“The Big Yellow One’s the Sun!”?

A video came through my Facebook feed, today, entitled “The Color of the Sun”. [Edit: WordPress doesn’t let me embed FB videos without modifying the html code of the theme I’m using, so you’ll just have to click on the link to watch it and come back in a minute. Or don’t, because I’m just about to spoil the entire thing.]

With fewer than 39 seconds to convey actual information, you know there’s going to be a lot that the person making this video left out. I want to rectify that at least a little bit.

Here’s the entire script, which I’ll break down point by point:

  • The Sun Isn’t Yellow (Space Myth Debunked!)
  • The Sun is actually all colors of the visible light spectrum combined. This makes white.
  • We see it as yellow, orange, or red because by the time light from the sun travels to Earth color wavelengths are scattered by our atmosphere.
  • So we see the colors individually.

“The Sun Isn’t Yellow”

There is a very quick way to make people realize that the Sun is white – point out that if it weren’t, objects we identify as white wouldn’t be white in sunlight. Were the Sun yellow, everything under it would be tinted differently – everything white from snow to wedding dresses would look yellow1 (Technically we wouldn’t know that, since we’ve only ever had the one version of sunlight. We’d just define that pure yellow color as “white” and go from there).

You can also tell them to stare at the Sun when it’s noon and ask them what color the Sun looks like, now. Actually, no. Don’t do that. But if it’s too late and someone’s already staring at the Sun for you, they’re likely to say it’s white…until their retinas burn out.

“The Sun is actually all colors of the visible light spectrum combined. This makes white.”2 

There’s an important bit of information that this video completely leaves out when it tells us the Sun emits the entirety of the visible spectrum:

It doesn’t emit all colors equally. (Also, it emits light outside of the visible spectrum, too, but that doesn’t affect the overall color because we can’t see any of those wavelengths.)

sunbb

Astronomers usually treat the Sun as a “blackbody” – an object that emits thermal radiation (i.e. light because of its temperature) and absorbs all light that hits it  – hence the black part of the name. All objects in the universe emit thermal radiation (including you), but we often can’t see it because the object isn’t hot enough to emit light in the visible part of the electromagnetic spectrum.

The above image shows a perfect blackbody matching the Sun’s average surface temperature (~5780 K, or ~9944 °F) – which wavelengths of light are emitted at which intensities. There’s a mathematical formula that describes this curve, but it’s not necessary to go into detail here.3  A blackbody is an unrealistic ideal – no object in the universe will have a curve matching a formula that only depends on one temperature4  – but for comparison, here are the blackbody curves for the Sun, Sirius – the brightest star in the night sky – and Proxima Centauri, the closest.

sunbb2.png

The peak wavelength tells us which color of light the star would theoretically emit the most; 501.3 nm is a blue-green. Given the asymmetric shape of the curve, the Sun’s real ‘curve’,4 and what wavelengths we sort into the various color labels, our Sun emits more green photons than any other. In general, the hotter the object, the shorter the wavelengths of the light it emits. This is why the hottest stars are blue (not red, as one might think from those childhood art lessons about warm and cool colors, or our general cultural associations for color and temperature – red=fire and blue=ice, etc.); they actually emit most of their light in the ultraviolet part of the spectrum, but we can’t see that.5

The Sun, you’ll hopefully notice, isn’t green, despite it shooting out a larger number of green photons than the other wavelengths. That’s because its peak is roughly in the middle of the visible spectrum, so it has more equal amounts of differently ‘colored’ photons emitted than a hotter or a cooler star does (which emit far more of one side of the visible band). Roughly equal amounts of different types of photons hitting the human eyeball is translated by our brains to mean ‘white’.

“We see it as yellow, orange, or red because by the time light from the sun travels to Earth color wavelengths are scattered by our atmosphere.”

Rather than looking at the space-based false-color images of the Sun from the video, here’s one taken from the ISS:

sunfromiss
Credit: NASA

Pretty darn white, right? The ISS orbits above essentially all of Earth’s atmosphere – air pressure up there’s about ten trillion times less dense than at sea level. But we can get pretty darn white photos of the Sun on the planet’s surface, too:

sun_white
Credit: Geoff Elston, CC BY 4.0

You need a solar filter to get this kind of image. As previously mentioned, it’s a very bad idea to stare at the Sun when it looks effectively white. It’s much safer when it’s lower in the sky, and is much more noticeably yellow.

The Mythbusters video is too short to explain why this happens. From their text, you might assume all the light gets scattered the exact same way in the exact same amount, which doesn’t help you understand the change in perceived color because why one color and not another? But lucky for me, that’s not the case.

Each wavelength of light has its own corresponding energy – shorter wavelength, higher energy. When the Sun’s photons enter our atmosphere – having previously travelled 150 million kilometers uninterrupted – they slam into various gas molecules (mostly nitrogen and oxygen). These particles are much smaller than the photons’ wavelengths, so when they collide the photons undergo a specific kind of scattering (called “Rayleigh scattering”); higher energy photons get scattered way more than lower energy ones. So, blue light gets sent every which way, making the sky appear blue.6 But that also means there’s less blue light following a more direct path from the Sun to you, which is why when you look directly at the Sun it appears yellow-ish.

While the Sun is rising/setting it’s at a lower angle in the sky, so the sunlight has to pass through more atmosphere before it reaches your eyes. This gives the blue light even more opportunity to scatter while the lower-energy wavelengths keep heading relatively straight for you. Thus, the Sun appears even redder and the sky does, too.

“So we see the colors individually.”

I don’t understand this line. Rayleigh scattering is not responsible for giving everything color. They likely only meant to refer to the color of the Sun, as anything that doesn’t emit its own visible light gets its color because white sunlight hits it and some of the colors get absorbed and some get reflected. But even if they were only referring to the Sun, they’re still wrong. The atmosphere is not a giant prism – separating white light into its constituent colors. We’re not walking around in rainbow light 24/7. This is a poorly constructed concluding sentence likely chosen to try and wrap things up quickly, but it failed in its execution.

A final note

The myth that the Sun is yellow is helped by the modern stellar classification system. Under this scheme, G-type stars like our Sun are referred to as ‘yellow’ because of the star Vega (surface temperature: 9602 K). In the 1950s, astronomers defined Vega (plus 5 other stars) to be ‘white’ based on the amount and type of light it emits, even though it would be a bluish-white to the human eye. Our Sun therefore sits in a category labeled ‘yellow’. The coolest G-type stars really are very very slightly yellow, but the Sun’s on the hotter end.

This gets translated into pretty much any illustration showing a range of star types, including those on college astronomy websites:

star-classes-surface-temperature-ranges
I see you, University of Chicago

1. As someone who regularly had to park in a campus structure that used those solid yellow lights (Sodium arc lamps, maybe?) at night and made my bright red car look a strange shade of grey, I know exactly what I’m talking about. 
2. In a way, they’ve said this backwards – we defined sunlight as white, and then discovered (Thanks, Newton) it was made of a spectrum of colors. Luckily they didn’t say white light is defined as adding up all the visible colors, because I’d have to point out the other ways to make white light. Actually, I’ll do it anyway. The obvious example is red + green + blue, which is how tv/computer/etc. screens make white light, but you can do it with only two, e.g. bluish-violet + yellow, as long as it’s a pair of complementary colors. 
3.  It’s called Planck’s Law: Iλ = 2hc2/λ5 × (e(hc/λkT)-1)-1)
4. Here’s what the Sun’s real ‘curve’ looks like compared to the ideal: 

640px-effectivetemperature_300dpi_e
Credit: Sch, CC BY-SA 3.0

It looks even more different at Earth’s surface, because the atmosphere absorbs certain wavelength bands, but that doesn’t affect the visible portion of the EM spectrum. 
5. This is also why incandescent lightbulbs waste a ton of energy – they emit a lot of infrared radiation which humans can’t see, defeating the purpose of the lightbulb’s existence. 
6. The sky isn’t violet for two reasons – the Sun emits fewer violet photons than blue ones, and the human eye is much better at detecting blue light than violet. Also, side note: this is why blue eyes are blue. 

PS – Shout-out to everyone who gets what the title references. Brivolthenumberseventheletterq fans, unite!

One comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s