If there were no atmosphere to make the sky blue, we’d all be incredibly dead, incredibly fast. But let’s set that aside for the moment.
If there were no atmosphere to make the sky blue, what colour would we look out and see? We’d see a (cloudless) black sky all day, no matter the hour. Our sun would still illuminate the greens of the grass and the blues of the oceans and the reds of our sports cars, but the Sun’s photons no longer have air molecules to scatter them before they reach these surfaces. The Sun’s light would be too bright for most stars on the day side to compete with it, so you’d have one thumb-sized white1 orb rising and falling through a sea of blackness, until your side of the planet turned away. Then you’d have a relatively familiar night (The stars wouldn’t twinkle, though, because there’s no atmospheric turbulence).
This is why we think of the universe as black. Or black with tiny flecks of white. Or black with tiny flecks of red, orange, yellow, white, and blue for those who remember that stars come in different colours.2 But there are other colours in the cosmos, too. Ionized atoms, gas, and dust in nebulae can produce a smorgasbord. The Hubble Space Telescope took this photo3 of a stellar nursery in the Carina Nebula, 7500 lightyears from Earth.
But what if we could add up all of the different coloured light in the known universe? What colour would we get? Astronomers from Johns Hopkins found out by surveying over 200,000 galaxies and adding all their light together. (They also had to compensate for the fact that galaxies from further away are redder than how they would appear if they were right next to ours – this is called “red-shifting” and is due to the fact that the universe is expanding.) These are all the colours:
And this is the result of the addition:
It is, perhaps, a bit underwhelming that the average color of the universe — if you could hold all the light in your hands — is a shade of beige. A software error actually had the astronomers believing that the average was a shade of light turquoise, for a while, would would have been far more interesting.
They named this shade “Cosmic Latte”; the Washington Post quoted one of the astronomers as saying they were looking for suggestions, and readers actually wrote in. Other suggestions included Cappuccino Cosmico, Big Bang Beige, Skyvory, and Primordial Clam Chowder.
What would you call the colour of the universe?
1. Yes, the Sun is white. Sunlight is how we define what white light is. If the Sun suddenly turned yellow, every coloured object would be slightly off in tint from what we’re used to, in the same way that things look different under fluorescent lighting. It may be because of some of the Sun’s blue light gets scattered away, or because we’re only able to get a proper look at the Sun for any longer than a moment when it’s lower in the sky and very clearly yellower (due to more significant atmospheric scattering) than it is when it’s high noon (Don’t stare at the Sun, kids.); but, whatever the reason, we’ve got it engrained in our brains to think of the Sun as yellow. ↩
2. But no green stars, despite green sitting smack dab in the middle of the visible part of the electromagnetic spectrum. That’s because all stars emit a range of light wavelengths. Blue stars are blue because most of the light they emit is actually invisible ultraviolet, with some blue light (and an insignificant amount of cooler colors). Red stars emit mostly infrared light with some red, less orange, and even less yellow, so the star looks red. A star that emits more green light than any other color – like the Sun, in fact – is going to be emitting lots of blue and yellow and orange and red so it all adds up to white-ish (the balance isn’t usually perfectly even). ↩
3. It’s actually a combination of three images – one taken with a blue filter (for detecting Oxygen), one with a green filter (Hydrogen and Nitrogen), and one with a red filter (Sulfur), and then adding them together. ↩