A star is a glowing ball of gas held together by its own gravity and powered by nuclear fusion at its center. The main interior regions of the Sun are the core, where nuclear reactions generate energy, the radiation zone, where the energy travels outward in the form of electromagnetic radiation, and the convection zone, where the Sun's matter is in constant convective motion.
The sharp solar disk visible from Earth marks the solar photosphere—the thin surface layer from which the Sun's light is emitted. Above the photosphere lies the chromosphere, which is separated from the solar corona by a thin transition zone in which the temperature increases from a few thousand to around a million kelvins.
The Sun's luminosity is the total amount of energy radiated from the solar surface per second. It is determined by measuring the solar constant—the amount of solar radiation reaching each square meter at Earth's distance from the Sun—and multiplying that amount by the area of an imaginary sphere of radius 1 A.U.
The mathematical model that best fits the observed properties of the Sun is the Standard Solar Model. Studies of helioseismology—oscillations of the solar surface caused by sound waves in the interior—provide further insight into the Sun's structure.
The effect of the solar convection zone can be seen on the surface in the form of granulation of the photosphere. As hotter, and therefore brighter, gas rises and cooler, dimmer gas sinks, a characteristic "mottled" appearance results. Lower levels in the convection zone also leave their mark in the form of larger patterns called supergranulation.
Most of the absorption lines seen in the solar spectrum are produced in the upper photosphere and the chromosphere. Studies of these allow scientists to determine the Sun's composition and the temperature structure of the solar atmosphere.
At about 10—15 solar radii, the gas in the corona is hot enough to escape the Sun's gravity, and the corona begins to flow outward as the solar wind. Most of the solar wind flows from low-density regions of the corona called coronal holes.
The steady component of the Sun's energy production is known as the quiet Sun. Superimposed on that is the much more erratic emission of the active Sun. Solar activity is generally associated with disturbances in the Sun's magnetic field. Sunspots are Earth-sized regions on the solar surface that are a little cooler than the surrounding photosphere. They are regions of intense magnetism.
Both the numbers and locations of sunspots vary in an 11-year sunspot cycle. At solar minimum, only a few spots are typically seen, and they lie far from the solar equator. At solar maximum, the number of spots is much greater, and they generally lie much closer to the equator. The sunspot cycle is quite irregular. Its length varies from 7 to 15 years. There have been times in the past when no sunspots were seen for long periods. The overall direction of the solar magnetic field reverses from one sunspot cycle to the next. The 22-year cycle that results when the direction of the field is taken into account is called the solar cycle.
Solar activity tends to be concentrated in active regions associated with sunspot groups. Prominences are loop- or sheetlike structures produced when hot gas ejected by activity on the solar surface interacts with the Sun's magnetic field. The more intense flares are violent surface explosions that blast particles and radiation into interplanetary space.
The Sun generates energy by "burning" hydrogen into helium in its core by the process of nuclear fusion. Nuclei are held together by the strong nuclear force. When four protons overcome their electromagnetic repulsion and are converted into a helium nucleus in the proton-proton chain, some mass is lost. The law of conservation of mass and energy requires that this mass appear as energy, eventually resulting in the light we see. Very high temperatures are needed for fusion to occur.
Some particles produced during the solar fusion process are the positron, or anti-electron, which quickly annihilates with electrons in the Sun's core to generate gamma rays, the deuteron, an isotope of hydrogen consisting of a proton and a neutron, and the neutrino, a near-massless particle that escapes from the Sun without any further interactions once it is created in the core. Neutrinos interact via the weak nuclear force.
Despite their elusiveness, it is possible to detect a small fraction of the neutrinos streaming from the Sun. The observations lead to the solar neutrino problem—substantially fewer neutrinos are observed than are predicted by theory. The resolution to this problem is unclear. A leading explanation is that neutrino oscillations convert some neutrinos into other (undetected) particles en route from the Sun to Earth.
1. The Sun is a rather normal star. (Hint)
2. The Sun's average density is greater than that of Earth. (Hint)
3. The Sun's diameter is about 10 times that of Earth. (Hint)
4. Observations of sunspots indicate that the Sun rotates differentially. (Hint)
5. In the solar convection zone, the gas is partly ionized. (Hint)
6. Convection involves cool gas rising toward the solar surface, and hot gas sinking into the interior. (Hint)
7. Most solar absorption lines are produced in the corona. (Hint)
8. There are as many absorption lines in the solar spectrum as there are elements present in the Sun. (Hint)
9. The faintness of the chromosphere is a direct result of its low temperature. (Hint)
10. The temperature of the solar corona decreases with increasing radius. (Hint)
11. Sunspots are regions of intense magnetic fields. (Hint)
12. Prominences are large flames erupting from the burning surface of the Sun. (Hint)
13. Positrons are the antiparticles of electrons. (Hint)
14. Nuclei are held together by the strong force. (Hint)
15. Neutrinos are hypothetical particles that are believed to exist but have never been detected experimentally. (Hint)
1. The part of the Sun we actually see is called the _____. (Hint)
2. Traveling outward from the surface, the two main regions of the solar atmosphere are the ____ and the _____. (Hint)
3. Below the solar surface, in order of increasing depth, lie the _____ zone, the _____ zone, and the _____. (Hint)
4. _____ seen on the surface of the Sun is evidence of solar convection. (Hint)
5. The Sun appears to have a well-defined edge because the thickness of the _____ is only 0.1 percent of the solar radius. (Hint)
6. The most abundant element in the Sun is _____. (Hint)
7. The second most abundant element in the Sun is _____. (Hint)
8. The two most abundant elements in the Sun make up about _____ percent of its composition (by number of atoms). (Hint)
9. The gas in the corona is highly _____. (Hint)
10. Sunspots appear dark because they are _____ than the surrounding gas of the photosphere. (Hint)
11. The sunspot cycle is _____ years long; the solar cycle is _____ years long. (Hint)
12. The entire solar luminosity is produced in the _____ (give the region) of the Sun. (Hint)
13. The net result of the proton—proton chain is that _____ protons are fused into a nucleus of _____ , two _____ are emitted, and energy is released in the form of _____. (Hint)
14. Energy is released in the proton—proton chain because mass is _____ in the process. (Hint)
15. The solar neutrino problem is the fact that astronomers observe too _____ neutrinos coming from the Sun. (Hint)
1. Name and briefly describe the main regions of the Sun. (Hint)
2. How massive is the Sun, compared with Earth? (Hint)
3. How hot is the solar surface? The solar core? (Hint)
4. How do scientists construct models of the Sun? (Hint)
5. What is helioseismology, and what does it tell us about the Sun? (Hint)
6. Describe how energy generated within the Sun reaches Earth. (Hint)
7. Why does the Sun appear to have a sharp edge? (Hint)
8. Give the history of "coronium," and tell how it increased our understanding of the Sun. (Hint)
9. What is the solar wind? (Hint)
10. What is the cause of sunspots, flares, and prominences? (Hint)
11. What fuels the Sun's enormous energy output? (Hint)
12. What are the ingredients and the end result of the proton—proton chain in the Sun? Why is energy released in the process? (Hint)
13. Why are scientists trying so hard to detect solar neutrinos? (Hint)
14. Describe some possible solutions to the solar neutrino problem. (Hint)
15. What would we observe on Earth if the Sun's internal energy source suddenly shut off? How long do you think it might take—minutes, days, years, millions of years—for the Sun's light to begin to fade? Repeat the question for solar neutrinos. (Hint)
1. Use the reasoning presented in Section 16.1 to calculate the value of the "solar constant" on Jupiter. (Hint)
2. Use Wien's law (Section 3.4) to determine the wavelength corresponding to the peak of the blackbody curve (a) in the core of the Sun, where the temperature is 107 K, (b) in the solar convection zone (105 K), and (c) just below the solar photosphere (104 K). What form (visible, infrared, 
ray, etc.) does the radiation take in each case? (Hint)
3. The largest-amplitude solar sound waves have periods of about 5 minutes. This is the time taken for the waves to cross from one side of the Sun to the other and back. Calculate the average speed of the wave. Compare the wave period with the orbital period of an object moving just above the solar photosphere. (Hint)
4. If convected solar material moves at 1 km/s, how long does it take to flow across the 1000-km expanse of a typical granule? Compare this with the roughly 10-minute lifetimes observed for most solar granules. (Hint)
5. Use Stefan's law (flux
T4, where T is the temperature in kelvins; see Section 3.4) to calculate how much less energy (as a fraction) is emitted per unit area of a 4500-K sunspot than from the surrounding 5800-K photosphere. (Hint)
6. The Sun's differential rotation is responsible for wrapping the solar magnetic field around the Sun (Figure 16.19). Using the data presented in the Sun Data table, calculate how long it takes for material at the solar equator to "lap" the material near the poles—that is, to complete one extra trip around the Sun's rotation axis.
7. How long does it take for the Sun to convert one Earth mass of hydrogen into helium? (Hint)
8. Assuming constant luminosity, calculate how long it would take the Sun to radiate its own mass into space. (Hint)
9. The solar wind carries mass away from the Sun at a rate of about 900,000 kg/s. Compare this rate with the rate at which the Sun loses mass in the form of radiation. (Hint)
10. The entire reaction sequence shown in Figure 16.26 generates 4.3 10-12 joules of electromagnetic energy and releases two neutrinos. Assume that neutrino oscillations transform half of these neutrinos into other particles by the time they travel 1 A.U. from the Sun. Estimate the total number of solar neutrinos passing through Earth each second. (Hint)
The projects given here all require a special solar filter. Such filters are easily purchased from various sources.
NEVER LOOK DIRECTLY AT THE SUN WITHOUT A FILTER!
1. An appropriately filtered telescope will easily show you sunspots. Count the number of sunspots you see on the Sun's surface. Notice that sunspots often come in pairs or groups. Come back and look again a few days later and you'll see that the Sun's rotation has caused spots to move, and the spots themselves have changed. If a sufficiently large sunspot (or, more likely, sunspot group) is seen, continue to watch it as the Sun rotates. It will be out of view for about 2 weeks. Can you determine the rotation of the Sun from these observations?
2. Solar granulation is not too hard to see. The atmosphere of Earth is most stable in the morning hours. Observe the Sun on a cool morning, 1 or 2 hours after it has risen. Use high magnification and look initially at the middle of the Sun's disk. Can you see changes in the granulation pattern? They are there, but are not always obvious or easy to see.
3. View some solar prominences and flares. Hydrogen-alpha filters are commercially available for small telescopes. They are quite expensive, but many science departments will have one. You can often see prominences and flares even during times of sunspot minimum. You are actually viewing the chromosphere rather than the photosphere, so the Sun looks quite different from its normal appearence.
