Episode 19 - Reading Starlight

Tomlin Hawthorne

I'd like you to make a prediction about the limits of knowledge.

Tomlin Hawthorne

Name one thing, one fact about the physical universe, that you're confident human beings will never find out. Not because it's too expensive, or too far off. Because it's impossible in principle.

Tomlin Hawthorne

Take a moment. Choose carefully.

Tomlin Hawthorne

In 1835 one of the cleverest men in France made his choice. He said that we might, one day, work out the shapes of the stars, their distances, their sizes, their movements. But there was one thing we could never know by any means whatever: what they're made of.

Tomlin Hawthorne

It was a very reasonable thing to say. A star is a point of light, trillions of miles away. You can't go there. You can't bring back a piece.

Tomlin Hawthorne

He died in 1857. Two years after that, two men in a laboratory in Germany did it. Today, how. And I'd hold on to your own prediction a little loosely.

Tomlin Hawthorne

Last time, a glass of water with a few drops of milk, and a torch. Blue from the side, orange down the beam. A sky and a sunset on the kitchen table. And a drop of water on your phone, showing you the red, green and blue dots that your brain turns into every colour you've ever seen on a screen.

Tomlin Hawthorne

So we know that white light is a mixture, and that a prism or a CD will spread it out into a spectrum. Today we're going to look at that spectrum much more closely. Because there's writing in it.

Tomlin Hawthorne

The Frenchman was Auguste Comte. You may well know him. He's the founder of positivism. He more or less invented the word sociology. His whole philosophy was that real knowledge comes only from observation, and that we should stop speculating about things we can't observe. So when he chose his example of the unknowable, he wasn't being lazy. He was applying his principles. You can observe where a star is. You cannot observe its chemistry.

Tomlin Hawthorne

He'd overlooked one thing. You can observe its light.

Tomlin Hawthorne

Our story starts twenty years before Comte's pronouncement, in Bavaria, with one of the more remarkable lives in science.

Tomlin Hawthorne

Joseph Fraunhofer was born in 1787, the eleventh child of a glassmaker. Both his parents were dead by the time he was eleven. He was apprenticed to a mirror-maker in Munich, a harsh man who wouldn't let him read or go to school.

Tomlin Hawthorne

In 1801, when Fraunhofer was fourteen, the house he lived and worked in collapsed. He was buried in the rubble. It took four hours to dig him out, and the rescue was watched by a crowd that happened to include the Prince Elector of Bavaria. The prince was moved, and gave the boy a purse of money. Fraunhofer used it to buy books, and a glass-grinding machine, and his way out of the apprenticeship.

Tomlin Hawthorne

Within fifteen years he was making the best lenses and prisms in the world. And in 1814, testing his glass, he passed sunlight through a narrow slit, then a very fine prism, and looked at the spectrum through a small telescope.

Tomlin Hawthorne

It wasn't a smooth rainbow. It was crossed by lines. Thin, sharp, dark lines, as though someone had ruled across it with a fine black pen. Hundreds of them. Fraunhofer counted five hundred and seventy-four, and mapped them carefully, and gave the darkest ones letters.

Tomlin Hawthorne

They were always in the same places. They were there in sunlight, in moonlight, in the light of the planets. When he pointed the instrument at the bright star Sirius, he found lines too, but different ones.

Tomlin Hawthorne

He had no idea what they were. He used them as markers, for calibrating glass. He died at thirty-nine, his lungs ruined by the fumes of the glass furnaces. On his grave it says, in Latin: he brought the stars nearer.

Tomlin Hawthorne

The lines sat in the textbooks for forty-five years, a known curiosity. Like Brown's jittering pollen.

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Heidelberg, 1859. Two professors. One is Robert Bunsen, a chemist: large, untidy, cheerful, blind in one eye from a laboratory explosion. He never married. He said he never had the time. The other is his friend Gustav Kirchhoff, a physicist: small, neat, precise.

Tomlin Hawthorne

Bunsen was interested in the old trick of identifying chemicals by the colour they give a flame. Throw salt on a fire and it flares yellow. Copper burns green. It's how fireworks are coloured. The trouble was that the flame itself had colour, which muddied everything. So Bunsen had his technician build a gas burner that mixes in air before the gas burns, and gives a hot, clean, almost colourless flame. Every school laboratory in the world has one. He never patented it.

Tomlin Hawthorne

Now he could hold a speck of some substance in the flame and see its colour clearly. But colours are hard to tell apart by eye. Two different reds look much the same. And Kirchhoff said: don't just look at the colour. Put it through a prism. Spread it out.

Tomlin Hawthorne

So they built an instrument out of a cigar box, two old telescopes and a prism. And what they saw changed everything.

Tomlin Hawthorne

A hot solid, like a glowing poker, gives a continuous rainbow. Every colour, smoothly. But a pure element, vaporised in the flame, doesn't. It gives a few sharp, bright lines of colour and darkness in between. And each element gives its own set.

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Sodium: two bright yellow lines, very close together. Always those two, always at exactly that place. Lithium: a strong red line. Potassium: a red and a violet. Every element they tried had its own pattern, like no other.

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It's a barcode. Each chemical element has a barcode written in light, and nothing can disguise it. A grain of salt too small to see will show the sodium lines.

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They promptly used it to discover two new elements nobody had known about, just from unfamiliar lines in mineral water. They named them for the colours of their lines: caesium, sky-blue, and rubidium, red.

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And here's the moment. Kirchhoff noticed that sodium's two bright yellow lines sit at exactly the same position in the spectrum as one of Fraunhofer's dark lines in sunlight. The one Fraunhofer had labelled D. Not close. Exactly.

Tomlin Hawthorne

And he worked out why. A hot gas gives out light at its own particular wavelengths. But the same gas, when it's cooler than a bright source shining through it, absorbs light at those same wavelengths. It takes out precisely the colours it would otherwise give off.

Tomlin Hawthorne

So the dark lines in sunlight are barcodes too. In negative. The Sun's surface is a blaze of every colour. And the light has to get out through the Sun's cooler outer layers. The atoms there each remove their own wavelengths. What reaches us is a rainbow with the fingerprints of every element in the Sun's atmosphere stamped out of it in black.

Tomlin Hawthorne

Fraunhofer's line D is sodium. There's sodium in the Sun. Others are iron. Calcium. Magnesium. Hydrogen.

Tomlin Hawthorne

There's a story about how the idea came to them. One evening, from the laboratory window, they saw a fire burning in the city of Mannheim, ten miles away across the plain. They turned the spectroscope on it, for fun, and found the lines of barium and strontium in the flames. And a few days later, out on one of their walks, Bunsen said: if we can tell what's burning in Mannheim, why shouldn't we do the same for the Sun? But people would say we must have gone mad, to dream of such a thing.

Tomlin Hawthorne

So: Comte's impossible question. Answered by a cigar box.

Tomlin Hawthorne

I can't resist one more story. Kirchhoff's banker, hearing about all this, was unimpressed. What use is gold in the Sun, he asked, if I can't bring it down to Earth? Some time later, Kirchhoff was awarded a medal for the work, by a British society, and the prize came with a sum in gold sovereigns. He took them to the banker and said: here is gold from the Sun.

Tomlin Hawthorne

It got better. In 1868 there was a total eclipse of the Sun, visible from India, and astronomers turned their spectroscopes on the glowing edge. They found a bright yellow line. Near the sodium lines, but not the same. It matched nothing. No element on Earth produced it.

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An English astronomer, Norman Lockyer, drew the bold conclusion. There's an element in the Sun that we don't have here. He named it after the Greek word for the Sun, helios. Helium.

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For twenty-seven years it was known only as a line in the Sun's spectrum. Then in 1895 a chemist in London found it on Earth, trapped in a uranium mineral. So the second most common element in the universe was discovered ninety-three million miles away, a generation before anyone held a sample.

Tomlin Hawthorne

Now the story moves to America, and to a group of people who are only lately getting their due.

Tomlin Hawthorne

By the 1880s the Harvard College Observatory was photographing the spectra of stars by the tens of thousands, on glass plates. Somebody had to examine every one. The director, Edward Pickering, got tired of his male assistants, and is supposed to have said that his Scottish maid could do a better job. So he hired her. Her name was Williamina Fleming, and she could. She ended up running a staff of dozens of women, who were known as the Harvard computers. They were paid about the same as factory hands, far less than a man would have got, and they were not permitted to use the telescopes.

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One of them, Annie Jump Cannon, who was almost entirely deaf, worked out the system that astronomers still use for sorting stars into types by their spectra. And then she applied it. She classified around three hundred and fifty thousand stars, by eye, with a magnifying glass. At her fastest she did three a minute.

Tomlin Hawthorne

And then, in 1923, a young Englishwoman arrived at Harvard. Cecilia Payne. She'd studied at Cambridge, where she'd completed the whole course, and where the university would not give her a degree, because it didn't award degrees to women. It didn't until 1948. So she came to America.

Tomlin Hawthorne

She had something the computers hadn't. She'd learned the new physics of the atom, which we'll get to in Part Six. And she saw that it could be used to read the barcodes properly. Not just which elements are present, but how much of each.

Tomlin Hawthorne

Everyone assumed the answer was obvious. The Sun's spectrum is full of lines from iron, calcium, sodium, the same things the Earth is made of. So the Sun must be a ball of much the same stuff as the Earth, only hotter.

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Payne showed that this was a mistake. The strength of a line depends on temperature as well as on amount. Iron shows up strongly because, at the Sun's temperature, iron atoms are very good at absorbing light. Not because there's a lot of iron.

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When she corrected for that, the result was astonishing. The stars are not made of Earth-stuff. They are almost entirely hydrogen and helium. Hydrogen, the lightest and simplest element, was about a million times more abundant than the metals. Everything the Earth is made of was a trace. A dusting.

Tomlin Hawthorne

She put this in her doctoral thesis in 1925. And the most eminent astronomer in America, Henry Norris Russell, read the draft and wrote to her that the result was clearly impossible.

Tomlin Hawthorne

She was twenty-five, a woman, a foreigner, with no position. So in the published thesis, next to her own calculation, there's a sentence saying that the enormous abundance she'd derived for hydrogen and helium is almost certainly not real.

Tomlin Hawthorne

Four years later Russell, by a different method, reached the same conclusion, and published. He did mention her. But for a long time the discovery was generally credited to him.

Tomlin Hawthorne

An astronomer later called her thesis the most brilliant ever written in the subject. In 1956 she became the first woman to be made a full professor at Harvard from within its own faculty. She'd been lecturing there for decades by then. For years her courses weren't listed in the catalogue.

Tomlin Hawthorne

And what she'd found is one of the largest facts there is. The universe is hydrogen and helium. Everything else, rock, water, iron, you, is the exception.

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One more thing the lines can do, and it sets up a great deal.

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Remember the ambulance from episode ten. A source moving toward you has its waves squeezed up; moving away, stretched out.

Tomlin Hawthorne

The barcode of an element is fixed. The sodium lines are at one exact place. So if you look at a star and find the whole barcode slid slightly toward the red end, every line shifted by the same fraction, there's only one explanation. The star is moving away from you. And the size of the shift tells you how fast.

Tomlin Hawthorne

So from a point of light, you can read off what it's made of, how hot it is, and how fast it's moving and in which direction. Very nearly everything we know about anything beyond the solar system, we know because of those lines. The telescopes in orbit now are reading the barcodes of the air on planets circling other stars. Every sentence in Part Eight of this series, about the Big Bang and the expanding universe, rests on what Kirchhoff and Bunsen did with a cigar box.

Tomlin Hawthorne

But I have to leave you with a hole in the story.

Tomlin Hawthorne

Why does each element have lines at all? Why should sodium give out light at just those two wavelengths and no others? In the physics we've covered so far, nothing explains it. An atom ought to be able to glow at any colour it likes. For fifty years the lines were used every day and understood by nobody.

Tomlin Hawthorne

The answer, when it came, tore up physics. You've had a clue already. It was in episode ten, and it involved a guitar.

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The myth of the week: that astronomers know what stars are made of because they've seen it. That a big enough telescope shows you. It doesn't. No telescope has ever shown a star as much more than a dot. What's known was inferred, by spreading the dot's light out and reading the gaps.

Tomlin Hawthorne

And, related: those magnificent images of nebulae in pink and teal. They're real data. But the colours have usually been chosen, to show which elements are where. One colour for hydrogen, one for oxygen. They're maps as much as photographs.

Tomlin Hawthorne

The question to take away is the one we started with.

Tomlin Hawthorne

Comte reasoned well and was wrong within a generation. What do we confidently call unknowable now? What's inside a black hole. What came before the Big Bang. What it's like to be a bat. Whether other universes exist.

Tomlin Hawthorne

Are some of those really beyond reach? Or are we Comte, unable to imagine the cigar box? And is there any way to tell which, from the inside?

Tomlin Hawthorne

Your challenge. You're going to be Kirchhoff. Take that CD from last week. In a dark room, or out at night, look at the reflection of different lamps in it, tilting it until you see the colours spread.

Tomlin Hawthorne

Try an old-fashioned bulb with a filament, or a candle. Try a fluorescent tube or an energy-saving bulb. Try an orange street lamp. Try a neon sign if you can find one.

Tomlin Hawthorne

Some of those will give you a smooth, continuous rainbow. And some will give you separate bands of colour with dark gaps between. You'll be seeing the barcodes. The street lamp is sodium. The fluorescent tube is mercury.

Tomlin Hawthorne

Next time, the last episode of Part Four. We come down from the stars to the wall socket. How did Faraday's wobbling needle become the largest machine ever built? It's a story of patents, publicity and a very dirty fight, and it'll leave us with a small puzzle about a microwave oven that classical physics cannot solve.