Hi, I’m Marley, the Astronomer here at the Space Centre

In astronomy, nearly all of our information comes from light. ‘Light’ refers to any wavelength of light in the electromagnetic spectrum. From radio waves to gamma rays, astronomers are able to learn so much about astronomical objects based on the type of light observed from them. In this blog, I want to talk about one way astronomers use light to learn about an object: redshift.

Light behaves like a wave, and thanks to the Doppler Effect we know that when an object is moving, waves can either be squished together, or stretched. The Doppler Effect describes what happens to soundwaves. When an ambulance drives past you, the siren has a high pitch as it is coming towards you, and a lower pitch as it speeds away. When the ambulance is coming towards you, the soundwaves coming from the siren are compressed together. As it leaves, the soundwaves are spread apart. The same principle applies to light. Astronomers call it redshift when the object is moving away, and blueshift when it is moving towards us.

To detect redshift, astronomers use spectroscopy. This is the study of the absorption and emission spectra of light. When a material is very hot, it will emit light in a continuous spectrum. As an example, think of a rainbow. If that light passes through a gas cloud, atoms in that cloud will absorb specific wavelengths of light. The resulting rainbow we would see would have a gap of colour missing. This is an absorption line spectrum. Astronomers can also see emission line spectra, where the entire rainbow is black, and we only see lines of colours at specific locations. When an object is redshifted, those lines appear at longer wavelengths. Astronomers know where these lines appear when an object is stationary. To determine the redshift (z) of an object, astronomers look at the difference between the wavelength they see the spectral lines at, and where they should be. Objects with a larger z value are further away from us.