The human eyes can only see a tiny fraction of the light around us. Light travels as a wave, covering the full spectrum of gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves. The only difference between the light we see, and the light we don’t, is the wavelength of the travelling light wave. Our eyes have evolved to observe only a narrow portion of total light (which we call visible light), with a wavelength of 400-700 billionths of a metre.
The Sun emits light across the full spectrum. The light our eyes see from the Sun is white light, comprising of all the colours of the rainbow. It is no surprise therefore, that when this sunlight gets split up into its wavelength components by a falling raindrop, that that’s exactly what we see projected on the sky – a rainbow. This white light is emitted at the Sun’s surface, known as its photosphere. Peak emission from the Sun occurs in this region of light too, close to the colour green.
But the Sun also emits many colours of light we can’t see. Radio waves, microwaves and X-rays are created by active regions in the Sun’s atmosphere, with the rare addition of gamma rays during energetic solar flare activity in these regions. Ultraviolet light is plentiful on the Sun too, emitted from layers in the Sun higher than the photosphere, both in the chromosphere (the lower solar atmosphere) and corona (the Sun’s full atmosphere). We can’t see these ‘colours’ of the Sun ourselves, but by measuring them with dedicated telescopes, we can use these wavelengths of light to observe layers of the Sun that we otherwise could never. The Sun’s corona, which can only ever be observed by our eyes during a total solar eclipse, is observed several times each minute by ultraviolet telescopes in space. Since the space age of satellites, observing at these wavelengths has provided us with unprecedented insights into the behaviour of the Sun, including the discovery of solar flares, coronal heating, and coronal mass ejections.
Below are images of our Sun this week, from Monday 22nd May. Measuring the different wavelengths of light allows us to observe these different regions of the Sun. Don’t get too caught up in the strange colours of these. Ultraviolet light is something we can’t see, so we use different colours to represent the different channels of ultraviolet images.
This article was written by Dr. Ryan French, solar physicist and author of The Sun: Beginner’s Guide to our Closest Star – out now. Order your copy to learn more about the captivating physics of the Sun, and how we’ve come to know what we know.