Fact Check: Is Our Sun Really White, Not Yellow?

A popular tweet about the true color of the sun has sparked a debate on Twitter as users question whether our closest star is really white or yellow.

The Claim

On September 12, Twitter account Latest In Space, which tweets space-related news and facts, tweeted: "Space Fact: The sun is actually white, but appears yellow because of Earth's atmosphere."

In less than 24 hours the tweet had gained more than 1,000 retweets and just under 10,000 likes, plus hundreds of replies.

"Not white!" wrote one user who indicated distinctly yellow images of the sun posted on NASA's website.

"Then why do we class it as a 'Yellow Dwarf' star?" asked another.

The Facts

Color is subjective. The way human eyes have evolved to see color means humans see things that might appear to be completely different in color to an animal, whose eyes are set up differently.

It's also true that some colors of light from the sun are more energetic than others. Most of the energy that the sun radiates is in the part of the light spectrum that we would perceive as green, for example.

But speaking purely in terms of how humans see the sun, it would be accurate to say that the sun's color is indeed white.

It may be natural to say that the sun is yellow because some of its most striking appearances in day-to-day life are when it turns the whole sky orange or red during sunset or sunrise. However, this is not because the sun is shining only yellow light at us.

The reason sunsets and sunrises appear red, yellow, or orange is because when the sun is lower in the sky its light must travel through more of Earth's atmosphere to reach the ground than when the sun is directly overhead.

The longer it takes for sunlight to travel through the atmosphere, the more shorter wavelengths of light—such as blue—will be scattered, while longer wavelengths—namely red—can continue for much longer distances. So the reason sunsets and sunrises appear red is because there is more red light left for us to see by the time the sunlight reaches our eyes. The same effect also causes red moons.

Outside of the scattering influence of our atmosphere, astronauts agree that sunlight looks white. "I can confirm this space fact," tweeted former NASA astronaut and International Space Station commander Scott Kelly in response to the Latest In Space tweet claiming the sun is white.

The reason sunlight in space looks white to human eyes is because the intense light given off by the sun is "essentially all colors mixed together," according to the Stanford Solar Center at Stanford University.

When all colors are mixed together, they appear white to human eyes. Proof that sunlight is many different colors of light mixed together is in rainbows. Rainbows are essentially sunlight that has been split into its constituent parts by passing through drops of rain. Similar effect can be achieved with a prism.

The reason sun images look yellow, or even green or blue in some NASA images, is that they have been edited or filtered that way to show certain details. The sun is indeed classed as a yellow dwarf star, but this is a misnomer—it does not mean the sun looks yellow to us. The name simply refers to the sun's medium size, common for this class of stars.

The Ruling

True.

When considered from the perspective of a human, and eliminating the scattering effect of Earth's atmosphere, sunlight is white. This has been confirmed by astronauts in space and can also be seen in photos of the sun seen from space.

FACT CHECK BY NEWSWEEK