Science Puzzle
How Do We Know What Stars Are Made Of?
In 1835 the French philosopher Auguste Comte declared that some things could never be known. Among his examples: we would never discover what the stars are made of, because we could never go there to collect a sample. We still haven't been to a star. Yet today astronomers can list the elements in the Sun and in stars thousands of light years away.
How can we tell what a star is made of without ever touching it?
The Answer
By splitting their light into a rainbow, called a spectrum, and reading the pattern of lines in it. Every element leaves its own fingerprint of lines.
Spread sunlight out with a prism and you get a rainbow, but look closely and it's crossed by hundreds of thin dark lines. The German optician Joseph von Fraunhofer mapped them carefully in 1814 without knowing what they were. In 1859 the German scientists Gustav Kirchhoff and Robert Bunsen cracked it. They heated elements in a flame and found that each one glows only at a few exact colours, giving bright lines in fixed places. Sodium, for example, glows a particular yellow. And when cooler gas sits in front of a hot light source, it does the opposite: it absorbs exactly those same colours, leaving dark lines.
So a dark line in sunlight at sodium's exact yellow means there's sodium in the Sun's outer layers, absorbing that colour. Match all the lines and you have the Sun's ingredients. In 1868 astronomers found lines in sunlight that matched no known element. They named the new element helium, after Helios, the Greek god of the Sun. It wasn't found on Earth until 1895.
Comte died in 1857, two years before Kirchhoff and Bunsen proved him wrong. The same technique now tells astronomers how hot stars are, how fast they're moving, and even what the air is made of on planets circling other stars.
The principle: Spectroscopy. Each element absorbs and gives out light only at its own particular wavelengths, so the pattern of lines in a star's spectrum reveals which elements it contains.