Dark lines in the rainbow
How We Know What Stars Are Made Of
A philosopher declared it permanently unknowable. He was wrong within forty years.
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Step 1 of 5
It was supposed to be the one unanswerable question.
In 1835 the philosopher Auguste Comte used the composition of the stars as his example of something humans could never possibly know. We can measure their shapes and distances, he wrote, and nothing else. Spectral lines were identified in 1859.
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Step 2 of 5
Split any light and you get a rainbow — but not a smooth one.
At a glance it is a smooth rainbow. Close up it is full of missing colours. Sunlight through a prism has hundreds of fine dark lines crossing it, at fixed positions. They had been noticed and carefully mapped decades before anyone knew what they meant.
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Step 3 of 5
Each element absorbs its own exact colours.
Emit and absorb are the same list read two ways. Heat sodium and it glows in two specific yellows. Shine white light through cool sodium and those same two yellows are missing. Every element has a pattern like this, unique to it and unchanging — a fingerprint made of colour.
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Step 4 of 5
So the gaps in starlight are a list of ingredients.
Line up a star's dark gaps against the fingerprints measured in a laboratory and you can read off what the star's outer layers contain. From a hundred light years away, without going anywhere.
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Step 5 of 5
It found an element in the Sun before anyone found it on Earth.
During an eclipse in 1868, a yellow line appeared that matched nothing known. It was named after the Greek for Sun: helium. It was not found on Earth for another twenty-seven years.
This is also how the composition of the whole universe was worked out — and how we know stars are mostly hydrogen.
The short version
Stars are read by splitting their light and matching the missing colours against elements measured in a laboratory.
Try it yourself
Look at a street lamp through the edge of a CD. The light splits into bands rather than a smooth rainbow — that is the lamp's fingerprint.