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Nobody has been to a star and nobody ever will. So how can anyone say the Sun is mostly hydrogen? Not by guessing — by reading the gaps in its light, which turn out to be a signature you cannot fake.
A star is a ball of gas at thousands of degrees, unimaginably far away. No probe has landed on one. No probe ever will — anything you sent would be vapour long before it arrived.
And yet a textbook will tell you, flatly, that the Sun is about three-quarters hydrogen. Somebody wrote that down as a fact.
If your reaction is how could they possibly know that, you are asking exactly the right question. In 1835 a well-regarded philosopher named Auguste Comte used the composition of the stars as his example of something humanity could never, in principle, find out. It was a sensible thing to say.
He was wrong within his own century, and the tool that proved him wrong was already sitting in a drawer when he said it.
Throw table salt into a flame and the flame turns yellow. Not yellow-ish — a very particular yellow, and always the same one. Copper gives green. Strontium gives the red in fireworks. Chemists have used this as a rough test for two hundred years.
Now do it properly. Spread the light out through a prism, so instead of one blur of yellow you see exactly where in the rainbow that yellow sits. Sodium turns out to give a line at almost exactly 589 nanometres and, if you look closely, a second one right beside it. Always those. Never anywhere else.
Every element does this, and every element's set is different. Hydrogen gives a ladder of lines. Calcium gives a hard pair in the violet. Iron gives so many that its pattern looks like a barcode.
That is what makes it evidence rather than a guess. You are not matching a colour. You are matching a whole pattern of exact positions, in an order nothing else produces.
Heat an element and it emits its own fixed set of colours. Shine light THROUGH that element, cooler, and it swallows the very same colours — leaving dark gaps where they should have been. A gap is a fingerprint too.
That second half is the trick that reads a star.
The hot, dense body of the Sun pours out every colour at once. On its way out, that light passes through the Sun's cooler outer atmosphere — and every element up there takes its own colours out of the beam.
So sunlight arrives here with hundreds of narrow dark lines cut into it. Each one is a receipt for something the light passed on the way.
Here are five of the strongest dark lines in the Sun's spectrum, in the order an investigator might pick them off. For each one, the position was measured first — the name came from matching it to something heated in a laboratory here.
A star's spectrum has a dark gap sitting at precisely the colour sodium glows in a flame. What does that gap tell you?
During a total eclipse in August 1868, astronomers turned their instruments on the thin layer of the Sun that only shows itself when the bright disc is covered. Pierre Janssen in India, and Norman Lockyer in London a couple of months later, both recorded a strong yellow line at about 588 nanometres.
It was close to sodium's pair but definitely not on it. And nothing in any laboratory on Earth had ever produced a line in that spot.
Lockyer and the chemist Edward Frankland took the position that it belonged to an element nobody had yet found here. They named it after the Greek word for the Sun: helium.
That was a bold claim and it stood unconfirmed for twenty-seven years. Then in 1895 William Ramsay released gas from a lump of uranium ore, passed its light through a prism, and found the line — in the same place, on a bench in London.
This is the part worth sitting with. A method that could invent an element out of wishful thinking would not have had to wait twenty-seven years to be right about one. The line was measured, the prediction was specific, and the world eventually produced the thing.
Why does the helium story count as evidence that reading spectra is a real measurement rather than clever guesswork?
Nobody invented this in one go. Put the steps in the order they happened — and notice how long the gap is between seeing the lines and understanding them.
Tap whichever you think happened next
This is what all those lines add up to, measured by weight. It is not a close-run thing.
| What the Sun's outer layers are made of | Percent by mass |
|---|---|
| Hydrogen | 73 |
| Helium | 25 |
| Everything else | 2 |
That third bar is doing a lot of quiet work. Every atom of carbon in you, every atom of iron in your blood, every atom of calcium in your teeth is in that sliver — and the Sun's sliver is typical.
It is also the number Cecilia Payne was talked out of believing in 1925. The lines said hydrogen, overwhelmingly, and the accepted view said the Sun should resemble Earth. The lines were right. They usually are, because unlike opinions they do not move.
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Auguste Comte, 'Cours de philosophie positive' (1835), gave the chemical composition of the stars as an example of knowledge permanently beyond reach. Joseph von Fraunhofer measured and catalogued 574 dark lines in the solar spectrum in 1814, labelling the strongest with letters A to K; they are still called Fraunhofer lines. Gustav Kirchhoff and Robert Bunsen established in 1859-1860 that each element has a characteristic line spectrum and that a cooler gas absorbs the same wavelengths it emits when hot. The sodium D lines lie at about 589.0 and 589.6 nanometres; the calcium H and K lines at about 393.4 and 396.8; the hydrogen Balmer lines at about 656.3, 486.1 and 434.0 nanometres. Helium was identified from a yellow line near 587.6 nanometres in the solar spectrum during the eclipse of 18 August 1868 by Pierre Janssen, and independently by Norman Lockyer that October; Lockyer and Edward Frankland named it. William Ramsay isolated it on Earth from uranium ore in 1895. Cecilia Payne's 1925 Radcliffe thesis 'Stellar Atmospheres' concluded that hydrogen and helium overwhelmingly dominate stellar composition. Henry Norris Russell persuaded her to state that the result was almost certainly not real, and published the same conclusion himself in 1929. Solar photospheric composition by mass: hydrogen about 73.8 percent, helium about 24.9 percent, all heavier elements together about 1.3 percent (Asplund, Grevesse, Sauval and Scott, 2009). Lines produced by Earth's own atmosphere are distinguished from a star's own lines because the star's shift with its motion relative to us while atmospheric lines do not.