1. Discovering the Universe Ninth Edition Discovering the Universe Ninth Edition Neil F. Comins William J. Kaufmann III CHAPTER 10 The Sun: Our Extraordinary Ordinary Star CHAPTER 10 The Sun: Our Extraordinary Ordinary Star

2. WHAT DO YOU THINK? 1. What percentage of the solar system’s mass is in the Sun? 2. Does the Sun have a solid and liquid interior like Earth? 3. What is the surface of the Sun like? 4. Does the Sun rotate? If so, how fast? 5. What makes the Sun shine? 6. Are matter and energy conserved?

3. In this chapter you will discover… why the Sun is a typical star why the Sun is a typical star how today’s technology has led to new understanding of solar phenomena, from sunspots to the powerful ejections of solar matter that sometimes enter our atmosphere how today’s technology has led to new understanding of solar phenomena, from sunspots to the powerful ejections of solar matter that sometimes enter our atmosphere that some features of the Sun generated by its varying magnetic field occur in cycles that some features of the Sun generated by its varying magnetic field occur in cycles how the Sun generates the energy that makes it shine how the Sun generates the energy that makes it shine new insights into the nature of matter from solar neutrinos new insights into the nature of matter from solar neutrinos

4. The Sun emits most of its visible light from a thin layer of gas, called the photosphere, as shown. Although the Sun has no solid or even liquid region, we see the photosphere as its “surface.” It is actually the top of the Sun’s convective zone. Astronomers always take great care when viewing the Sun by using extremely dark filters or by projecting the Sun’s image onto a screen.

5. Limb Darkening The Sun’s edge, or limb, appears distinctly darker and more orange than does its center, as seen from Earth. This occurs because we look through the same amount of solar atmosphere at all places. As a consequence, we see higher in the Sun’s photosphere near its limb than when we look at its central regions. The higher photosphere is cooler and, because it is a blackbody, darker and more orange than the lower, hotter region of the photosphere.

6. Solar Granulation High-resolution photographs of the Sun’s surface reveal a blotchy pattern, called granulation. Granules, which measure about 1000 km across, are convection cells in the Sun’s photosphere. Inset: Gas rising upward produces the bright granules. Cooler gas sinks downward along the darker, cooler boundaries between granules. This convective motion transports energy from the Sun’s interior outward to the solar atmosphere.

7. Solar Granulation At lower resolution, the Sun’s surface appears relatively smooth. Inset: Viewed near the Sun’s limb, granules are seen to bulge upward at their centers as a result of the convection that creates them.

8. The Chromosphere This photograph of the chromosphere was taken by the Hinode (Japanese for “sunrise”) satellite. The dark bumps are the tops of granules, and the light regions are hotter gases in spicules. The spicules on the edge or limb of the Sun give a sense of the height of these gas jets.

9. Spicules and Supergranules Spicules appear in this photograph of the Sun’s chromosphere. The Sun appears rose-colored in this image because it was taken through an H α filter that passes red light from hydrogen and effectively blocks most of the photosphere’s light. Surrounded by spicules, supergranules are regions of rising and falling gas in the chromosphere. Each supergranule spans hundreds of granules in the photosphere below. Inset: This is a view of spicules from above.

10. Spicules and Supergranules The spicules are jets of gas that surge upward into the Sun’s outer atmosphere. This schematic diagram shows a spicule and its relationship to the solar atmosphere’s layers. The photosphere is about 400 km thick. The chromosphere above it extends to an altitude of about 2000 km, with spicules jutting up to nearly 10,000 km above the photosphere. The outermost layer, the corona, extends millions of kilometers above the photosphere.

11. The Solar Corona (a) This visible-light photograph was taken during the total solar eclipse of July 11, 1991. Numerous streamers are visible, extending millions of kilometers above the solar surface. (b) This X-ray image of the Sun’s corona, taken by the Yohkoh satellite in 1999, provides hints of the complex activity taking place on and in the Sun. The million-degree gases in the corona emit the X rays visible here.

12. The Solar Corona This graph shows how temperature varies with altitude in the Sun’s chromosphere and corona and in the transition region between them (white). Note that both the height and temperature scales are nonlinear.

13. Sunspots (a) This dark region on the Sun is a typical isolated sunspot. Granulation is visible in the surrounding, undisturbed photosphere. (b) This high-resolution photograph shows a sunspot group in which several sunspots overlap.

14. The Sunspot Cycle The number of sunspots on the Sun varies with a period of about 11 years. The most recent sunspot maximum occurred in 2001, and the most recent sunspot minimum occurred in 2007 (and continued through 2010).

15. The Sunspot Cycle The active Sun has many sunspots (this photo was taken in 1979). The Sun has many fewer sunspots when it is not active (this photo, showing a time with no sunspots, was taken in 1989).

16. The Sun’s Rotation This series of photographs taken in 1999 shows the same sunspot group over one-third of a solar rotation. Note how the sunspot groups have changed over this time. By observing a group of sunspots from one day to the next in this same manner, Galileo found that the Sun rotates once in about 4 weeks. Sunspot activity also reveals the Sun’s differential rotation: The equatorial regions rotate faster than the polar regions.

17. Locations of Sunspots Throughout the Sunspot Cycle This “butterfly” diagram of sunspot locations shows that they occur at changing latitudes throughout each cycle. From most common locations to least, the diagram is color coded yellow, orange, and black.

18. Zeeman Splitting by a Sunspot’s Magnetic Field (a) The black line drawn across the sunspot indicates the location toward which the slit of the spectroscope was aimed. (b) In the resulting spectrogram, one line in the middle of the normal solar spectrum is split into three components by the Sun’s magnetic field. The amount of splitting between the three lines is used to determine the magnetic field’s strength. Typical sunspots have magnetic fields some 5000 times stronger than Earth’s magnetic field.

19. Helioseismology (a) This computer-generated image shows one of the myriad ways in which the Sun vibrates because of sound waves resonating in its interior. The regions that are moving outward are blue; those moving inward are red. The cutaway shows how deep these oscillations are believed to extend. (b) This cutaway picture of the Sun shows how the rate of solar rotation varies with depth and latitude. Red and yellow denote faster-than-average motion; blue regions move more slowly than average. The pattern of differential surface rotation, which varies from 25 days at the equator to 35 days near the poles, persists at least 19,000 km down into the Sun’s convective layer. Sunspots preferentially occur on the boundaries between different rotating regions. Earthlike jet streams and other wind patterns have also been discovered in the Sun’s atmosphere.

20. Babcock’s Magnetic Dynamo Differential rotation wraps a magnetic field around the Sun. Convection under the photosphere tangles the field, which becomes buoyant and rises through the photosphere, creating sunspots and sunspot groups. Insets: In each group, the sunspot that appears first as the Sun rotates has the same polarity as the Sun’s magnetic pole in that hemisphere. The Sun’s magnetic fields are revealed by the radiation emitted from the gas they trap. These ultraviolet images show coronal loops up to 160,000 km (100,000 mi) high, with gases moving along the magnetic field lines at speeds of 100 km/s (60 mi/s).

21. Active Sun in H α This photograph shows the chromosphere and corona during a solar maximum, when sunspots are abundant. The image was taken through a filter allowing only light from H α emission to pass through. The hot, upper layers of the Sun’s atmosphere are strong emitters of H α photons. A few large sunspots are evident. Most notable are features that do not appear at the solar minimum, such as the snakelike features shown here called filaments, bright areas called plages, and prominences (filaments seen edge on) observed at the solar limb.

22. Prominences (a) A huge prominence arches above the solar surface. The radiation that exposed this picture is from singly ionized helium at a wavelength of 30.4 nm, corresponding to a temperature of about 50,000 K. (b) The gas in these prominences was so energetic that it broke free from the magnetic fields that shape and confine it. This eruptive prominence occurred in 1999 (and did not strike Earth, which is shown for size).

23. A Coronal Hole This X-ray picture of the Sun’s corona was taken by the SOHO satellite. A huge coronal hole dominates the lower right side of the corona. The bright regions are emissions from sunspot groups.

24. A Flare Solar flares, which are associated with sunspot groups, produce energetic emissions of particles from the Sun. This image, taken in 2000 by SOHO, shows a twisted flare (upper left part of figure) in which the Sun’s magnetic field lines are still threaded through the region of emerging particles.

25. Origin of a Solar Flare This Hinode satellite image of the Sun shows a developing sunspot colliding with a preexisting sunspot. The interacting magnetic fields funnel hot gases rapidly away from the Sun as a solar flare.

26. A Snapshot of the Sun’s Global Magnetic Field By following the paths of particles emitted by a solar flare, astronomers have begun mapping the solar magnetic field outside the Sun. The field guides the outflowing particles, which, in turn, emit radio waves that indicate the position of the field. These data were collected by the Ulysses spacecraft in 1994. Ulysses was the first spacecraft to explore interplanetary space from high above the plane of the ecliptic.

27. A Coronal Mass Ejection An X-ray image of a coronal mass ejection from the Sun taken by SOHO.

28. A Coronal Mass Ejection Two to 4 days later, the highest-energy gases from the ejection reach 1 AU. If they come our way, most particles are deflected by Earth’s magnetic field (blue). However, as shown, some particles leak Earthward, causing aurorae, disrupting radio communications and electric power transmission, damaging satellites, and ejecting some of Earth’s atmosphere into interplanetary space.

31. Hydrostatic Equilibrium (a) Matter deep inside the Sun is in hydrostatic equilibrium, meaning that upward and downward forces on the gases are balanced. (b) When the forces on the divers in water are in hydrostatic equilibrium, they neither sink nor rise.

32. The Solar Model Thermonuclear reactions occur in the Sun’s core, which extends to a distance of 0.25 solar radius from the center. In this model, energy from the core radiates outward to a distance of 0.7 solar radius. Convection is responsible for energy transport in the Sun’s outer layers.

33. The Solar Model The Sun’s internal structure is displayed here with graphs that show how the luminosity, mass, temperature, and density vary with the distance from the Sun’s center. A solar radius (the distance from the Sun’s center to the photosphere) equals 696,000 km.

34. A Solar Neutrino Experiment Located 2703 m (6800 ft) underground in the Creighton nickel mine in Sudbury, Canada, the Sudbury Neutrino Observatory is centered around a tank that contains 1000 tons of water. Occasionally, a neutrino entering the tank interacts with one or another of the particles already there. Such interactions create flashes of light, called Cerenkov radiation. Some 9600 light detectors sense this light. The numerous silver protrusions are the back sides of the light detectors prior to their being wired and connected to electronics in the lab (seen at the bottom of the photograph).

35. Summary of Key Ideas

36. The Sun’s Atmosphere The thin shell of the Sun’s gases we see are from its photosphere, the lowest level of its atmosphere. The gases in this layer shine nearly as a blackbody. The photosphere’s base is at the top of the convective zone. The thin shell of the Sun’s gases we see are from its photosphere, the lowest level of its atmosphere. The gases in this layer shine nearly as a blackbody. The photosphere’s base is at the top of the convective zone. Convection of gas from below the photosphere produces features called granules. Convection of gas from below the photosphere produces features called granules. Above the photosphere is a layer of hotter, but less dense, gas called the chromosphere. Jets of gas, called spicules, rise up into the chromosphere along the boundaries of supergranules. Above the photosphere is a layer of hotter, but less dense, gas called the chromosphere. Jets of gas, called spicules, rise up into the chromosphere along the boundaries of supergranules. The outermost layer of thin gases in the solar atmosphere, called the corona, extends outward to become the solar wind at great distances from the Sun. The gases of the corona are very hot, but they have extremely low densities. The outermost layer of thin gases in the solar atmosphere, called the corona, extends outward to become the solar wind at great distances from the Sun. The gases of the corona are very hot, but they have extremely low densities.

37. The Active Sun Some surface features on the Sun vary periodically in an 11-year cycle. The magnetic fields that cause these changes actually vary over a 22-year cycle. Some surface features on the Sun vary periodically in an 11-year cycle. The magnetic fields that cause these changes actually vary over a 22-year cycle. Sunspots are relatively cool regions produced by local concentrations of the Sun’s magnetic field protruding through the photosphere. The average number of sunspots and their average latitude vary in an 11-year cycle. Sunspots are relatively cool regions produced by local concentrations of the Sun’s magnetic field protruding through the photosphere. The average number of sunspots and their average latitude vary in an 11-year cycle. A prominence is gas lifted into the Sun’s corona by magnetic fields. A solar flare is a brief, but violent, eruption of hot, ionized gases from a sunspot group. Coronal mass ejections send out large quantities of gas from the Sun. Coronal mass ejections and flares that head our way affect satellites, communication, and electric power, and cause aurorae. A prominence is gas lifted into the Sun’s corona by magnetic fields. A solar flare is a brief, but violent, eruption of hot, ionized gases from a sunspot group. Coronal mass ejections send out large quantities of gas from the Sun. Coronal mass ejections and flares that head our way affect satellites, communication, and electric power, and cause aurorae. The magnetic dynamo model suggests that many transient features of the solar cycle are caused by the effects of differential rotation and convection on the Sun’s magnetic field. The magnetic dynamo model suggests that many transient features of the solar cycle are caused by the effects of differential rotation and convection on the Sun’s magnetic field.

38. The Sun’s Interior The Sun’s energy is produced by the thermonuclear process, called hydrogen fusion, in which four hydrogen nuclei release energy when they fuse to produce a single helium nucleus. The Sun’s energy is produced by the thermonuclear process, called hydrogen fusion, in which four hydrogen nuclei release energy when they fuse to produce a single helium nucleus. The energy released in a thermonuclear reaction comes from the conversion of matter into energy, according to Einstein’s equation, E = mc 2. The energy released in a thermonuclear reaction comes from the conversion of matter into energy, according to Einstein’s equation, E = mc 2. The solar model is a theoretical description of the Sun’s interior derived from calculations based on the laws of physics. The solar model reveals that hydrogen fusion occurs in a core that extends from the center to about a quarter of the Sun’s visible radius. The solar model is a theoretical description of the Sun’s interior derived from calculations based on the laws of physics. The solar model reveals that hydrogen fusion occurs in a core that extends from the center to about a quarter of the Sun’s visible radius.

39. The Sun’s Interior Throughout most of the Sun’s interior, energy moves outward from the core by radiative diffusion. In the Sun’s outer layers, energy is transported to the Sun’s surface by convection. Throughout most of the Sun’s interior, energy moves outward from the core by radiative diffusion. In the Sun’s outer layers, energy is transported to the Sun’s surface by convection. Neutrinos were originally believed to be massless. The electron neutrinos generated and emitted by the Sun were originally detected at a lower rate than is predicted by our model of thermonuclear fusion. The discrepancy occurred because electron neutrinos have mass, which causes many of them to change into other forms of neutrinos before they reach Earth. These alternative forms are now being detected. Neutrinos were originally believed to be massless. The electron neutrinos generated and emitted by the Sun were originally detected at a lower rate than is predicted by our model of thermonuclear fusion. The discrepancy occurred because electron neutrinos have mass, which causes many of them to change into other forms of neutrinos before they reach Earth. These alternative forms are now being detected.

40. Key Terms 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) limb darkening magnetic dynamo neutrino photosphere plages plasma positron prominence radiative zone solar cycle solar flare solar luminosity solar model solar wind spicule sunspot sunspot maximum sunspot minimum supergranule thermonuclear fusion transition zone Zeeman effect

41. WHAT DID YOU THINK? What percentage of the solar system’s mass is in the Sun? What percentage of the solar system’s mass is in the Sun? The Sun contains about 99.85% of the solar system’s mass. The Sun contains about 99.85% of the solar system’s mass.

42. WHAT DID YOU THINK? Does the Sun have a solid and liquid interior like Earth? Does the Sun have a solid and liquid interior like Earth? No. The entire Sun is composed of hot gases. No. The entire Sun is composed of hot gases.

43. WHAT DID YOU THINK? What is the surface of the Sun like? What is the surface of the Sun like? The Sun has no solid surface. Indeed, it has no solids or liquids anywhere. The level we see, the photosphere, is composed of hot, churning gases. The Sun has no solid surface. Indeed, it has no solids or liquids anywhere. The level we see, the photosphere, is composed of hot, churning gases.

44. WHAT DID YOU THINK? Does the Sun rotate? If so, how fast? The Sun’s surface rotates differentially, varying between once every 35 days near its poles and once every 25 days at its equator.

45. WHAT DID YOU THINK? What makes the Sun shine? Thermonuclear fusion in the Sun’s core is the source of the Sun’s energy.

46. WHAT DID YOU THINK? Are matter and energy conserved? By themselves, they are not always conserved. Nuclear fusion converts matter into energy. Energy can also be converted into matter. The sum of the matter (multiplied by c 2 ) and energy is always conserved.