Date: February 24, 2011
Title: A Journey to the Centre of the Sun
Podcaster: L. Cate Kendal
Link: http://www.legofusker.net/astronomy.htm
Description: This podcast will take you on a short journey down through the different layers of the Sun, and give a brief outline of how the four forces of nature are at work in the Sun to produce its energy.
Bio: L Cate Kendal is a science writer, theoretical physicist and amateur astronomer living in Scotland. She has written for the magazine Astronomy Now, and does science outreach volunteer work with the Royal Observatory in Edinburgh.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Greg Dorais, and is dedicated to the Chabot Space And Science Center in Oakland California, home of Bill Nye’s Climate Lab, Space Explorers Summer Camp, and so much more. At Chabot Space And Science Center, the universe is yours to experience. Set amid 13 trail-laced acres of East Bay parkland, with glorious views of San Francisco Bay and the Oakland foothills, Chabot is a hands-on celebration of sights, sounds, and sensations. Find out more about the Chabot Space And Science Center at www.chabotspace.org.
Transcript:
Hello, I’m L Cate Kendal and welcome to this podcast. Today we are going on a short journey to the centre of the Sun. In less than ten minutes!
We’ll skip over the large, but not empty, gap between us and the Sun, then descend through the layers of the Sun’s atmosphere. From there, we’ll continue deeper through to the very central region where nuclear fusion occurs. On our way there we will see how the four fundamental forces of nature all work to get the sun shining.
So lets go!
On earth we are used to the sun being the source of heat and light, but we still feel its influence in two other ways that we are perhaps less aware of. There is the solar wind, the tenuous stream of very energetic particles that pour off the surface of the Sun, and are sometimes blown off in great gusts by solar flares, like the one that was detected earlier this February.
And of course, Earth is also influenced by the Sun’s gravity that keeps us in our comfortable orbit. And that is the first and most familiar of the forces we encounter.
Gravity may be very weak, but it has an insidious quality always trying to pull matter together. The more material comes together, the greater the density and the larger the pull of gravity.
The Sun is mostly made of hydrogen, with some helium, as a much smaller amount of other elements. Most of this is a highly ionised gas called plasma. All the particles in the Sun are in constant motion, colliding with each other, and with photons, the denser the gas, the more collisions occur. But although the Sun is made of gas, it still weighs well over three hundred thousand times the mass of the Earth. And that’s a lot of mass, and a lot of gravity.
But detail is there, and at about 3 million kilometres from the centre, we meet the top-most part of the sun, called the corona. This is the part that makes a solar eclipse such a beautiful sight, when the light from the main body of the Sun is cut off, and the pearly white glow of the corona is all we can see. This is the very outer section of the Sun’s atmosphere. It’s called an atmosphere mainly because it is less dense than the gas below it, but there’s no sharp distinction between levels as on Earth, there is no solid surface on the Sun.
The atmosphere has two more layers, and the one just underneath the corona is the chromosphere. This is a thin region, that’s also hot, but produces a lot of emission and absorption lines from the plasma that can tell us what ions and atoms are there. Helium, for example, was first identified from its emission lines in the chromosphere. Like the corona, this region is also difficult to see, but in some eclipse pictures, it can be seen as a narrow pinkish-red band. But most of the time it’s overwhelmed by the emissions from the region below.
This is the photosphere, and it the observable “surface” of the sun at about one and a half million kilometres from the centre, and where almost all the sun’s electromagnetic radiation comes from. It’s complex, with a lot of structure, with patterns of cell-like granules over the surface, as well as sunspots. NASA’s STEREO spacecraft has just taken a 360-degree picture of the photosphere.
We can’t “see” deeper than the photosphere because light can’t pass through the material, and that in itself tells us something about the behaviour of the plasma there. Although we can’t see what’s beneath the photosphere, we use models to understand what is happening.
Below the photosphere is the convection zone, and this region is mostly powered by gravity. Here, most of the energy in the sun is transported outwards by the physical movement of the plasma. The hot, less dense material will rise and the cooler, denser material will sink, setting up convection cells that are constantly in motion. Gravity not only keeps this motion going, but is still acting to squash the plasma together, and as well as the temperature rising as we go deeper, the density does as well. As we get further inside the sun, convection becomes less good at transferring energy, and the Sun must find another way. We have reached the radiative zone.
The sun’s big problem is getting its energy out, as any energy produced in the centre can’t just travel straight out. The plasma is too dense. And unlike the zone above it, it’s thought that the radiative zone isn’t in convective motion and energy isn’t transferred by the gravitational force, but by the electromagnetic force. Here the photons undergo countless electromagnetic interactions with the surrounding plasma, like scattering, absorption and emission. Of course, these processes occur in other parts of the sun too, but they’re the dominant process in the radiative zone.
Finally, at the base of the radiative zone, about half a million or so kilometres from the centre is the start of the Sun’s core, where nuclear fusion occurs. One of the give away signs that we had made it to the core would be a sharp increase in the amount of helium, and the decrease of hydrogen.
At the centre of the sun the collision between hydrogen nuclei, or protons, sometimes have enough energy to get them to overcome the electromagnetic repulsive force of their positive charges, and stick together, under the strong force. The strong force is just that, very, very strong compared to the others, but it only acts for an extremely short distance, so the hydrogen nuclei have to get very close to each other to feel it’s effect. This can only happen in the very hot and very dense core.
But the two protons can’t stay like that; it’ not stable so either they fly apart, or one of them must undergo a change and become a neutron. This reaction is governed by the weak force, which changes a proton into a neutron, a positron (a positive electron) and a neutrino. The weak force, as it’s name suggests is weak compared to the strong force, but by weak force interaction, particles can be transformed into different ones, and that is what saves the situation here. A proton and neutron can stay quite happily bound together in a substance called deuterium. This is the very first step in the process of making helium out of hydrogen.
As you can imagine, the processes in nuclear fusion are complicated, and I’m not going to speak about them in detail. However, at each step, energy is released, either in the form of photons or as the kinetic energy of the particles that are made. It’s this energy that powers the Sun.
So here we are at our goal, the core of the Sun, the hottest and most dense place in the solar system, the place where all our energy ultimately comes from. We’ve encountered the four forces of nature and seen how they all act to get the Sun shining, either directly in nuclear fusion, the strong and the weak forces, or by helping get the energy out the electromagnetic and gravitational forces.
And here I think is a good place to end this podcast. I hope you have enjoyed the journey. Thank you for listening.
End of podcast:
365 Days of Astronomy
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