# Seeing The Sun in Stereo

16 July 2006

## Interview with

Dr Chris Davis, Rutherford Appleton Laboratory

## Fig_1_STEREO_spacecraft_A.jpg

 STEREO spacecraft A

Chris S - Thanks for joining us tonight to talk all about the sun. It's something that we very much take for granted. I remember a mathematics probability textbook I had that said, 'There are some things that are certain, such as the sun rising tomorrow. Probability equals 1.' But what actually is the sun and is it always going to rise?

Chris D - Well it's our nearest star. The Sun is a huge ball of mainly hydrogen gas and it's burning this hydrogen by the process of nuclear fusion to form helium. As far as we can tell it's been burning for the last four and a half billion years and will continue to burn for another four and a half billion years, so that's as near to a probability of one as you can get I would think.

Chris S - The reason the Sun is losing weight comes down to Einstein and e=mc^2 doesn't it?

Chris D - Well it certainly is losing weight because it's sticking hydrogen atoms together to make helium atoms and that releases energy and you can convert energy to mass using the famous e=mc^2 relationship. But it's atmosphere is so hot that it's boiling. It's like looking at a pan of boiling water and the steam rising from the surface. That's because those particular particles have enough energy to leave the fluid in the saucepan, and it's the same with the surface of the Sun. If you look at it with special cameras in space you can see that it's just a writhing mass. It looks likes a pan of rice on the boil and this can send material into space in a continuous stream known as the solar wind.

Chris S - One statistic I've heard is that it takes 8 minutes for light from the Sun to reach the Earth. But then I was speaking to a professor of astrophysics who said that the light we're getting out of the Sun is already a million years old and it's been bombarding itself all around inside the Sun a million years before it escapes and makes its way in that 8 minutes to us.

Chris D - Well yes, the 8 minutes is in consequential. The light is generated in the centre of the Sun, as it is only here where the temperature is great enough and the pressure is great enough to actually produce the nuclear reactions that can fuse hydrogen into helium. The material is so dense there that the light that's given out is immediately absorbed by something else, emitted again and absorbed by something else. So it's an incredibly large arcade game where this thing is being ricocheted backwards and forwards inside the Sun. It does take about a million years for that one single photon of light to get from the centre out to the surface where the density of gas decreases to the extent where the material becomes transparent and it can shine out into space.

Chris S - Can we talk a little bit about what's going on at the centre of the Sun to power it? What is the process that's happening and how is it doing that?

Chris D - The Sun is an enormous ball of gas and it's being pulled together under its own gravity, which is then squishing the materials in the centre to incredible densities. This actually forces the particles in the soup so close together that they can actually fuse to form to a helium atom from two hydrogen atoms. The temperature is at about 16 million degrees at the centre of the Sun so you have to have this huge ball of gas pressing down on it to produce all the conditions necessary. When you take two atoms and join them together, you release energy and that's what gives the Sun the light and the heat that it emits.

Kat - So how do we actually measure things about the Sun like its size and its heat and the light that comes out of it?

Chris D - Well there have been lots of observations that have been going on for many hundreds of years because the Sun was obviously a religious object for people like the Aztecs and the Egyptians. So we've got a lot of information about the Sun and particularly solar eclipses and that tells us about the solar atmosphere for example. When the moon moves in front of the Sun it blocks out a large disc and allows us to see the feint solar wind which is the gas blowing out into space. So they made observations like that and were able to predict solar eclipses. But as we've moved towards the space age, we can actually start to look at the Sun in different wavelengths. The Sun is the colour it is because the surface of the Sun is at about 5800 degrees. The colour tells us the temperature. This is just like a blacksmith who knows when the metal is the right temperature because it is white hot or red hot. He uses the colour the hot metal is producing to tell him what temperature it is.

Kat - And how do we study other things about the Sun? What exactly are solar flares and sun spots and all these weird phenomena that are associated with the Sun?

Chris D - Well the Sun is like most stars and has a magnetic field like the Earth. That magnetic field is pretty stable and we know it flips up and down every hundred thousand years. But the Sun is a fluid and it has peculiar properties. The equator of the Sun rotates about every five days while the poles of the Sun rotate every thirty days.

Kat - Tell us a bit now about the new mission you're working on called STEREO. What's all this about?

Chris D - With the space missions we've been sending out of late, we've been getting a much more detailed view of the surface of the Sun. You can get active regions on the surface of the Sun where the Sun's magnetic field gets contorted and twisted and pops out through the surface of the Sun. It's these that are the root of solar flares and also are the root of things called coronal mass ejections. These are a very violent storms that come out into space and ascending dense clouds of this gas towards us. But the trouble is that we want to be able to predict these coronal mass ejections and the direction they're going to go in and how fast they're travelling. Although SOHO has been a very successful mission, it's only one view in space, and it's very difficult to deconstruct that convoluted 3D nature. So with the STEREO mission we're going to send two spacecraft out: one ahead of the Earth's orbit and one just behind the Earth just inside the Earth's orbit. After a couple of months they'll be far enough apart that they can look back at the Sun and view the Sun in three dimensions and try to work out this complicated twist of spaghetti loops that you can see coming out of the surface.