Why The Universe Isn't Green

Despite how it may seem, our night sky is not a black canvas. If we stare at the stars long enough, our eyes adjust to the gloom and more and more stars appear as if from nowhere. If our eyes were sensitive enough, like the Hubble’s huge camera, we would see a view filled with bright stars, vast clouds of gas and incredibly distant galaxies. Every single patch of the universe our eyes could resolve would be burning with light. But this leads to the question: what colour is our universe?

The light from a star begins in nuclear reactions in the stars core, usually where hydrogen is fused to larger helium atoms. The particles of light, known as photons, that are released may take millions of years to tunnel their way to the surface, as they bounce around between densely-packed atoms within the star. When they escape, they give the star a distinctive colour that depends on the amount of energy being released across each square meter of its surface. The coolest stars glow red-hot, like a horseshoe fresh from a blacksmith’s furnace. Hotter stars produce colour across the entire visible spectrum of light and glow white-hot, like Tungsten light bulbs. Some of the biggest stars in the universe are so incredibly hot that they glow blue.

But there is no such thing as a green star. No matter how deep you stare into the universe, none will have the green hue we are so used to here on Earth. This is not some sort of cosmic coincidence, but is instead due to how the eyes of animals have evolved over millions of years. As everyone knows from school biology, all plants use a molecule called chlorophyll to convert energy from sunlight into sugars and important carbohydrates. This one molecule forms the pillar on which all complex life feeds upon; be it tiny insects or docile jellyfish, deadly snakes or intelligent apes.

Chlorophyll is a fussy molecule. Of the whole spectrum of colours (which extends far beyond the tiny part we can see), chlorophyll only uses two specific blocks of light: red and blue. In between these two broad regions of light is a tiny segment of the rainbow that photosynthesis cannot use. All plant leaves and algal cells reflect this narrow band of light, each in a slightly different way. This is what we see as green, and over millions of years our eyesight has become incredibly accurate at viewing it. And when the difference between a good meal and being poisoned is the tiniest change in the shade of green, it is easy to see why.

But physics could not care less about how human eyes are built. All stars glow across a broad range of light – the white that we see in most stars is due to the light of the whole rainbow being squashed together. However, the green colour our eyes are focused on is far too narrow to be seen in stars. So, while the earth may be teeming with turquoise, emerald and olive greens, the universe itself is, astonishingly, purple.