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OU Lecture 2007: The dirty window

Updated Tuesday, 26th June 2007

Astronomers have great difficulty looking at the stars. John Zarnecki explains why.

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Let’s just digress for a minute and ask ourselves, as astronomers, as planetary scientists, why is it we want to go up there? If we take a look at the sky – this is a picture taken by my colleague Phil Rosenberg from his back garden in Wolverton here in Milton Keynes – you can see the stars. There’s the Plough, the famous shape there. If you go somewhere a bit darker – up in the Lake District for example – and look at the same bit of the sky you get a slightly different view. You can see the Plough there but hundreds of stars now in the background. But they are twinkling. They are distorted by the atmosphere. For an astronomer it's like looking through a dirty window. Now this is another picture; this is the Sombrero Galaxy. This is taken by Steve Otter and Ollie Butters, two of our PhD students. They were working on the Open University course, "Observing the Universe" in the observatory in Majorca – a better view there – very good observing conditions. And you are beginning to see some structure in the Galaxy.

But if, instead of going a few hundred miles south to a nice site, you actually go a few hundred miles up above the atmosphere, this is what happens. This is the same object taken from a telescope on a satellite. You are seeing much more detail there. You are beginning to see the structure in the Galaxy and that lane of dust in the plane of the Galaxy. Now, I have been saying “see the Galaxy” but when we say “see” we don’t always mean “see” – as we do with the eye – in visible light, because visible light is only a tiny part of the entire gamut of radiation that exists. There is radio, radiation, X-rays, infra red, ultra violet – many, many types of radiation – and these are conveying lots of information about objects in our Universe.

Now most of this radiation is absorbed by our atmosphere, which is great for us as human beings because a lot of this radiation of course is very harmful. But it's not so great if you are an astronomer. You want to see this radiation because it's telling you so much. So lets have a look at this Sombrero Galaxy in other radiation, in other light. This is an image in infra-red light, taken again from above the atmosphere. It's beginning to look a little bit different and that dust lane as we call it around the Galaxy is now glowing rather brightly. That’s the region in red because dust emits lots of infra red radiation. Now here it is again, seen in X-rays. And now it's looking very, very different. That’s because normal stars and dust – they just don’t emit in X-rays. We are now seeing completely different objects and processes – in fact some of the most exotic and violent events in the Universe. We are seeing massive stars and we are seeing material being sucked into black holes, being heated to incredible temperatures. So by being able to see different radiation we see a whole range of different processes and different types of objects. Now remember, these images, these last few images, would be inaccessible from the Earth. So we need to go above the atmosphere. How? Well, we put our telescopes on orbiting satellites and get rid of the effect of the atmosphere.


With thanks to:

  • Steve Otter and Olly Butters
  • T. Credner & S. Kohle
  • NASA/ESA/JPL-Caltech

The Open University Lecture 2007

This is part 2 of 10

Next: My first five minutes in space





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