What stardust is made of...

Have you ever wondered what the Sun is made of? Well, Graihagh Jackson has and so she asked Andrew Norton to talk her through it...

Graihagh - That night, it was a beautifully clear and warm evening. I was in my back garden, gazing up at the night sky and all the tiny twinkles. If I'm lucky (or unlucky depending on how you look at it) I might be able to count up to 3,000 stars...

But this is just the tip of a vast cosmic iceberg. There are 100 billion stars in the milky way... and then there are a further 100 billion galaxies.

AHH - shooting star!! That's supposed to be good luck right?

I was always told that it mean a new baby was falling to Earth (presumably from heaven?) ready to be born. But I have heard that shooting stars representing souls that have been released from purgatory and are not off on their way to heaven. Of course, they're not stars or lost souls at all, just meteors burning up as they hit the Earth's atmosphere.

It wasn't meteor that got me all starry eyed though but the idea that we're all made up of stardust.

Andrew - Absolutely, all those elements other than hydrogen, of which we're made, have indeed been formed in the cause of earlier generations of stars thrown out into space and then incorporated into the solar system and the planet earth when it formed and we are a result of that.

Graihagh - That is, of course,  Andrew Norton...

Andrew - Professor of Astrophysics education at the Open University.  I always say I got interested in astronomy because when I was a child, on TV we these great programmes called Dr Who and Star Trek. Now I know they're still on TV now...

Graihagh - I know Star Trek! I watched Captain Janeway is what my childhood is filled with.

Andrew - So that was it, that's what got me interested in astronomy and, bizarrely, I've managed to make a living out of it every since so it suits me fine.

Graihagh - Let's boldly go to where man has been before, on this occasion Paris in 1935 where a tutor at *ahem* L'Ecole Polytechnique by the name of Auguste Comte wrote:

"there is no conceivable means by which we shall one day determine the chemical composition of the stars"

But shortly after his death in 1957, two german physicists - one Robert Bunsen, who you may have heard of, and a Gustav Kirchhoff - proved Comte wrong.

Andrew - We don't need to go to the sun to find out what it's made of. We can look at the light which the sun emits and the key to it is that different elements emit particular patterns of light, a particular spectrum of light a particular set of wavelengths of frequencies if you like, so hydrogen gas emits light at particular frequencies that we can recognise - particular colours. So when we look at the spectrum of light from the sun or, indeed, any other star, we can see that recognisable pattern of so-called "spectral lines," that really are the fingerprint of a particular element. In fact, the gas helium was not discovered on Earth. It was discovered on the Sun. That's where the name helium comes from, the Greek word "helios," I guess, is the word for the sun.

Graihagh - Really, that's awesome!

Andrew - So back in the 19th century, I guess it was, people looked at the spectrum of the sun and saw these particular lines (spectral lines) that they didn't recognise as corresponding to any element known on Earth and that element was called helium. That's where it was first discovered in the spectrum of light from the sun and now, indeed, in other stars.

Graihagh - Now you've got me thinking back to my chemistry lessons at school and I seem to remember things like copper burns blue and sodium burns yellow. I can't remember any others but is it a bit like that?

Andrew - It is essentially that. There's a dominant colour, if you like, from particular elements but it's the detailed pattern of light that really is the key so you mentioned sodium. If you look at the spectrum of light from sodium, when sodium is in a gassier state, we see strong lines in the yellow part of the spectrum. Neon has strong lines in the red part of the spectrum - we see that in neon signs. But every element, right across the periodic table of the elements, will have it's own particular combination of spectral line.

There is another way actually that we can tell what's in the Sun and that's not by us going to the sun, it's because the sun emits particles in our direction, the so called "solar wind." And, in principle, we can detect those particles of the solar wind out in orbit around the Earth and we could in that way measure the particles directly from the Sun as well. So that's a nice way to confirm, if you like, what we're seeing in the spectra is physically there in terms of the atoms blowing off the surface of the sun.

Graihagh - Umm that's really cool. I'm just thinking a bit like literally stardust - someone can actually hold it.

Andrew - Absolutely. That's what it is - it is stardust, yes.

Graihagh - So you would attach this spectrometer to a telescope and voila but, sadly, I don't really have one of them. So can you tell me what is our Sun made of?

Andrew - Just like other stars it's pretty much three quarters hydrogen, one quarter helium, and a tiny bit of everything else.

Graihagh - Most stars are incredibly far away and we don't know a huge amount about them but what we do know, we know because of our star, the Sun - the creator of our planet and to which we owe our existence.

Andrew - I always find it amazing that deep down in the core of the sun we can figure out the temperature is 15 million degrees, so that's pretty amazing I think. Another fact about the sun: you could fit a million planet Earths inside the sun. It's about a million times the volume of the planet Earth. And what about a third fact... Umm... Well I suppose a third fact would be that the Sun is about halfway through it's life. It's about 5 billion years old and it's got about another 5 billion years to go.