What colour is the Sun? © NASA/SOHO

What colour is the Sun really? Hint: it’s not yellow

Colours of stars depends on their surface temperature as well as the wavelength the human eye can perceive.

Not the yellow hue you’ll find in your average school textbook.

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The Sun emits light over a whole range of wavelengths (or colours). In fact, it does so in all parts of the electromagnetic spectrum, apart from gamma rays. The peak in the Sun’s spectrum can be used to derive its surface temperature, about 5,780 Kelvin (roughly 5,500°C). The same process can be used to establish the surface temperatures of the stars.

The peak wavelength in a spectrum also generally determines an object’s apparent colour. So, for example, cooler stars appear red and hotter stars appear blue, with orange, yellow and white stars in between. For the Sun, the spectrum actually peaks at a wavelength that we would normally describe as green.

However, across the narrow range of the visible spectrum the amount of light emitted at each wavelength is almost exactly the same. But more crucially, the human eye doesn’t perceive light by averaging the various colours of the spectrum together. So, a very slight excess of green light doesn’t look green to the human eye – it looks white. The Sun would have to emit only green light for our eyes to perceive it as green.

This means the actual colour of the Sun is white. So, why does it generally look yellow? This is because the Earth’s atmosphere scatters blue light more efficiently than red light. This slight deficit in blue light means the eye perceives the colour of the Sun as yellow.

The more atmosphere the Sun’s light passes through, the more blue light is scattered. Hence, during sunrises and sunsets there is a much greater percentage of red light in the Sun’s spectrum, giving often spectacular results.

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Asked by: Darren Richardson, Wimbledon

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