The Physics of Visible Star Light

Globular cluster NGC 6717. The colour of light from the stars is a general indicator of surface temperature.

The colour of a star tells us how hot it burns. From the dull red of Arcturus to the brilliant blue of Rigel, you can actually see these subtle colour differences with your own eyes when looking up at the night sky.

Just like an iron cast into the blacksmith’s forge, which slowly changes from red to white hot, stars emit light at different frequencies depending on their overall luminosity and energy output.

The same relationship between colour and temperature is noted when metal objects are heated.

The Planck-Einstein equation E = hf is a basic way of understanding this. E is energy, f is frequency and h is the famous Planck’s constant. Higher frequency light (blue) is more energetic than lower frequency light (red) and therefore hotter and more luminous stars tend to appear more blue. Meanwhile cooler stars whose external atmospheric envelopes has expanded (red giants like Betelgeuse) appear redder.

A simple way to highlight the colour of star light is to take your smartphone camera or DSLR and manually defocus it on a target star. This will emphasise the colour and you can even produce beautiful star trails like the one below by taking a movie or long exposure star trail.

Star trail image credit – Amanda Cross

Why do we See what we See?

The western highlands of Scotland, bathed in the visible light of our home star.

When we think about the vast array of electromagnetic radiation all around us – from Gamma rays, X-rays , UV, Microwaves and Radio waves – a natural question to ask is why do human eyes see in a very narrow band we call ‘visible light’?

The answer is undoubtably tied to the energy output of our nearest star – the Sun. Its peak radiation just happens to be at this ‘visible’ band of radiation. I’ve illustrated this below with a black body radiation profile of our Sun.

Our eyes have therefore evolved to ‘see’ this particular narrow range of otherwise insignificant wavelengths. There’s nothing inherently important about visible light – in fact it makes up a tiny 0.0035 percent of the entire electromagnetic spectrum.

Understanding this makes me wonder about the potential sensory apparatus of life that might have evolved elsewhere in the universe. Other stars with different stellar classifications to our Sun have different peak radiation profiles.

If we had evolved next to a source of intense gamma rays for instance, we would very likely be completely blind to visible light but adept at observing small granular differences in the intensity of gamma radiation.