Spectroscopy

How we learn about stars, nebulae and planets by splitting and analysing their light

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Modern telescopes are capable of providing much more than mere images of distant objects. One of the biggest tools that astronomers use to study the Universe is spectroscopy, the science of splitting light (visible or otherwise) into its component wavelengths, known as a spectrum. Different materials reflect, absorb and emit light at characteristic wavelengths. This means that the object that emitted the light, plus any gas the light travels through and any surface it reflects off, all imprint a signature on the spectrum. We can study this signature to understand the properties of planets, stars, galaxies and nebulae.

Instruments that split light like this are called spectroscopes or spectrographs, with the resulting spectrum typically plotted as the light's intensity against wavelength. These instruments can make use of a prism, whereby light entering the prism is bent (or refracted), with different wavelengths being bent by different amounts. Many may recall seeing this demonstrated in simple school science experiments, where white light is directed towards a prism. The light enters, is split and exits as a rainbow of colours, with shorter wavelengths towards the blue end and longer towards wavelengths the red. The natural rainbows we see in the sky are also created by the same process, with water droplets in the air acting as prisms to refract and split sunlight. As well as prisms, spectrographs can incorporate gratings, which use a process known as diffraction to split the light.

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