Chapter 1. Overview
1.1 Tutorial: Solar Interior
Considering the Sun's Interior
The image at right shows the Sun's photosphere as seen from Earth's surface. The photosphere is the dense layer of the Sun below which we cannot see.
Question 1.1
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A Journey Into the Sun
Although we cannot directly see beneath the photosphere, we can use the laws of physics to infer what must be going on there. In particular, astronomers create computer models that describe how luminosity, mass, temperature and density must be changing with increasing distance from the center. The table below shows that the Sun's luminosity is completely generated within one-quarter of the radius and that most of the Sun's mass is concentrated inside one-half of the Sun's radius. The temperature inside the Sun is highest in the center at nearly 16 million degrees and the temperature decreases with distance from the center. Not surprisingly, the Sun's density is greatest at the center, 14 times denser than lead, and the Sun's density decreases as you move out toward the photosphere.
1.1.2 Tutorial: Solar Interior
Self Test: A Theoretical Model of the Sun
Box Title?
Question 1.2
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Question 1.3
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Question 1.4
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Question 1.5
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1.1.3 Tutorial: Solar Interior
The Sun's Center
Why might the Sun shine?
- Initial High Temperature: If the Sun was formed very hot, maybe it is just slowly cooling down and releasing energy.
- Gravitational Contraction: If the Sun has immense gravity, maybe it is compressing and heating its interior gases.
- Fuel Burning: If the Sun shines, maybe it is burning coal or some other familiar energy source deep inside.
- Thermonuclear Fusion: If the center is hot enough and dense enough, maybe hydrogen atoms are combining to make helium and releasing energy according to E=mc2.
Over history, scientists have fiercely debated what causes the Sun and the stars to shine. The box at right lists several mechanisms proposed over the years. As it turns out, only the process of thermonuclear fusion can provide enough energy over the long term to account for the intense brightness of the Sun. According to Einstein's mass-to-energy conversion equation, E=mc2, the Sun must be converting 600 billion kilograms of hydrogen into helium every second. Given that the Sun's mass is 333,000 times more than Earth, there is enough hydrogen inside the Sun to keep this process going at its current rate for a total of about 10 billion years. So far, the Sun has only used about one-half of its easily available hydrogen supply.
If the Sun's center is the location of highest temperature and highest density, it might not surprise you that the center is where the Sun's thermonuclear fusion power source is located. At temperatures in excess of 10 million degrees, as in the center of the Sun where the temperature is nearly 16 million degrees, hydrogen atoms combine to make helium atoms in a thermonuclear reaction called the proton-proton chain.
As shown in the figure below, three principal steps define this process. First, two hydrogen atoms combine to create a deuterium isotope, a neutrino, and a positron. A deuterium isotope is simply a hydrogen atom with an extra neutron in its nucleus, a neutrino is a nearly massless particle that escapes the Sun quickly, and a positron is a subatomic particle that quickly annihilates by colliding with a nearby electron, releasing a gamma ray. In the second step, the deuterium isotope combines with another hydrogen atom that creates a helium-3 isotope and another gamma ray. The helium-3 isotope has one neutron less than the more familiar helium-4 atom that has two neutrons. Finally, two helium-3 isotopes combine to make a stable helium-4 atom and release two hydrogen atoms in the process that can be used again and again. This process accounts for the vast majority of the Sun's energy output.
1.1.4 Tutorial: Solar Interior
Energy Transport Inside the Sun
Once energy leaves the Sun's core, it takes a very long time for the energy, in the form of gamma rays released in the proton-proton chain, to make its way 696,000 km out to the photosphere. As gamma-ray photons are released, they do not travel very far before colliding with a nearby ion. When they bounce off that ion, they may or may not move in a direction pointing outward before they collide with yet another ion. The result is that it can take nearly a million years for photons to make their way out of the Sun.
Because the density is highest in the inner half of the Sun, collisions among ions and photons increase the temperature. This energy escapes the core through the process of radiative transfer by transferring energy to the more distant and cooler regions. At larger distances from the core, the Sun's density is considerably less, and a different energy transfer process takes place. Hotter regions expand and then float towards the photosphere because they are less dense. The overlying cooler regions then sink towards the center, and the process repeats itself. This churning motion where hot material moves upward and cooler material sinks is called convection. The result is that there are three distinct regions to the Sun's interior: the core where thermonuclear fusion is taking place, the radiative zone where energy is transferred via radiative transfer, and the outer convective envelope where material is actually moving up and down in circular cells.
1.1.5 Tutorial: Solar Interior
Self Test: The Sun's Internal Structure
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Question 1.6
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Question 1.7
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Question 1.8
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1.1.6 Tutorial: Solar Interior
Helioseismology
In much the same way that geologists on Earth can infer the hidden structure of Earth's interior by looking carefully at how seismic waves from earthquakes move through the Earth, astronomers can determine the Sun's interior structure by monitoring how waves pass through the Sun.
Helioseismology allows astronomers to probe the solar interior using the Sun's own vibrations
Seismic waves move throughout the solar interior causing the surface to move up and down by about 10 meters every 5 minutes. These waves bounce back and forth through the solar interior. Careful analysis has shown that the Sun rotates differently on the inside than it does on the outside. The outer 20 percent of the Sun seems to have a variable rotation rate. Material at the poles spins around the Sun in about 35 days whereas material at the equator moves around the Sun in about 25 days. Inside, the Sun seems to be moving like a rigid sphere in 27 days. Helioseismologists have also been able to determine the exact depth of the convection zone overlying the radiative zone using this technique.
1.1.7 Tutorial: Solar Interior
The Solar Neutrino Problem
Searching for Neutrinos beyond the Textbooks
an essay by John N. Bahcall
In attempting to understand the Sun, physicists, chemists, and astronomers have been confronted with a mystery—the case of the missing neutrinos. In the early 1960s, Ray Davis and I proposed to test the theory of how the Sun shines. Ray, a chemist at Brookhaven National Laboratory, had developed a neutrino detector that uses a cleaning fluid containing chlorine. Using standard theories of physics and astronomy, I calculated the rate at which neutrinos are produced in the Sun. I could then predict the rate at which neutrinos should be captured in the largest detector Ray could build. If my calculations matched experiment, they would confirm that the Sun shines by nuclear fusion in its interior.
The actual experiment used 100,000 gallons of perchloroethylene, about enough to fill an Olympic swimming pool. Ray and his collaborators put their detector in a deep gold mine, to shield it from other particles that hit the surface of the Earth. To everyone’s surprise, Ray’s chlorine detector captured many fewer neutrinos than I had predicted. The results were challenged and checked repeatedly over the following three decades, but always with the same result: many neutrinos appear to be missing! The case of the missing neutrinos has grown stronger with time. Three other experiments, each with a different type of detector, searched for neutrinos from the Sun. They all found fewer than I predicted.
What could be wrong? Where have the neutrinos gone? There are three possibilities. Either the experiments are wrong, the standard model of how the Sun (and other stars) shine is wrong, or something happens to the neutrinos after they are produced. New experiments are now under way in Japan, in Italy, and in Canada to test these hypotheses.
1.1.8 Tutorial: Solar Interior
The Solar Neutrino Problem
Searching for Neutrinos beyond the Textbooks (continued)
Most people working in the field think that the last of the three explanations is most likely to be correct: Only physics beyond the standard textbooks can describe what has happened to solar neutrinos. Somehow, most physicists think, neutrinos created in the solar interior change into neutrinos that are more difficult to detect as they pass out of the Sun and travel to the Earth. They change their personalities, so to speak! If this is correct, it would be the first experimental demonstration of a process beyond the standard model of particle physics.
So far, evidence for the new physics is only circumstantial. We know only that the results differ markedly from predictions based on our understanding now of how the Sun shines. Future experiments designed to search for a "smoking gun"—unequivocal evidence of processes not in the physics textbooks—will use the fact that neutrinos come in different types. Most easily detected are the so-called electron-type neutrinos; more difficult to detect are muon-type and tau-type neutrinos.
Suppose some of these neutrinos from the Sun convert into neutrinos that are easier to detect as they pass through the Earth at night on their way to the detector. The change would make the Sun appear brighter (in terms of neutrinos) at night than in the day . If that were seen, it would provide a dramatic demonstration that unconventional physics is occurring.
If such a smoking gun is found, it could offer a clue to new laws of particle physics. Recent experiments in Japan using neutrinos produced in the Earth’s atmosphere by energetic particles from outside our solar system (cosmic rays) have already shown that neutrinos have a tiny mass, which allows them to convert from one type to another. Observations of solar neutrinos may lead us to further revise the laws which govern the smallest scales of matter.
1.1.9 Tutorial: Solar Interior
The Solar Neutrino Problem
Searching for Neutrinos beyond the Textbooks (continued)
I do not know the correct explanation for the missing neutrinos. However, the particle-physics (split-personality) explanation has a mathematical beauty and simplicity that are very attractive. If the deity has not chosen this solution to the mystery, then he or she has missed an excellent opportunity to enrich the laws of the universe.
However, the real message of the experiments on solar neutrinos is even more remarkable. It is that working on the frontier of science, you may stumble across something that is beautiful and unexpected. We do not yet know precisely what we have discovered with solar neutrinos, but we do know that future experiments will solve the mystery for us. Their outcome might point the way to a better understanding of fundamental physics and of the stars.
John N. Bahcall
Neutrinos from the Sun have been detected, but in smaller numbers than expected.
1.1.10 Tutorial: Solar Interior
Reconsidering the Sun's Interior
Earlier in this tutorial, you described your concept of the Sun's interior as the following:
(ANSWER FROM FIRST SLIDE/QUESTION WOULD BE SHOWN HERE)
Again, describe the interior of the Sun and be sure to use and explain terms including:
- luminosity
- percent mass
- temperature
- density
- thermonuclear fusion
- proton-proton chain
- core
- radiative zone
- convection zone
- helioseismology
- the neutrino problem