Image source:   solarviews.com/raw/vss/VSS00031.jpg

 

As we saw on the previous page, the sun puts out a tremendous amount of energy. We will study the internal structure of the sun by considering how the energy is created and transported.

@ 2014 Pearson Education, Inc.

 

By definition, the core is the region of the sun where fusion takes place. The core is the only place where it is hot enough for fusion to happen. As the temperature increases, the particles move faster and faster. Protons are positively charged particles, and repel each other via the electromagnetic force. As they are flying around, when they get close to each other, they veer apart, due to this repulsion. As the temperature gets higher, and they move faster, they get closer together before they veer apart. Deeper and deeper within the star, as it gets hotter, eventually the protons get close enough that the strong nuclear force dominates the electromagnetic force and they stick together. We call this fusion.

© 2014 Pearson Education, Inc. When the two protons stick together, one of them is converted into a neutron. Actually what is going on here is that protons are made up of elementary particles called quarks. Quarks typically come in sets of three. Protons have two up quarks and one down quark. Neutrons are made of two down quarks and an up quark. When a proton is converted into a neutron, one of the up quarks is converted into a down quark. The product of this fusion is called a deuteron, a proton bound to a neutron. Also, when the fusion event happens, a neutrino and a positron are ejected, as well as a photon. We can write this as a sort of equation: P + P = D + n + e+ + g Here, the Greek symbol n (nu) stands for the neutrino, the g (gamma) stands for the photon, and the e+ stands for the positron. A positron is just like an electron, except that it is made of antimatter instead of normal matter. In collisions like this, physical quantities like mass, energy, charge, spin, matter/antimatter all must be conserved. Since the neutron is neutral (has no charge) a particle with a positive charge, the positron, must be emitted. Since the positron is an antimatter particle, a matter particle, the neutrino, must be emitted. The conservation of energy means there is also something else very important happening here. If you add up the masses of the deuteron, positon and neutrino, the total mass is less than the mass of the original two protons. This matter was converted to pure light energy. The amount of light energy can be found by relating mass to energy using Einstein's famous equation: E = mc2 The c in this equation is the speed of light, which is a very big number. c2 is a huge number. This means that transforming a small amount of matter creates a large amount of energy, as indicated in the animation below. In fact, to produce the energy we calculate coming from the sun, fusion in the core fuses 600 billion tons of hydrogen into helium every second.

Video courtesy of NASA: http://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=11084  simulation showing the creation of light via proton-proton fusion.

@ 2014 Pearson Education, Inc.

 

The fusion is the core of the sun doesn't stop with the deuteron. The process continues until a Helium nucleus is created. The process, graphically shown above, is called the proton-proton chain. As you can see, two deuterons each undergo fusion to another proton, forming helium-3 nuclei. The helium-3 nuclei fuse together to create a helium-4 nucleus, kicking out two protons. You can also see that each positron emitted in the formation of a deuteron quickly finds its matter counterparts, an electron, and both are annihilated in a flash of light. More correctly, their combined mass creates a high energy gamma-ray photon.

 

Since there is a spread of energies for the incident protons, the light created can have any wavelength. This is why the light created in the core of the sun has a continuous spectrum.

@ 2014 Pearson Education, Inc.

 

As the photons make their way out of the core, they encounter matter particles and are absorbed and re-emitted. Since it is so hot, nuclei are ionized and relatively few collisions take place between free protons or electrons. In other words, the deep interior of the sun is transparent to photons. The photons can move about a centimeter before being absorbed and randomly emitted. This kind of motion is called a random walk. Scientists calculate that it takes from a few hundred thousand to a few million years for a photon created in the core of the sun to reach the sun's surface, much of this time spent in the random walk through the radiation zone. This region of the sun's interior is called the radiation zone because the energy is mainly transported via radiation, or the movement of photons.

 

As the photons make their way outward in the star, the plasma is also getting relatively cooler. More and more, photons encounter atoms that have bound nuclei, which absorb the photons. The outer limit of the radiation zone is defined where the matter has become opaque to photons, and the energy is largely transported via convection.

Image source

 

Convection takes place just outside the radiation zone. Convection is the transport of energy via the mass motion of the fluid. Think of it as many particles migrating together. Convection on earth is responsible for much of our weather. Simply put, hot air rises and cool air sinks. When air gets hotter, the molecules are moving faster. This makes the the pressure higher, which means that the air can push on the surrounding air, becoming less dense and more buoyant. Hot, buoyant air is lighter, so it rises. Conversely, cool air sinks.

 

The same process takes place in the convection zone in the sun, except that the fluid in question here is hot hydrogen gas. Giant convection cells of gas rise to the surface, cooler gas sinks.

Video source: www.astro.uio.no/~matsc/shp/results/fly.html

3D magnetohydrodynamical simulation (work together with Dr. Bob Stein, Michigan State University, USA).

 

The granulation we see on the surface of the sun is evidence of the convection that happens beneath. This simulation shows how the granules evolve over time as the hot fluid wells up to the surface, cools off, and sinks again. This simulation shows what would happen to the granules over about half an hour. The bright areas are the hotter areas, and the darker areas are cooler. Here, we see what it would look like, first looking straight down and then looking across the surface of the sun.

The above images are not simulations, but actual time lapse video of the solar surface taken by the SOHO satellite. The videos have been taken using different wavelengths of light, such that the video on the right shows a view that is deeper into the sun's surface. You can see the churning up of the hot plasma changing over time as the plasma cools and sinks.

Credit & Copyright: G. Scharmer (ISP, RSAS) et al., Lockheed-Martin Solar & Astrophysics Lab

 

This photograph of the sun's surface, taken edge-on, shows the 3-D nature of the granules. The granular features here are about 1000 kilometers across in size.

 

The surface of the sun is also called the "photosphere." This is because it is the layer of the sun where the light becomes free to travel away. The temperature has lowered to the point where most of the electrons are bound to nuclei, so that only photons with very specific energies can be absorbed. This happens over a relatively thin layer of the sun, only a few hundred miles. This is why the sun seems to have a sharply defined edge.

 

Recall that absorption lines in the spectrum are caused when light with a continuous spectrum passes through a cooler gas. The dark lines in the spectrum correspond to excited states of the elements present. Now, you probably would not think of the surface of the sun as a cool gas, but it is a lot cooler than the material underneath it. The spectral lines are created as the light passes through the atmosphere of the sun, beginning with the photosphere. As we saw in the chapter on spectroscopy, there are very many absorption lines in the solar spectrum, giving us a rich source of information about the chemical makeup of the sun as well as many other properties.

These large dark regions on the sun are called sunspots. They appear dark because they are cooler than the surrounding material. The dark sunspot is about 4500 - 5500 K, while the surrounding material is about 5800 K, so even though the sunspot is cooler than the surrounding region of the photosphere, it is by no means cool compared to temperatures we encounter in daily life.

@ 2014 Pearson Education, Inc.

 

Sunspots always come in pairs. They show the locations where the magnetic field of the sun breaks through the photosphere. The sun has a very strong magnetic field. Since the plasma is very electrically conducting, the magnetic field lines are  "frozen" into the field.

http://sohowww.nascom.nasa.gov/gallery/Movies/animations.html

 

When the hot, buoyant plasma floats upward toward the surface, the field lines are dragged with it. Giant "bubbles" of plasma break the surface and rise above it. This creates great looping structures  on the sun's surface. This animation from NASA depicts what happens as the magnetic field lines break through the surface of the sun. We cannot see the field lines, but what we can see are charged particles like protons and electrons traveling along the field lines, emitting light.

Energy transport in the sun

  • Fusion in the core creates photons
  • Core is ionized – electrons are not bound to nuclei
  • Radiation zone
      • Ionized plasma – no bound electrons
      • Photons have a random walk motion
  • Convection zone
      • Temperature decreases
      • Electrons become bound to nuclei
      • Atoms absorb photons but do not release energy easily  since it is “cool” and dense
      • Energy transported via motion of atoms
  • Photosphere
      • Density decreases
      • Material is transparent to photons

@ 2014 Pearson Education, Inc.

 

Since the sun is not solid, some parts can rotate at different speeds than others. We call this "differential" rotation. The sun rotates faster on the equator than it does at the poles, opposite from Keplarian rotation. Jupiter and Saturn show similar behavior, while Uranus and Neptune rotate faster at the poles than at the equator.

The differential rotation of the sun pulls the magnetic field lines around with the fluid, so that the orientation of the field is opposite for the northern hemisphere, as compared to the southern hemisphere.

Video source: svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=12144

 

This video shows the entire sixth year of images from the Solar Dynamic Observatory, from Jan. 1, 2015, to Jan. 28, 2016, as one time-lapse video. If you watch carefully, you can see the differential rotation of the sun.

http://solarscience.msfc.nasa.gov/SunspotCycle.shtml

 

The above graphs show the sunspot activity as a function of time and location. The bottom graph shows the number of sunspots counted on the sun. You can see that the sun is very active at times and very quiet at other times. There seems to be a connection with sunspots and weather on Earth, but the correlation is not well understood. For example, it was reported that there were practically no sunspots present during the Maunder Minimum, from about 1645 and 1715. This time period also saw a rapid advancement of glaciers, and was called "the little ice age" in Europe.

 

The upper graph shows the location of sunspots on the sun's surface as a function of time, also known as the "butterfly pattern." You can see that there is a correlation between the two graphs, and that the sunspots move with respect to their latitude line.

 

One more important thing to note about the solar magnetic field is that though the amplitudes and frequency of appearance of sunspots appears to follow an 11-year cycle, the actual cycle is 22 years, and that every 22 years the magnetic poles of the sun switch. That is, the north magnetic pole becomes the south magnetic pole, and vice versa.

 

Earth also undergoes magnetic field switching, but on a much longer and less regular time frame. If you find this interesting, check out this video.

 http://www.crystalinks.com/sun.html

 

Active regions on the edges of sunspots can trigger all kinds of behavior, like these plasma tornadoes on the sun.

© 2014 Pearson Education, Inc. When the two protons stick together, one of them is converted into a neutron. Actually what is going on here is that protons are made up of elementary particles called quarks. Quarks typically come in sets of three. Protons have two up quarks and one down quark. Neutrons are made of two down quarks and an up quark. When a proton is converted into a neutron, one of the up quarks is converted into a down quark. The product of this fusion is called a deuteron, a proton bound to a neutron. Also, when the fusion event happens, a neutrino and a positron are ejected, as well as a photon. We can write this as a sort of equation: P + P = D + n + e+ + g Here, the Greek symbol n (nu) stands for the neutrino, the g (gamma) stands for the photon, and the e+ stands for the positron. A positron is just like an electron, except that it is made of antimatter instead of normal matter. In collisions like this, physical quantities like mass, energy, charge, spin, matter/antimatter all must be conserved. Since the neutron is neutral (has no charge) a particle with a positive charge, the positron, must be emitted. Since the positron is an antimatter particle, a matter particle, the neutrino, must be emitted. The conservation of energy means there is also something else very important happening here. If you add up the masses of the deuteron, positon and neutrino, the total mass is less than the mass of the original two protons. This matter was converted to pure light energy. The amount of light energy can be found by relating mass to energy using Einstein's famous equation: E = mc2 The c in this equation is the speed of light, which is a very big number. c2 is a huge number. This means that transforming a small amount of matter creates a large amount of energy, as indicated in the animation below. In fact, to produce the energy we calculate coming from the sun, fusion in the core fuses 600 billion tons of hydrogen into helium every second.
© 2014 Pearson Education, Inc. When the two protons stick together, one of them is converted into a neutron. Actually what is going on here is that protons are made up of elementary particles called quarks. Quarks typically come in sets of three. Protons have two up quarks and one down quark. Neutrons are made of two down quarks and an up quark. When a proton is converted into a neutron, one of the up quarks is converted into a down quark. The product of this fusion is called a deuteron, a proton bound to a neutron. Also, when the fusion event happens, a neutrino and a positron are ejected, as well as a photon. We can write this as a sort of equation: P + P = D + n + e+ + g Here, the Greek symbol n (nu) stands for the neutrino, the g (gamma) stands for the photon, and the e+ stands for the positron. A positron is just like an electron, except that it is made of antimatter instead of normal matter. In collisions like this, physical quantities like mass, energy, charge, spin, matter/antimatter all must be conserved. Since the neutron is neutral (has no charge) a particle with a positive charge, the positron, must be emitted. Since the positron is an antimatter particle, a matter particle, the neutrino, must be emitted. The conservation of energy means there is also something else very important happening here. If you add up the masses of the deuteron, positon and neutrino, the total mass is less than the mass of the original two protons. This matter was converted to pure light energy. The amount of light energy can be found by relating mass to energy using Einstein's famous equation: E = mc2 The c in this equation is the speed of light, which is a very big number. c2 is a huge number. This means that transforming a small amount of matter creates a large amount of energy, as indicated in the animation below. In fact, to produce the energy we calculate coming from the sun, fusion in the core fuses 600 billion tons of hydrogen into helium every second.
© 2014 Pearson Education, Inc. When the two protons stick together, one of them is converted into a neutron. Actually what is going on here is that protons are made up of elementary particles called quarks. Quarks typically come in sets of three. Protons have two up quarks and one down quark. Neutrons are made of two down quarks and an up quark. When a proton is converted into a neutron, one of the up quarks is converted into a down quark. The product of this fusion is called a deuteron, a proton bound to a neutron. Also, when the fusion event happens, a neutrino and a positron are ejected, as well as a photon. We can write this as a sort of equation: P + P = D + n + e+ + g Here, the Greek symbol n (nu) stands for the neutrino, the g (gamma) stands for the photon, and the e+ stands for the positron. A positron is just like an electron, except that it is made of antimatter instead of normal matter. In collisions like this, physical quantities like mass, energy, charge, spin, matter/antimatter all must be conserved. Since the neutron is neutral (has no charge) a particle with a positive charge, the positron, must be emitted. Since the positron is an antimatter particle, a matter particle, the neutrino, must be emitted. The conservation of energy means there is also something else very important happening here. If you add up the masses of the deuteron, positon and neutrino, the total mass is less than the mass of the original two protons. This matter was converted to pure light energy. The amount of light energy can be found by relating mass to energy using Einstein's famous equation: E = mc2 The c in this equation is the speed of light, which is a very big number. c2 is a huge number. This means that transforming a small amount of matter creates a large amount of energy, as indicated in the animation below. In fact, to produce the energy we calculate coming from the sun, fusion in the core fuses 600 billion tons of hydrogen into helium every second.

© 2014 Pearson Education, Inc.

 

When the two protons stick together, one of them is converted into a neutron. Actually what is going on here is that protons are made up of elementary particles called quarks. Quarks typically come in sets of three. Protons have two up quarks and one down quark. Neutrons are made of two down quarks and an up quark. When a proton is converted into a neutron, one of the up quarks is converted into a down quark.

 

The product of this fusion is called a deuteron, a proton bound to a neutron. Also, when the fusion event happens, a neutrino and a positron are ejected, as well as a photon. We can write this as a sort of equation:

 

P + P = D + n + e+ + g

 

 

Here, the Greek symbol n (nu) stands for the neutrino, the g (gamma) stands for the photon, and the e+ stands for the positron. A positron is just like an electron, except that it is made of antimatter instead of normal matter. In collisions like this, physical quantities like mass, energy, charge, spin, matter/antimatter all must be conserved. Since the neutron is neutral (has no charge) a particle with a positive charge, the positron, must be emitted. Since the positron is an antimatter particle, a matter particle, the neutrino, must be emitted.

 

The conservation of energy means there is also something else very important happening here. If you add up the masses of the deuteron, positon and neutrino, the total mass is less than the mass of the original two protons. This matter was converted to pure light energy. The amount of light energy can be found by relating mass to energy using Einstein's famous equation:

 

E = mc2

 

The c in this equation is the speed of light, which is a very big number. c2 is a huge number. This means that transforming a small amount of matter creates a large amount of energy, as indicated in the animation below. In fact, to produce the energy we calculate coming from the sun, fusion in the core fuses 600 billion tons of hydrogen into helium every second.