When you see photos from a modern telescope of a planet, nebula, or distant galaxy, it’s easy to be dazzled by the detailed and intricately beautiful images. But those pictures tell only a small part of the story.
What astronomers usually get the most excited about is a property of the light we can’t perceive with our eyes at all: the spectrum. This secret code embedded in starlight can tell us not only what a celestial object is made of, but can also, for the most distant objects in the Universe, tell us the story of our own cosmic past.
As far back as the 18th century, scientists experimenting with lighting chemicals on fire discovered that each substance produced its own pattern of colours when it was burning. It turns out that each element or molecule, when heated, can emit light at certain colours specific to that species.
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 electron 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.
When the atom is disturbed, electrons can move between the levels. If you heat a substance (or light it on fire), that kicks electrons up to higher energy levels. When it cools, the electrons fall back down and emit photons (particles of light) at specific energies, which correspond to specific colours.
When you shine light through a gas, the atom can absorb photons at those special colours to boost electrons up. A white light that’s passed through arrives on the other side with dark gaps where the photons were stolen by the gas.
When astronomers in the early 1900s applied this principle to astronomical observation, they were suddenly able, just by spreading starlight through a prism, to say exactly what the stars were made of. Each element has a specific pattern of absorption or emission lines, like a unique bar code; identifying those patterns can tell us exactly what elements that light has been passing through.
Now, telescopes around the world, and some in space, use spectroscopy to figure out which elements make up the Universe around us. But these spectra can tell us even more than that, in an indirect but astonishing way.
It has to do with the fact that even though light can be said to be made of particles and photons, it also acts like a wave. And just like with sound, when a light wave is moving quickly away from you, it gets stretched out, going to a lower frequency, or longer wavelength.
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With sound, this is why a siren drops in pitch when it rushes past you. We can measure how quickly stars are moving toward or away from us by the way their spectral lines shift along the spectrum, to longer or shorter wavelengths, since the whole barcode shifts together.
When something is moving away from us, the shift is to longer wavelengths – toward red if it’s visible light – so that’s known as redshift. If it’s moving toward us, it’s blueshifted. Just seeing a blue or red tint isn’t a conclusive sign of motion, though, since lots of things can affect the colour of a star. It’s the shifted barcode that really gives the game away.
When it comes to very distant galaxies, though? They’re all redshifted.
In this case, it’s not that they’re rushing away from us, exactly, though it looks like that from here. It’s really the fact that the Universe is stretching out – expanding – in between here and there. And as the universe stretches, it stretches out the light, shifting it to redder wavelengths.
This is why the James Webb Space Telescope is an infrared telescope. The galaxies it’s trying to see are redshifted so extremely that they fall out of the visible part of the spectrum entirely, and can only be seen in the infrared.
Measuring the redshifts of distant galaxies tells us how long the light has been travelling and (indirectly) how far away the galaxy is. But it also gives us an invaluable tool to measure the expansion of the Universe at different points in cosmic history.
Astronomers have a pretty tough job, trying to understand and explain objects we’ll never touch or sample, based only on pinpricks of light in the sky. But thanks to the quirks of electron behaviour, and, ultimately, the rules of quantum mechanics), we can gather an astounding amount of information from each light ray that crosses the cosmos to reach us.
Sometimes that data helps us work out the processes of stellar evolution or search for exoplanet atmospheres for traces of life. At other times, those light rays carry with them the story of the cosmos itself.
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