What color is the Sun? Be careful – it’s a trick question!
If you Google “What color is the Sun?” you’ll find that it is white. This probably runs counter to what you were taught in grade school – remember those bright, yellow smiley-face Suns you used to draw in the corner of you masterful work of art? Yeah – me, too.
The truth is that when it rises or sets, the Sun appears yellow (or orange, or red) because of the way light rays – or more specifically, photon packets – are dispersed through the atmosphere as they approach the Earth’s surface.
The core of the Sun, which is 25% of its area – but only 2%(!) of the entire Sun’s mass – is where the action starts. Like bumper cars, hydrogen atoms run into each other under the tremendous force of gravity at the center of our star. This hydrogen fusion creates helium 4, and this process generates photons which begin their journey from the core through the radiative zone, one micron at a time. This trip through the radiative and convection zones, then through the photosphere and chromosphere, and finally into the corona (the part of the Sun only visible on Earth during a solar eclipse), can take anywhere from 10,000 to 170,000 years. The final leg of this journey is the eight seconds it takes light (energy) to reach the surface of the Earth from the Sun. So, it’s entirely possible that if the Sun is shining on your face as you read this on your phone, you are viewing it in sunlight that was first generated 170,000 years and eight seconds ago.
So, if you’re viewing the Sun in the early morning or late evening, is it correct to say that the sun is yellow? Or orange? Or red?
And what about at midday, when the Sun is (more or less) directly overhead? In the morning, the shorter (blue & white) wavelengths of the electromagnetic spectrum are being diffused, causing the Sun to look orange. But when the Sun is high in the sky, the shorter wavelengths are dispersed, bouncing around the sky above your head and making the sky appear blue. The Sun itself takes on a blueish-white hue. Trust me on this – don’t look directly at it just to prove me wrong (or right).
So far, we’ve come up with a handful of colors – white, yellow, orange, red, blue-white…but which is it? What actual color is our actual Sun, minus all of the atmospheric interference and distractions?
Before we get there, let’s talk a little about the Kelvin scale.
In 1848, William Thomson (later made Lord Kelvin, from which the scale takes its name) devised a method of recording temperatures wherein absolute zero was denoted with a ‘0’ and subsequent degrees were incremented in a scale commensurate with the Celsius temperature scale. This “Kelvin scale” has since been widely used in the fields of engineering, astronomy, and science, particularly as a means of measuring noise temperature and color temperature.
A “blackbody” is a theoretical substance that absorbs (and transmits) the entire electromagnetic spectrum, and the gradations of the Kelvin scale are based on measurements of light refracted when a blackbody maintains a given temperature. (As an aside, a “whitebody” serves the opposite purpose, denoting a surface that reflects the entire electromagnetic spectrum. I know you were curious about that.)
Kelvin temperatures of 2000-3000K (we don’t use the term ‘degrees’ when discussing Kelvin temperatures) are characterized by reds-oranges-yellows (think sunrise and sunset); 3100-4500K are your yellow-whites (think late morning and afternoon); and 4600-6500K covers whites-blues (think midday). Anything less than 3000K is considered to be warm white; above that but less than 4500K is cool white; and anything over that is what we might consider daylight on a bright, sunny day.
So after all of that, have we come to a consensus on the color of the sun?
White? Red? Orange? Yellow? Warm white? Cool white?
I did mention that it was a trick question, didn’t I?
The truth of the matter is that the sun doesn’t have an inherent color. Electromagnetic waves themselves have no physical properties that would give them color.
Color is purely a physical perception, a product of photons hitting the rods and cones of the retina in the eye. Humans have three cones, whereas dogs (for example) only have two cones. As a result, we humans are able to perceive a wider range of color than our canine friends. While dogs have better nocturnal vision than we do, their vision is usually in the 20/75 region, meaning we see much more detail than they do on the whole.
So, make no mistake – color is all in your (and my) head.
The Sun – and everything else in the universe, for that matter – is colorless. It is only through a fluke of evolution and natural selection that we are able to perceive it as a vibrant, beautiful rainbow of color.
More about the color of the sun: http://solar-center.stanford.edu/SID/activities/GreenSun.html
More about Kelvin color temps: http://www.westinghouselighting.com/color-temperature.aspx
Even more about Kelvin temps: https://www.larsonelectronics.com/blog/2017/12/12/led-lighting/lighting-101-color-temperature-what-is-the-kelvin-scale
Curious about blackbodies? https://en.wikipedia.org/wiki/Black_body
What is color, you ask? https://physics.info/color/
That’s cool, but how does it work? https://www.livescience.com/32559-why-do-we-see-in-color.html
Many thanks to JJ for inspiring this post.
And as always, thanks for reading!