Rights: University of Waikato. All Rights Reserved. Published 1 April 2009 Download

Dr Melanie Johnston-Hollitt, from Victoria University of Wellington, describes how she uses different parts of the electromagnetic spectrum (X-rays and radio waves) to explore galaxy clusters.


When you observe the universe in different wavelengths you get to see different things. So, what I am talking about here is you are using different parts of the electro magnetic spectrum, so different types of light to try and understand how all the different pieces of the universe fit together.

And one of the things that we can do with galaxy clusters is we can look at them in the optical, which is the same light that you see with, so you see the individual galaxies.

But you can also look at them in X-ray wavelengths. So just like the X-rays that are used in hospitals, we can look out and see X-rays emitted from space and this shows us the hot gas that surrounds a cluster and all the galaxies that sit inside it are embedded in that. So, we use X-ray telescopes which sit in space to look at hot plasma, so hot gas of the order of between 1 and 10 million degrees. X-ray telescopes have to be space balls, so they are satellites that orbit the Earth and the reason for that is that X-rays don’t penetrate the Earth’s atmosphere. And we are very, very lucky that they don’t because if our atmosphere weren’t there and we were being bombarded by X-rays, we probably wouldn’t be here at all. So we have to put the telescopes in space. So they are very, very expensive, but actually incredibly useful for the detection of this hot X-ray emitting gas that we usually find in galaxy clusters.

A radio telescope is just another of the tools that astronomers can use to try and understand this jigsaw of the universe and, again, what we are doing with a radio telescope is we’re looking at a different part of the electro magnetic spectrum. In this case we are looking at a part with very long wavelengths so you are talking of things between the order of centimetres up to many metres. And, so, this is just the same type of thing, so we are looking at photons, but just a different part of the electro magnetic spectrum – just as an X-ray telescope, or an optical telescope, or an ultra violet telescope, infrared, and so have you. The reason that we can have radio telescopes on the surface of the Earth is that radio waves can penetrate the Earth’s atmosphere. They get through quite easily and this is because the wavelength of radio waves is such that the atmosphere is transparent to them. In fact I have observed with radio telescopes in the rain and it is not a problem, depending on the wavelength.

Sarah Brough
Antennas of the Australian Telescope Compact Array, J.Masterton, © CSIRO
NASA/MPIA/Calar Alto Observatory, Oliver Krause
NASA/STScI/AURA/Hubble Heritage