Reading starlight

Nobody has ever been to a star. Nobody has even sent a machine to one, apart from our own Sun. Yet we know what stars are made of.

The secret is in their light. If you pass sunlight through a prism it spreads into a band of colours. Look very closely at that band and you find it is crossed by hundreds of thin dark lines, like a barcode.

Each kind of atom makes its own set of lines. Hydrogen has one pattern. Iron has another. So the lines are like fingerprints. Find the pattern, and you know that atom is there.

In this way people found a new element in the Sun before anyone had found it on Earth. They named it helium, after the Greek word for the Sun.

In 1835 the philosopher Auguste Comte chose the make-up of the stars as his example of something people could never know. He was wrong within thirty years.

Where the lines come from. An atom can only take in or give out light of certain exact wavelengths, and each element has its own list. The hot, dense body of a star shines at every wavelength. On the way out, that light passes through the star’s thinner outer layers, and the atoms there remove their own particular wavelengths. What reaches us is a full band of colour with narrow gaps in it.

Joseph von Fraunhofer mapped hundreds of these dark lines in sunlight in 1814 without knowing what they were. In 1859 Gustav Kirchhoff and Robert Bunsen showed in the laboratory that each element, heated, gives its own set of lines, and that the Sun’s dark lines matched them. From then on a telescope and a prism were a chemistry kit for the sky.

What the lines say.

From the lines’	You learn
Positions, as a pattern	Which elements are there
Relative strengths	The temperature, and how much of each element
Shift, all together, towards red or blue	Whether the star is moving away or towards us, and how fast
Width	How fast the star spins, and the pressure of its gas

Two discoveries. In 1868 astronomers saw a line in the Sun’s light that matched no known element. They concluded it was a new one, and called it helium. It was not found on Earth until 1895.

In 1925 Cecilia Payne worked out from the strengths of the lines that stars are made overwhelmingly of hydrogen and helium, with everything else a small trace. This was so unexpected that she was persuaded to describe her own result as probably not real. It was right. By mass the Sun is about 73 per cent hydrogen and 25 per cent helium.

Motion. When a source of waves moves towards you the waves are squeezed, and when it moves away they are stretched. You hear this as a siren drops in pitch on passing. With light, the whole pattern of lines shifts towards the blue end for something approaching and towards the red end for something receding. The size of the shift gives the speed.

This is how the first planet round a Sun-like star was found, in 1995. Michel Mayor and Didier Queloz saw the lines of the star 51 Pegasi slide to and fro every 4.2 days, as an unseen planet tugged it back and forth at about 50 m a second. More than a thousand planets have been found the same way since. The method is delicate: the Earth moves the Sun by only 9 cm a second.

On the largest scale, the light of nearly every galaxy is shifted towards the red, and more so the further away the galaxy is. That discovery, in the 1920s, was the first sign that the universe is expanding.

Reading the air of other worlds. When a planet passes in front of its star, a little of the starlight shines through the planet’s atmosphere on the way to us, and the gases there leave their own faint lines in it. In 2022 the Webb telescope used this to make the first clear detection of carbon dioxide in the air of a planet round another star.