On the line – August 2008

Rob: Hello, I am Rob Edwards and I am speaking to Dr Claire Bretherton at the Royal Observatory Greenwich. With many of us going away at this time of year, I am going to be asking Claire some questions about what's visible in the night sky close to home and further afield. My first question, Claire, is: from central London I can see about half a dozen stars - how many stars will I be able to see if I go away from the city?

Claire: When you are in the city there is a lot of light pollution, so you can really only see the very brightest objects. But if you get further out, even just out into the suburbs, you can see a lot more. If you go somewhere really dark, you may see as many as 3,000 stars just with your own eyes.

If you are lucky when you go somewhere dark enough, you might even see a milky white band across the sky. This is our own galaxy, the Milky Way, made up of individual stars, although you probably will not be able to see the stars themselves. We live about two thirds of the way out from the centre of the Galaxy. In the summer skies, we are really lucky, because we are looking towards the center of the Milky Way, which means it is much brighter and a broader band across the sky.

Rob: How do I know where to look to find the Milky Way?

Claire: The Milky Way can be quite difficult to spot if you are not in a really dark place. There are three signs that can guide your eye. These make up a triangle shape that dominates the skies at this time of year, called the Summer Triangle. Our eyes and our brains are actually very good at picking up these geometric shapes, so it is very easy to spot. So if you find the Summer Triangle pattern of stars, then the Milky Way runs straight through the middle of it.

Rob: Can we see anything beyond our Galaxy in the night sky?

Claire: If we go somewhere dark, then we have got a good chance. Just on the outskirts of our Galaxy there are around 150 globular clusters, each containing hundreds of thousands of ancient stars, which were formed when the Universe was young. Just over towards the right hand side of the band of the Milky Way galaxy, and about two thirds of the way up the sky, you may just be able to pick out a faint and fuzzy patch, which is M13 - a globular cluster in the constellation of Hercules.

Also, it's a good time for spotting galaxies. A little bit later in the night you may be able to pick out the Andromeda galaxy, which is the nearest large, spiral galaxy to our own. It's around two and a half million light years away, which means that the light we are seeing now, actually left that galaxy two and a half million years ago. If you are lucky enough to be heading down to the southern hemisphere this year, then you may also see the two Magellanic Clouds: the Large and the Small Magellanic Cloud. These are what we call irregular dwarf galaxies, and they are two of the closest galaxies to our own Milky Way.

Rob: My final question is: if I'm not going somewhere remote or exotic, what else should I look out for in the night sky?

Claire: The one thing that you can see really easily, even from the city at the moment, is the planet Jupiter. You can see it roughly towards the south, around 10 o'clock, once it has got properly dark. It is so bright, you really can not mistake it for anything else. Jupiter is the largest planet in our Solar System - so big that you could fit the Earth inside it 1,300 times. We know it has at least 63 moons. The four largest moons: Io, Europa, Ganymede and Callisto, are actually also visible through binoculars or a small telescope.

Rob: Dr Claire Bretherton, thank you very much.