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Objectives Explore the structure of the Sun. The Sun Describe the solar activity cycle and how the Sun affects Earth. Compare the different types of spectra.

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Presentation on theme: "Objectives Explore the structure of the Sun. The Sun Describe the solar activity cycle and how the Sun affects Earth. Compare the different types of spectra."— Presentation transcript:

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2 Objectives Explore the structure of the Sun. The Sun Describe the solar activity cycle and how the Sun affects Earth. Compare the different types of spectra. –photosphere –chromosphere –corona –solar wind –sunspot Vocabulary –solar flare –prominence –fusion –fission –spectrum

3 Astronomers still rely on computer models for an explanation of the interior of the Sun because the interior cannot be directly observed. The Sun Through observations and probes, such as the Solar Heliospheric Observatory (SOHO) and the Ulysses mission, astronomers have begun to unravel the mysteries of the Sun. The Sun

4 Properties of the Sun The Sun is the largest object in the solar system, in both size and mass. The Sun –The Sun contains more than 99 percent of all the mass in the solar system, which allows it to control the motions of the planets and other objects. –Models show that the density in the center of the Sun is about 1.50 × 10 5 kg/m 3.

5 Properties of the Sun The solar interior is gaseous throughout because of its high temperature—about 1 × 10 7 K in the center. The Sun Many of the gases are in a plasma state, meaning that they are completely ionized and composed only of atomic nuclei and electrons. The outer layers of the Sun are not quite hot enough to be plasma.

6 The Sun’s Atmosphere The photosphere, approximately 400 km in thickness, is the lowest layer of the Sun’s atmosphere. The Sun –This is the visible surface of the Sun because most of the light emitted by the Sun comes from this layer. –The average temperature of the photosphere is about 5800 K.

7 The Sun’s Atmosphere The chromosphere, which is above the photosphere and approximately 2500 km in thickness, has a temperature of nearly K at the top. The Sun The corona, which is the top layer of the Sun’s atmosphere, extends several million kilometers southward from the top of the chromosphere and has a temperature range of 1 million to 2 million degrees K.

8 The Sun’s Atmosphere Solar Wind The Sun –Gas flows outward from the corona at high speeds and forms the solar wind. –Solar wind consists of charged particles, or ions, that flow outward through the entire solar system, bathing each planet in a flood of particles. –The charged particles are trapped in two huge rings in Earth’s magnetic field, called the Van Allen belts, where they collide with gases in Earth’s atmosphere, causing an aurora.

9 Solar Activity The Sun’s magnetic field disturbs the solar atmosphere periodically and causes new features to appear in a process called solar activity. The Sun Sunspots are cooler areas that form on the surface of the photosphere due to magnetic disturbances, which appear as dark spots.

10 Solar Activity Solar Activity Cycle The Sun –The number of sunspots changes regularly, and on average reaches a maximum number every 11.2 years. –The length of the solar activity cycle is 22.4 years. The solar activity cycle starts with minimum spots and progresses to maximum spots. The Sun’s magnetic field then reverses in polarity, and the spots start at a minimum number and progress to a maximum number again. The magnetic field then switches back to the original polarity and completes the solar activity cycle.

11 Solar Activity Other Solar Features The Sun –Coronal holes, often located over sunspot groups, are areas of low density in the gas of the corona. –Solar flares are violent eruptions of particles and radiation from the surface of the Sun that are associated with sunspots. –When these particles reach Earth, they can interfere with communications and damage satellites. –A prominence, sometimes associated with flares, is an arc of gas that is ejected from the chromosphere, or gas that condenses in the inner corona and rains back to the surface.

12 Solar Activity Impact on Earth The Sun –Some scientists have found evidence of subtle climate variations within 11-year periods. –There were severe weather changes on Earth during the latter half of the 1600s when the solar activity cycle stopped and there were no sunspots for nearly 60 years. –Those 60 years were known as the “Little Ice Age” because the weather was very cold in Europe and North America during those years.

13 The Solar Interior Fusion occurs within the core of the Sun where the pressure and temperature are extremely high. The Sun –Fusion is the combining of lightweight nuclei, such as hydrogen, into heavier nuclei. –Fission, the opposite of fusion, is the splitting of heavy atomic nuclei into smaller, lighter atomic nuclei. In the core of the Sun, helium is a product of the process in which hydrogen nuclei fuse. At the Sun’s rate of hydrogen fusing, it is about halfway through its lifetime, with about another 5 billion years left.

14 The Solar Interior Energy from the Sun The Sun –The quantity of energy that arrives on Earth every day from the Sun is enormous. –Above Earth’s atmosphere, 1354 J of energy is received in 1 m 2 per second (1354 W/m 2 ). –Not all of this energy reaches the ground because some is absorbed and scattered by the atmosphere.

15 The Solar Interior Solar Zones The Sun –Energy produced in the core of the Sun gets to the surface through two zones in the solar interior. In the radiative zone, which is above the core, energy is transferred from particle to particle by radiation, as atoms continually absorb energy and then re-emit it. Above the radiative zone, in the convective zone, moving volumes of gas carry the energy the rest of the way to the Sun’s surface through convection.

16 The Solar Interior Solar Zones The Sun

17 Spectra A spectrum is visible light arranged according to wavelengths. The Sun There are three types of spectra: –A continuous spectrum is a spectrum that has no breaks in it that can be produced by a glowing solid or liquid, or by a highly compressed, glowing gas. –An emission spectrum has bright lines in it called emission lines that depend on the element being observed. –An absorption spectrum has dark lines called absorption lines which are caused by different chemical elements that absorb light at specific wavelengths.

18 Spectra Absorption is caused by a cooler gas in front of a source that emits a continuous spectrum. The Sun By comparing laboratory spectra of different gases with the dark lines in the solar spectrum, it is possible to identify the elements that make up the Sun’s outer layers.

19 Spectra A continuous spectrum is produced by a hot solid, liquid, or dense gas. When a cloud of gas is in front of this hot source, an absorption spectrum is produced. A cloud of gas without a hot source behind it will produce an emission spectrum. The Sun

20 Solar Composition The Sun consists of hydrogen, about 73.4 percent by mass, and helium, 25 percent, as well as a small amount of other elements. The Sun This composition is very similar to that of the gas giant planets. The Sun’s composition represents that of the galaxy as a whole.

21 Section Assessment 1.Match the following terms with their definitions. ___ photosphere ___ corona ___ chromosphere ___ sunspot The Sun A.the middle layer of the Sun’s atmosphere B.the outermost layer of the Sun’s atmosphere C.cooler region on the Sun’s surface that forms due to magnetic irregularities D.the lowest layer of the Sun’s atmosphere D B A C

22 Section Assessment 2.How can we determine what gases are in the outer layers of the Sun’s atmosphere? The Sun Dark bands in the solar spectrum represent light that has been absorbed by the gases of its atmosphere. By comparing laboratory spectra of different gases with the dark lines in the solar spectrum, it is possible to identify the elements that make up the Sun’s outer layers.

23 ______ The Sun contains more than 99 percent of all mass in the solar system. ______ Most visible light from the sun originates in the chromosphere. ______The energy released by the Sun originates through nuclear fission. ______Mass can be converted into energy. Section Assessment 3.Identify whether the following statements are true or false. The Sun true false true

24 End of Section 1

25 Objectives Describe star distribution and distance. Classify the types of stars. Summarize the interrelated properties of stars. –constellation –binary star –parallax –apparent magnitude –absolute magnitude –luminosity –Hertzsprung-Russell diagram –main sequence Vocabulary Measuring the Stars

26 Groups of Stars Constellations are the 88 groups of stars named after animals, mythological characters, or everyday objects. Measuring the Stars –Circumpolar constellations can be seen all year long as they appear to move around the north or south pole. –Summer, fall, winter, and spring constellations can be seen only at certain times of the year because of Earth’s changing position in its orbit around the Sun.

27 Groups of Stars Star Clusters Measuring the Stars –Although stars may appear to be close to each other, very few are gravitationally bound to one other. –By measuring distances to stars and observing how they interact with each other, scientists can determine which stars are gravitationally bound to each other. –A group of stars that are gravitationally bound to each other is called a cluster. In an open cluster, the stars are not densely packed. In a globular cluster, stars are densely packed into a spherical shape.

28 Groups of Stars Binaries Measuring the Stars –A binary star is two stars that are gravitationally bound together and that orbit a common center of mass. Accurate measurements can show that its position shifts back and forth as it orbits the center of mass. In an eclipsing binary, the orbital plane of a binary system can sometimes be seen edge-on from Earth. –More than half of the stars in the sky are either binary stars or members of multiple-star systems. –Astronomers are able to identify binary stars through several methods.

29 Stellar Position and Distances Astronomers use two units of measure for long distances. Measuring the Stars –A light-year (ly) is the distance that light travels in one year, equal to × km. –A parsec (pc) is equal to 3.26 ly, or × km.

30 Stellar Position and Distances To estimate the distance of stars from Earth, astronomers make use of the fact that nearby stars shift in position as observed from Earth. Measuring the Stars Parallax is the apparent shift in position of an object caused by the motion of the observer. As Earth moves from one side of its orbit to the opposite side, a nearby star appears to be shifting back and forth.

31 Stellar Position and Distances The distance to a star, up to 500 pc using the latest technology, can be estimated from its parallax shift. Measuring the Stars

32 Basic Properties of Stars The basic properties of stars include diameter, mass, brightness, energy output (power), surface temperature, and composition. Measuring the Stars The diameters of stars range from as little as 0.1 times the Sun’s diameter to hundreds of times larger. The masses of stars vary from a little less than 0.01 to 20 or more times the Sun’s mass.

33 Basic Properties of Stars Magnitude Measuring the Stars –One of the most basic observable properties of a star is how bright it appears. –The ancient Greeks established a classification system based on the brightnesses of stars. –The brightest stars were given a ranking of +1, the next brightest +2, and so on.

34 Basic Properties of Stars Apparent Magnitude Measuring the Stars –Apparent magnitude is based on the ancient Greek system of classification which rates how bright a star appears to be. –In this system, a difference of 5 magnitudes corresponds to a factor of 100 in brightness. –Negative numbers are assigned for objects brighter than magnitude +1.

35 Basic Properties of Stars Absolute Magnitude Measuring the Stars –Apparent magnitude does not actually indicate how bright a star is, because it does not take distance into account. –Absolute magnitude is the brightness an object would have if it were placed at a distance of 10 pc.

36 Basic Properties of Stars Luminosity Measuring the Stars –Luminosity is the energy output from the surface of a star per second. –The brightness we observe for a star depends on both its luminosity and its distance. –Luminosity is measured in units of energy emitted per second, or watts. –The Sun’s luminosity is about 3.85 × W.

37 Spectra of Stars Stars also have dark absorption lines in their spectra and are classified according to their patterns of absorption lines. Measuring the Stars

38 Spectra of Stars Classification Measuring the Stars –Stars are assigned spectral types in the following order: O, B, A, F, G, K, and M. –Each class is subdivided into more specific divisions with numbers from 0 to 9. –The classes correspond to stellar temperatures, with the O stars being the hottest and the M stars being the coolest. –The Sun is a type G2 star, which corresponds to a surface temperature of about 5800 K.

39 Spectra of Stars Classification Measuring the Stars –All stars, including the Sun, have nearly identical compositions—about 73 percent of a star’s mass is hydrogen, about 25 percent is helium, and the remaining 2 percent is composed of all the other elements. B5 star F5 star K5 star M5 star –The differences in the appearance of their spectra are almost entirely a result of temperature effects.

40 Spectra of Stars Wavelength Shift Measuring the Stars –Spectral lines are shifted in wavelength by motion between the source of light and the observer due to the Doppler effect. If a star is moving toward the observer, the spectral lines are shifted toward shorter wavelengths, or blueshifted. If the star is moving away, the wavelengths become longer, or redshifted.

41 Spectra of Stars Wavelength Shift Measuring the Stars

42 Spectra of Stars Wavelength Shift Measuring the Stars –The higher the speed, the larger the shift, and thus spectral line wavelengths can be used to determine the speed of a star’s motion. –Astronomers can learn only about the portion of a star’s motion that is directed toward or away from Earth.

43 Spectra of Stars H-R Diagrams Measuring the Stars –A Hertzsprung-Russell diagram, or H-R diagram, demonstrates the relationship between mass, luminosity, temperature, and the diameter of stars. –An H-R diagram plots the absolute magnitude on the vertical axis and temperature or spectral type on the horizontal axis.

44 Spectra of Stars H-R Diagrams Measuring the Stars –The main sequence, which runs diagonally from the upper-left corner to the lower-right corner of an H-R diagram, represents about 90 percent of stars. –Red giants are large, cool, luminous stars plotted at the upper-right corner. –White dwarfs are small, dim, hot stars plotted in the lower- left corner.

45 Spectra of Stars H-R Diagrams Measuring the Stars

46 Section Assessment 1.Match the following terms with their definitions. ___ binary star ___absolute magnitude ___ luminosity ___ parallax Measuring the Stars A.the energy output from the surface of a star per second B.when two stars are gravitationally bound and orbit a common center of mass C.the brightness an object would have if placed at a set distance D.an apparent shift in the position of an object caused by the motion of the observer B C A D

47 Section Assessment Measuring the Stars 2.How can astronomers measure the speed at which a star is moving? Spectral lines are shifted in wavelength by motion between the source of light and the observer. If a star is moving toward the observer, spectral lines are blueshifted. If a star is moving away, spectral lines are redshifted. The higher the speed, the larger the shift.

48 Section Assessment 3.Identify whether the following statements are true or false. Measuring the Stars ______ The full Moon has less brightness than Venus on the absolute magnitude scale. ______ Luminosity of stars is a relatively consistent stellar property. ______Around two-thirds of the stars in the sky are either binary stars or members of multi-star systems. ______The Sun is part of the main sequence. true false true

49 End of Section 2

50 –nebula –protostar –neutron star Objectives Explain how astronomers learn about the internal structure of stars. Describe how the Sun will change during its lifetime and how it will end up. Compare the evolutions of stars of different masses. Vocabulary Stellar Evolution –supernova –black hole

51 Basic Structure of Stars The mass and the composition of a star determine nearly all its other properties. Stellar Evolution –Hydrostatic equilibrium is the balance between gravity squeezing inward and pressure from nuclear fusion and radiation pushing outward. –This balance, which is governed by the mass of a star, must hold for any stable star; otherwise, the star would expand or contract.

52 Basic Structure of Stars Fusion Stellar Evolution –Inside a star, the density and temperature increase toward the center, where energy is generated by nuclear fusion. –Stars on the main sequence all produce energy by fusing hydrogen into helium, as the Sun does. –Stars that are not on the main sequence either fuse different elements in their cores or do not undergo fusion at all.

53 Basic Structure of Stars Fusion Stellar Evolution –Fusion reactions involving elements other than hydrogen can produce heavier elements, but few heavier than iron. –The energy produced according to the equation E = mc 2 stabilizes a star by producing the pressure needed to counteract gravity.

54 Stellar Evolution and Life Cycles A star changes as it ages because its internal composition changes as nuclear fusion reactions in the star’s core convert one element into another. Stellar Evolution As a star’s core composition changes, its density increases, its temperature rises, and its luminosity increases. When the nuclear fuel runs out, the star’s internal structure and mechanism for producing pressure must change to counteract gravity.

55 Stellar Evolution and Life Cycles Star Formation Stellar Evolution –A nebula (pl. nebulae) is a cloud of interstellar gas and dust. –Star formation begins when the nebula collapses on itself as a result of its own gravity. –As the cloud contracts, its rotation forces it into a disk shape. –A protostar is a hot condensed object that forms at the center of the disk that will become a new star.

56 Stellar Evolution and Life Cycles Star Formation Stellar Evolution

57 Stellar Evolution and Life Cycles Fusion Begins Stellar Evolution –Eventually, the temperature inside a protostar becomes hot enough for nuclear fusion reactions to begin converting hydrogen to helium. –Once this reaction begins, the star becomes stable because it then has sufficient internal heat to produce the pressure needed to balance gravity. –The object is then truly a star and takes its place on the main sequence according to its mass.

58 –It takes about 10 billion years for a star with the mass of the Sun to convert all of the hydrogen in its core into helium. –When the hydrogen in its core is gone, a star has a helium center and outer layers made of hydrogen-dominated gas. –Some hydrogen continues to react in a thin layer at the outer edge of the helium core. The Sun’s Life Cycle What happens during a star’s life cycle depends on its mass. Stellar Evolution

59 The Sun’s Life Cycle –The energy produced in the thin hydrogen layer forces the outer layers of the star to expand and cool and the star becomes a red giant. Stellar Evolution –While the star is a red giant, it loses gas from its outer layers while its core becomes hot enough, at 100 million K, for helium to react and form carbon. –When the helium in the core is all used up, the star is left with a core made of carbon.

60 The Sun’s Life Cycle A Nebula Once Again Stellar Evolution –A star of the Sun’s mass never becomes hot enough for carbon to react, so the star’s energy production ends at this point. –The outer layers expand once again and are driven off entirely by pulsations that develop, becoming a shell of gas called a planetary nebula. –In the center of a planetary nebula, the core of the star remains as a white dwarf made of carbon.

61 The Sun’s Life Cycle Pressure in White Dwarfs Stellar Evolution –A white dwarf is stable because it is supported by the resistance of electrons being squeezed close together and does not require a source of heat to be maintained. –A star that has less mass than that of the Sun has a similar life cycle, except that helium may never form carbon in the core, and the star ends as a white dwarf made of helium.

62 A massive star undergoes many reaction phases and produces many elements in its interior. Life Cycles of Massive Stars A massive star begins its life high on the main sequence with hydrogen being converted to helium. Stellar Evolution The star becomes a red giant several times as it expands following the end of each reaction stage.

63 A massive star loses much of its mass during its lifetime. White dwarf composition is determined by how many reaction phases the star went through before reactions stopped. Life Cycles of Massive Stars As more shells are formed by the fusion of different elements, the star expands to a larger size and becomes a supergiant. Stellar Evolution

64 Life Cycles of Massive Stars Supernovae Stellar Evolution –A star that begins with a mass between about 8 and 20 times the Sun’s mass will end up with a core that is too massive to be supported by electron pressure. –Once no further energy-producing reactions can occur, the core of the star violently collapses in on itself and protons and electrons in the core merge to form neutrons. –A neutron star results from the resistance of neutrons to being squeezed, which creates a pressure that halts the collapse of the core.

65 Life Cycles of Massive Stars Supernovae Stellar Evolution –A neutron star has a mass of 1.5 to 3 times the Sun’s mass but a radius of only about 10 km. –Infalling gas rebounds when it strikes the hard surface of the neutron star and explodes outward. –A supernova (pl. supernovae) is a massive explosion in which the entire outer portion of the star is blown off and elements that are heavier than iron are created.

66 Life Cycles of Massive Stars Supernovae Stellar Evolution

67 Life Cycles of Massive Stars Black Holes Stellar Evolution –A star that begins with more than about 20 times the Sun’s mass will not be able to form a neutron star. –The resistance of neutrons to being squeezed is not great enough to stop the collapse, so the core of the star simply continues to collapse forever, compacting matter into a smaller and smaller volume. –A black hole is a small, extremely dense remnant of a star whose gravity is so immense that not even light can escape its gravity field.

68 Section Assessment 1.Match the following terms with their definitions. ___ nebula ___ protostar ___ supernova ___ black hole Stellar Evolution A.a cloud of interstellar gas and dust B.small, extremely dense remnant of a star with immense gravity C.a hot, condensed object that eventually will begin nuclear fusion. D.a massive explosion that blows off the outer portion of a massive star A C D B

69 Section Assessment 2.How is a neutron star different from a white dwarf? Stellar Evolution A white dwarf is created when the resistance of electrons to being squeezed stops the inward collapse of a star’s core. A neutron star is created when the original star does not lose enough mass to become a white dwarf. The pressure of the collapsing core causes protons and electrons to merge to form neutrons. The resistance of neutrons to be being squeezed halts the collapse of the core forming the neutron star.

70 Section Assessment Stellar Evolution 3.Identify whether the following statements are true or false. ______ The Sun will likely produce a supernova. ______ Black holes are likely smaller than 10 km in diameter. ______Planets form from planetary nebula. ______All stable stars have hydrostatic equilibrium. ______The Sun will become a red giant in about 5 million years. false true false true

71 End of Section 3

72 Chapter Resources Menu Study Guide Section 30.1 Section 30.2 Section 30.3 Chapter Assessment Image Bank

73 Section 30.1 Main Ideas The Sun contains most of the mass in the solar system and is made up primarily of hydrogen and helium. Astronomers learn about conditions inside the Sun by a combination of observation and theoretical models. The Sun’s atmosphere consists of the photosphere, the chromosphere, and the corona. The Sun has a 22-year activity cycle caused by reversals in its magnetic field polarities. Sunspots, solar flares, and prominences are active features of the Sun. The solar interior consists of the core, where fusion of hydrogen into helium occurs, and the radiative and convective zones. Section 30.1 Study Guide

74 Section 30.2 Main Ideas Positional measurements of the stars are important for measuring distances through stellar parallax shifts. Stellar brightnesses are expressed in the systems of apparent and absolute magnitude. Stars are classified according to the appearance of their spectra, which indicate the surface temperatures of stars. The H-R diagram relates the basic properties of stars: class, mass, temperature, and luminosity. Section 30.2 Study Guide

75 Section 30.3 Main Ideas The mass of a star determines its internal structure and its other properties. Gravity and pressure balance each other in a star. If the temperature in the core of a star becomes high enough, elements heavier than hydrogen but lighter than iron can fuse together. Stars such as the Sun end up as white dwarfs. Stars up to about 8 times the Sun’s mass also form white dwarfs after losing mass. Stars with masses between 8 and 20 times the Sun’s mass end as neutron stars, and more massive stars end as black holes. A supernova occurs when the outer layers of the star bounce off the neutron star core, and explode outward. Section 30.3 Study Guide

76 1.Which atmospheric layer of the Sun is visible under normal conditions? a.photospherec.convective zone b.coronad.chromosphere Multiple Choice Chapter Assessment The photosphere is the lowest layer of the Sun’s atmosphere. The chromosphere and corona are the upper two layers of the sun’s atmosphere which are usually only visible during a solar eclipse.

77 2.The apparent shift in the position of an object caused by the motion of the observer is called ____. a.luminosityc.parallax b.magnituded.a parsec The distance to a star can be estimated from its parallax shift. In this case, the motion of the observer is the change in position as the Earth orbits the Sun. Using the parallax technique, astronomers can find accurate distances up to 500 pc. Multiple Choice Chapter Assessment

78 Multiple Choice 3.What is the first fusion reaction in all stars? a.helium to oxygenc.hydrogen to helium b.silicon to irond.oxygen to carbon Chapter Assessment Hydrogen to helium is the first fusion reaction in all stars. In the next reaction, when the core consists primarily of helium, it is fused to form oxygen. Stars will continue to generate sequentially heavier elements, though usually no heavier than iron, as its mass allows.

79 Multiple Choice 4.What causes sun spots? a.prominence b.magnetic irregularities c.solar flares d.solar wind Chapter Assessment Sun spots are caused by magnetic irregularities in the photosphere. Prominence, solar flares, and solar wind are all associated with sun spots.

80 Multiple Choice 5.Which of the following star classifications represents the highest temperature? a.Gc.F b.Kd.B Chapter Assessment Stars are assigned spectral types in the following order from hottest to coolest: O, B, A, F, G, K, and M. Each class is subdivided into more specific divisions with number from 0 to 9. The Sun is a type G2 star.

81 Short Answer 6.What happens halfway through the solar activity cycle? Chapter Assessment Halfway through the 22-year solar activity cycle, the Sun’s magnetic field reverses so that the north magnetic pole becomes the south magnetic pole.

82 Short Answer 7.What is a binary star? How common are they? Chapter Assessment A binary star is when two stars are gravitationally bound together and orbit a common center of mass. More than half of the stars in the sky are either binary stars or members of multi-star systems.

83 True or False 8.Identify whether the following statements are true or false. ______ The Sun has roughly the same density as Jupiter. ______ A parsec is roughly one-third of a light year. ______About 90 percent of all stars are main sequence stars. ______ The Sun will someday become a white dwarf made of helium. ______ A neutron star has a radius of only about 10 km. Chapter Assessment true false true false true

84 Image Bank Chapter 30 Images

85 Image Bank Chapter 30 Images

86 Image Bank Chapter 30 Images

87 Image Bank Chapter 30 Images

88 Image Bank Chapter 30 Images

89 Help To navigate within this Interactive Chalkboard product: Click the Forward button to go to the next slide. Click the Previous button to return to the previous slide. Click the Chapter Resources button to go to the Chapter Resources slide where you can access resources such as assessment questions that are available for the chapter. Click the Menu button to close the chapter presentation and return to the Main Menu. If you opened the chapter presentation directly without using the Main Menu this will exit the presentation. You also may press the Escape key [Esc] to exit and return to the Main Menu. Click the Help button to access this screen. Click the Earth Science Online button to access the Web page associated with the particular chapter with which you are working. Click the Speaker button to hear the vocabulary term and definition when available.

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