of one object with others in the field of vision. If the sky looks blue, that could also make the sun look yellower. While I hear the claim that the sun is yellow pretty often, however, I don’t buy it. For one thing, if the sun were actually yellow, white paper—which reflects light pretty well in all colors—should look yellow in sunlight, too. But it looks white.

Also, it’s really hard to look directly at the sun to judge its color. That’s a good thing! Infrared light from the sun can damage our retinas, so evolution has encouraged us not to stare at it. It’s hard to tell the color of something when you can’t look at it. And at this point, I should be very clear that you should never look directly at the sun. It can permanently burn your retinas in tiny spots, so it’s very dangerous to your vision.

The only time we can safely look at the sun without protection is when it’s very low to the horizon and dimmed by atmospheric haze (and even then, you should be careful). At sunrise or sunset, it tends to have even more blue and green light scattered away, so it really does look yellow, orange and even red. This pattern could also be why people tend to think it’s yellow.

As for conspiracy theorists who claim the sun has changed color, it really hasn’t. That’s the kind of thing astronomers would have noticed, and we’re not known for being able to keep our mouths shut when cool astronomical phenomena transpire. More likely they’re just misremembering.

But then that’s the beauty of color. It really is in the eye of the beholder.

(Phil Plait is a professional astronomer and science communicator in Colorado.)