How Light's 'Secret Code' Reveals the Story of the Cosmos- The starlight we can see tells us a lot about the Universe, but it's the parts we can't see that contain the biggest revelations

When you see photos from a modern telescope of a planet, nebula or distant galaxy, it's easy to be dazzled by the detail. But what astronomers get really excited about is a property of the light our eyes can't see: the spectrum. This secret code embedded in starlight can tell us not only what a celestial object is made of, but also the story of our cosmic past.Back in the 18th century, scientists discovered that each substance produced its own pattern of colours when it was burning. It turns out that each element, when heated, emits light at certain colours specific to that substance.

- By Dr Katie Mack - illustration by Olena Shmahalo

When you see photos from a modern telescope of a planet, nebula or distant galaxy, it's easy to be dazzled by the detail. But what astronomers get really excited about is a property of the light our eyes can't see: the spectrum. This secret code embedded in starlight can tell us not only what a celestial object is made of, but also the story of our cosmic past.

Back in the 18th century, scientists discovered that each substance produced its own pattern of colours when it was burning. It turns out that each element, when heated, emits light at certain colours specific to that substance.

These 'emission lines' show up as bright bands of colour when the light is spread out in a prism or diffraction grating, creating a telltale pattern that can be used to identify the substance. If you'd rather not set your sample on fire, you can also identify it by shining a white light through a gas of the stuff: you'll see the same pattern of lines, but this time in the form of dark gaps in the spectrum, known as 'absorption lines'.

In both cases, the lines are caused by electrons shifting between energy levels. Every element has a specific arrangement of levels its electrons can be in, which you can picture (somewhat inaccurately) as concentric orbits around the nucleus, with lower energies closer in and higher energies farther out.