Cerenkov radiation
chromosphere
convective zone
core (of the Sun)
corona
coronal hole
coronal mass ejection
differential rotation
filament
granule
helioseismology
hydrogen fusion
hydrostatic equilibrium
limb (of the Sun)
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limb darkening
magnetic dynamo
neutrino
photosphere
plages
plasma
positron
prominence
radiative zone
solar cycle
solar flare
solar luminosity (L⊙)
solar model
solar wind
spicule
sunspot
sunspot maximum
sunspot minimum
supergranule
thermonuclear fusion
transition zone
Zeeman effect
Review Questions
The answers to computational problems, which are preceded by an asterisk (*), appear at the end of the book.
1. Describe the features of the Sun’s atmosphere that are always present.
2. Describe the three main layers of the solar atmosphere and how you would best observe them.
3. Name and describe seven features of the active Sun. Which two are the same, seen from different angles?
4. Describe the three main layers of the Sun’s interior.
*5. When will the next sunspot minimum and sunspot maximum occur after the maximum in 2013 and the minimum in 2007? Explain your reasoning.
6. Why is the solar cycle said to have a period of 22 years, even though the sunspot cycle is only 11 years long?
7. How do astronomers detect the presence of a magnetic field in hot gases, such as the field in the solar photosphere?
8. Describe the dangers in attempting to observe the Sun. How have astronomers learned to circumvent these hazards?
9. Give an everyday example of hydrostatic equilibrium not presented in the book.
10. Give some everyday examples of heat transfer by convection and radiative transport.
11. What do astronomers mean by a “model of the Sun”?
12. Why do thermonuclear reactions in the Sun take place only in its core?
13. What is hydrogen fusion? This process is sometimes called “hydrogen burning.” How is hydrogen burning fundamentally unlike the burning of a log in a fireplace?
14. Describe the Sun’s interior, including the main physical processes that occur at various levels within the Sun.
15. What is a neutrino, and why are astronomers so interested in detecting neutrinos from the Sun?
Advanced Questions
The answers to computational problems, which are preceded by an asterisk (*), appear at the end of the book.
*16. Using the mass and size of the Sun, calculate the Sun’s average density. Compare your answer to the average densities of the outer planets. (Hint: The volume of a sphere of radius r is
πr3.)
*17. Assuming that the current rate of hydrogen fusion in the Sun remains constant, what percent of the Sun’s mass will be converted into helium over the next 5 billion years? How will this affect the chemical composition of the Sun?
*18. Calculate the wavelengths at which the photosphere, chromosphere, and corona emit the most radiation. Explain how the results of your calculations suggest the best way to observe these regions of the solar atmosphere. (Hint: Use Wien’s law and assume that the average temperatures of the photosphere, chromosphere, and corona are 5800 K, 50,000 K, and 1.5 × 106 K, respectively.)
19. When we are near a sunspot maximum, the Hubble Space Telescope must be moved to a higher orbit. Why? (Hint: Think about how the increased solar energy affects Earth’s atmosphere.)
20. Explain how the Sun can be emitting more energy today than shortly after it formed even though its surface temperature has remained roughly constant.
Discussion Questions
21. Discuss the extent to which cultures around the world have worshiped the Sun as a deity throughout history. Why do you think our star inspires such wide-spread veneration?
22. Discuss some of the difficulties of correlating solar activity with changes in the terrestrial climate.
23. Describe some advantages and disadvantages of observing the Sun (a) from space and (b) from Earth’s South Pole. What kinds of phenomena and issues do solar astronomers want to explore from both Earth-orbiting and Antarctic observatories?
What If…
24. The Sun were not rotating? What about it would be different?
25. The typical solar wind were much stronger (say, 100 times stronger) than it is now? What differences would there be in the solar system?
26. The typical solar wind were much weaker (say, 100 times weaker) than it is now? What differences would there be in the solar system?
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27. The Sun’s brightness and heat output periodically changed, as they do in many stars? What differences would there be in the solar system? Unless your instructor gives you another time frame, assume that the light and heat vary together in a cycle that lasts 1 year.
Web Questions
28. Search the Web for all solar neutrino experiments. Make a list of them and indicate which are currently operating and which are still under construction. Summarize the results found by the active detectors.
29. Search the Web for information about features of the solar atmosphere called sigmoids. What are they? What causes them? How do sigmoids provide a way to predict coronal mass ejections?
30.
Determining the Lifetime of a Solar Granule. Access and view the video “Granules on the Sun’s Surface” in Chapter 10 of the Discovering the Universe Web site. You will use it to determine the approximate lifetime of a solar granule. Select an area on the Sun’s image and slowly and rhythmically repeat Start, Stop, Start, Stop until you can consistently predict the appearance and disappearance of granules. While keeping your rhythm, move to a different area of the video and continue monitoring the appearance and disappearance of granules. When you are confident that you have the timing right, move your eyes to the clock shown in the video (or work with a partner). Using your Start-Stop cycle, determine the length of time between the appearance and disappearance of the granules and record your answer.
Got It?
31. Does the Sun shine by burning gas, like methane? Justify your answer.
32. What causes sunspots?
33. Does the Sun rotate? Justify your answer.
34. Does the Sun have a solid or liquid surface, like the Earth?
35. Does the Sun have an atmosphere? Explain your reasoning.
Observing Projects
36. Use a telescope to view the Sun, but ONLY when it is equipped with an appropriate and safe solar filter, or by projecting the Sun’s image onto a screen or sheet of white paper. Do not look directly at the Sun! Looking at the Sun causes blindness. Do you see any sunspots? If so, sketch their appearance. Can you distinguish between the umbrae and penumbrae of the sunspots? Can you see limb darkening? Can you see granulation?
37. If you have access to a telescope equipped with a special Hα filter, specifically designed for viewing the Sun safely at the color of the Balmer-alpha hydrogen spectral line, use this instrument to examine the solar surface. How does the filtered appearance of the Sun differ from that in white light (see Question 36)? What do sunspots look like in Hα? Can you see any prominences? Can you see any filaments? Are the filaments in the Hα image close to any sunspots seen in white light?
38.
Use Starry Night™ to measure the Sun’s rotation. Select Favourites > Explorations > Solar Rotation to display the Sun as seen from about 0.008 AU above its surface, well inside the orbit of Mercury. Use the Time controls to stop the Sun’s rotation at a time when a line of longitude on the Sun makes a straight line between the solar poles, preferably a line crossing a recognizable solar feature. Note the date and time. Run time forward and adjust the date and time to place this selected meridian in this position again. (a) What is the rotation rate of the Sun as shown in Starry Night™? Note that this demonstration does not show one important feature of the Sun, namely its differential rotation, where the equator of this fluid body rotates faster than the polar regions. (b) To which region of the Sun does your measured rotation rate refer? It is this differential rotation that is thought to generate the magnetic fields and active regions that make the Sun an active star. Occasional emission of high-energy particles from these active regions can disturb the Earth’s environment and disrupt electrical transmission systems. Examine this image of the Sun and compare it to real images seen in textbooks, the Internet, or from the links in the Solar Images layer in the LiveSky pane. (c) In particular, how does the distribution of sunspots and active regions on this image compare to the distribution of these regions on the real sun?
39.
Use the Starry Night™ program to examine simulations of various features that appear on the surface of the Sun. Select Favourites > Explorations > Solar Surface to show a simulated view of the visible surface of the Sun as it might appear from a spacecraft. Stop Time Flow and use the location scroller to examine this surface. (a) Which layer of the Sun’s atmosphere is shown in this part of the simulation? (b) List the different features that are visible in this view of the Sun’s surface. Click and hold the Decrease current elevation button in the toolbar to move to a location on the surface of the Sun, from which you can look out into the chromosphere. (The Viewing Location panel will indicate the location on the Sun’s surface.) This simulated view of the chromosphere is at the color of the wavelength of the Balmer-alpha spectral line of hydrogen. The opacity of the gas at this wavelength means that you can see the structure of the hot chromosphere that lies above the visible surface. Use the hand tool or cursor keys to change the gaze direction to view different features of the Sun, zooming in when necessary for a closer look at features on the horizon. (c) Provide a detailed description of the various features visible in this simulation of the Sun’s surface. You can see current solar images from both ground and spacebased solar telescopes by opening the LiveSky pane if you have an Internet connection on your computer.