Have you ever wondered how astronomers know what objects in space are made of? Since it’s inconvenient to impossible to travel to most of the universe, how do we examine stars and planets to find out whether there’s hydrogen or carbon dioxide or kryptonite (not a real element) there? If you’ve ever seen a rainbow you’ve seen a simple visual example of the answer and the basics of spectroscopy in action. Of course, astronomers like to take it much further, always asking how, why, but how though? Relax, I’m going to tell you.
It’s fair to say that Spectroscopy (the science) and Spectrometry (the method of measuring interactions between light and matter), were the key to understanding what the universe is made of. In our last article we discussed how most of the elements in the universe were created by stars, including much of the makeup of our own bodies. Thanks to spectroscopy, we now know that those same elements and molecules exist on other worlds and in other galaxies. A great example of this science in action is the discovery of helium in the chromosphere of the sun, before it was discovered on earth. Its discovers didn’t know it was helium at the time. They just saw its spectral signature.
Every atom or molecule behaves differently in how it emits, reflects or absorbs light of various wavelengths across the electromagnetic spectrum. When we see a ‘normal’ rainbow we are seeing the way water droplets reflect sunlight from the rear of each droplet (from the observers perspective), and then refract it into the spectrum of visible light that your eyes and brain perceive as colour. The various elements absorb light differently too.
When a continuous light source is passed through a cloud of gas and analysed by a spectrograph, it shows distinctive dark lines within the spectrum known as absorption lines. When the light of distant stars is analysed by a spectrograph, we see many of the same bright emission lines and dark absorption lines at the same positions in the spectrum as we do in our own sun or when passed through gases on earth. In the meantime, since there’s been plenty of rain around, you might see a rainbow. When you do, you might impress your peers by exclaiming “Look, a localised, personalised visible demonstration of the refraction of sunlight into its constituent wavelengths and validation of the science of spectroscopy in the sky”.
