This Document Contains Chapters 21 to 24 Chapter 21 Origins of Modern Astronomy Origins of Modern Astronomy opens by examining the astronomical observations and contributions made by early civilizations and the Greek philosophers, including Aristotle, Aristarchus, Hipparchus, as well as Ptolemy. An in-depth examination of the birth of modern astronomy centers on the contributions of Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei, and Sir Isaac Newton. Following a brief overview of the constellations, a system for locating stars in the sky is presented. The primary motions of Earth are also described in detail. The chapter concludes with discussions of the phases of the Moon, lunar motions, and eclipses. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 21.1 Explain the geocentric view of the solar system and describe how it differs from the heliocentric view. 21.2 List and describe the contributions to modern astronomy of Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, Galileo Galilei, and Isaac Newton. 21.3 Compare the equatorial system of coordinates used to establish the position of the stars with longitude and latitude. Explain how the positions of stars are described using declination and right ascension. 21.4 Describe the two primary motions of Earth and explain the difference between a solar day and a sidereal day. 21.5 Sketch the changing positions of the Earth–Moon system that produce the regular cycle we call the phases of the Moon. 21.6 Sketch the positions of the Earth–Moon system that produce a lunar eclipse, as well as a solar eclipse. TEACHING ORIGINS OF MODERN ASTRONOMY During the first class on this topic, give students an overview of what the night sky will be like for the coming week. Have them go outside, if the weather permits, and encourage them to make their own observations and record what they have seen. Have your students reflect on the night sky. Have them try to think of it from the perspective of ancient civilizations who did not have the benefit of modern technology regarding astronomy. Have students research the concept of time and calendars and try to reconcile a calendar based on a lunar cycle with one based on an Earth’s annual orbit cycle. Point out some examples of calendars used by past civilizations, such as ancient Arab cultures using a strictly lunar-based calendar or the French Revolutionary calendar that had 12 months of equal days. Have your students find other examples of calendars unlike our own that were based on astronomical phenomena. Instruct students to select a star or constellation that will be visible rising in the night sky as you begin this unit. You may need to select the star or constellation for them and give them detailed instructions on where to find it and what it looks like. Then have your students note its location in the sky at the same time over the course of a week or two. They should observe that this star or constellation will rise about a half hour earlier after one week, illustrating the difference between a sidereal and a solar day. Have students make their own sundials or bring a sundial to class. Discuss how sundials work and calibrate it to your local latitude. A common misconception is that the noon Sun is directly overhead every day, regardless of your location on Earth. Discuss the tilt of the Earth and have students determine the furthest latitudes where it is possible for the noon Sun to ever be overhead (23.5o North and South). Let students figure this out and struggle with it a bit if needed; it is easier for them to correct this misconception in their own minds if they can reconcile it for themselves. Have your students do the Astronomy Interactives (see Additional Resources) on their computers. These simulations of planetary motion and phases of the Moon can give students a better grasp of these motions than if you were to just explain the motions to them. You can illustrate phases of the Moon with Styrofoam balls that are 4 to 6 inches in diameter and a light bulb. Have your students work in small groups and move the balls relative to the light bulb to see how much of the balls are illuminated by the light bulb “Sun.” Have them draw their own conclusions about how the phase of the Moon work. Discuss the political and cultural forces at work during the times of Ptolemy, Galileo, Copernicus, Brahe, and Kepler. Have students put these in context of what these scientists were able to accomplish despite the politics, Church teachings, and culture of the times. Many students have difficulty understanding the concept of precession. You can do a simple demonstration using a toy top or a gyroscope to show how the axis can change, or experience precession. CONCEPT CHECK ANSWERS Concept Check 21.1 Why did the ancients believe that celestial objects had some influence over their lives? Answer: They realized that seasonal changes were related to the positions of celestial bodies, so they figured that these objects could control other aspects of their lives. What is the modern explanation of “guest stars” that suddenly appear in the night sky? Answer: Supernovae. Explain the geocentric view of the universe. Answer: Earth is a motionless sphere at the center of the universe. The Sun, Moon, and planets orbit the Earth and beyond these objects is a celestial sphere to which stars are attached. In the Greek model of the universe, what were the seven wanderers, or planetai? How were they different from stars? Answer: They were planets and differed from stars because their appearances and motions in the sky were different. Describe what produces the retrograde motion of Mars. What geometric arrangements did Ptolemy use to explain this motion? Answer: Earth orbits the Sun faster than Mars; when Earth passes Mars in orbit, Mars appears to be moving backwards, or retrograde. Ptolemy explained this motion with epicycles. Concept Check 21.2 What major change did Copernicus make in the Ptolemaic system? Why was this change philosophically different? Answer: Copernicus proposed a heliocentric model with the Sun at the center of the solar system. It was philosophically different because it was considered heretical to not view Earth as the center of the universe. What data did Tycho Brahe collect that was useful to Johannes Kepler in his quest to describe planetary motion? Answer: Observations about the locations of Mars. Who discovered that planetary orbits are ellipses rather than circles? Answer: Johannes Kepler. Does Earth move faster in its orbit near perihelion (January) or near aphelion (July)? Answer: Perihelion. Explain why Galileo’s discovery of a rotating Sun supports the Copernican view of a Sun-centered universe. Answer: He tracked the movement of sunspots and their rotation, supporting the idea that the planets rotate around the Sun. His observations of the phases of Venus also support this. Newton discovered that the orbits of the planets result from opposing forces. Briefly explain these forces. Answer: The force of inertia keeps objects moving in their original direction and the force of gravitation pulls objects towards each other. Concept Check 21.3 How do modern astronomers use constellations? Answer: They use them to roughly identify the area of the night sky they are observing. How many constellations are currently recognized? Answer: Eighty-eight. How are the brightest stars in a constellation denoted? Answer: They are named in order of brightness by letters of the Greek alphabet with the name of the parent constellation. Briefly describe the equatorial system. Answer: It divides the celestial sphere into coordinates similar to Earth’s latitude and longitude system. This sphere appears to rotate around an imaginary line that extends from Earth’s axis, so the north celestial pole is near the North Star. Concept Check 21.4 Describe the three primary motions of Earth. Answer: Rotation – Earth rotates on its axis once every 24 hours. Revolution – Earth orbits the Sun once per year. Precession – Earth’s axial tilt slowly changes direction every 26,000 years. Explain the difference between the mean solar day and the sidereal day. Answer: The mean solar day is the interval between noon on one day and noon the next day. The sidereal day is how long it takes Earth to make one rotation, which is 23 hours, 56 minutes, 4 seconds. Define the ecliptic. Answer: This is the apparent path of the Sun during the course of one year. Why does axial precession have little effect on the seasons? Answer: Earth’s tilt changes only slightly on a short-term basis. Concept Check 21.5 Compare the synodic month with the sidereal month. Answer: The synodic month is 29.5 days, or how long it requires for the Moon to pass through all its phases. The sidereal month is how long it takes the Moon to orbit Earth, which is 27.3 days. What is the approximate length of the cycle of the phases of the Moon? Answer: 29.5 days. What phenomenon results from the fact that the Moon’s periods of rotation and revolution are the same? Answer: The same side of the Moon always faces Earth. The Moon rotates very slowly (once in 27 1/3 days) on its axis. How does this affect the lunar surface temperature? Answer: There is a high temperature on the day side of the Moon and a very low temperature on the night side. What is different about the crescent phase that precedes the new-Moon phase and that which follows the new-Moon phase? Answer: The side on which the crescent appears is different. What phase of the Moon occurs approximately 1 week after the new Moon? 2 weeks? Answer: 1 week after the new Moon is the first quarter. 2 weeks after is the full Moon. Concept Check 21.6 Sketch the locations of the Sun, Moon, and Earth during a solar eclipse and during a lunar eclipse. Answer: See Figures 21.26 and 21.27. Solar Eclipse Lunar Eclipse How many eclipses normally occur each year? Answer: Four. Solar eclipses are slightly more common than lunar eclipses. Why, then, is it more likely that your region of the country will experience a lunar eclipse than a solar eclipse? Answer: A lunar eclipse is visible to anyone on the side of Earth facing the Moon but a solar eclipse is only visible for a geographically narrow zone that is never wider than 275 kilometers, the diameter of the Moon’s shadow. How long can a total eclipse of the Moon last? How about a total eclipse of the Sun? Answer: A total eclipse of the Moon can last 4 hours and a total eclipse of the Sun can last 7 minutes. GIVE IT SOME THOUGHT ANSWERS Refer to Figure 21.3 and imagine that Eratosthenes had measured the difference in the angles of the noonday Sun between Syene and Alexandria to be 10 degrees instead of 7 degrees. Consider how this measurement would have affected his calculation of Earth’s circumference as you answer the following questions. Would this new measurement lead to a more accurate calculation? Would this new measurement lead to an estimate for the circumference of Earth that is larger or smaller than Eratosthenes’s original estimate? Answer: No, it would not be more accurate. This new measurement would lead to an estimate of Earth’s circumference that is larger than Eratosthenes’s original estimate. Use Kepler’s third law to answer the following questions: Determine the period of a planet with a solar distance of 10 AU. Determine the distance between the Sun and a planet with a rotational period of 5 years. Imagine two bodies, one twice as large as the other, orbiting the Sun at the same distance. Which of the bodies, if either, would move faster than the other? Answer: 31.6 years. 4.6 AU. Neither of the two bodies would move faster than the other – Kepler’s third law states that the orbital period of an object is mathematically related to the mean solar distance. Galileo used his telescope to observe the planets and moons in our solar system. These observations allowed him to determine the positions and relative motions of the Sun, Earth, and other objects in the solar system. Refer to Figure 21.15A, which shows an Earth-centered solar system, and Figure 21.15B, which shows a Sun-centered solar system, to complete the following: Describe the phases of Venus that an observer on Earth would see for the Earth-centered model of the solar system. Describe the phases of Venus that an observer on Earth would see for the Sun-centered model of the solar system. Explain how Galileo used observations of the phases of Venus to determine the correct positions of the Sun, Earth, and Venus. Answer: Venus would only be visible in a crescent phase. All phases of Venus will be visible. Galileo’s observations of the phases of Venus provided definitive evidence that the Sun is the center of the solar system – the particular sequence of the phases could only occur with a heliocentric solar system. Refer to the accompanying diagram, which shows three asteroids (A, B, and C) that are being pulled by the gravitational force exerted on them by their partner asteroid shown on the left. How will the strength of the gravitational force felt by each asteroid (A, B, and C) compare? (Assume that all these asteroids are composed of the same material.) Answer: A will feel the strongest force, B will feel the weakest gravitational force, C will feel a force between that of A and B. Refer to the accompanying diagram, which shows two pairs of asteroids, Pair D and Pair E. Is it possible for the asteroids in Pair D to be experiencing the same degree of gravitational force as the asteroids in Pair E? Explain your answer. Answer: Yes. Even though the asteroids in pair A are much smaller, they are closer together than those in pair B. The asteroids in pair B are much larger, but being much further apart would perhaps give them the same gravitational attraction as those in pair A. Imagine that Earth rotates on its axis at half its current rate. How much time would be required to capture the time-lapse photo shown in Figure 21.19? Answer: It would take twice as long. If we were able to reverse the direction of Earth’s rotation, would the solar day be longer, shorter, or the same? Answer: The length of the solar day would be the same. Refer to the accompanying photo of the Moon to complete the following: When you observe the phase of the Moon shown, is the Moon waxing or waning? What time of day can this phase of the Moon be observed? Answer: The Moon is waning. Approximately 3:00 am to 3:00 pm. Imagine that you are looking up at a full Moon. At the same time, an astronaut on the Moon is viewing Earth. In what phase will Earth appear to be from the astronaut’s vantage point? Sketch a diagram to illustrate your answer. Answer: An astronaut on the Moon during the full-Moon phase would look towards Earth and see the “new” phase—none of the lit side of Earth would be visible. If the Moon’s orbit were precisely aligned with the plane of Earth’s orbit, how many eclipses (solar and lunar) would occur in a 6-month period of time? If the Moon’s orbit were tilted 90 degrees with respect to the plane of Earth’s orbit, how many eclipses (solar and lunar) would occur in a 6-month period? Answer: 6 solar and 6 lunar eclipses would occur in 6 months. If the Moon’s orbit were tilted 90 degrees, there would be no eclipse. EXAMINING THE EARTH SYSTEM ANSWERS Currently, Earth is closest to the Sun (perihelion) in January (147 million kilometers [91.5 million miles]) and farthest from the Sun in July (152 million kilometers [94.5 million miles]). As a result of the precession of Earth’s axis, 12,000 years from now perihelion (closest) will occur in July, and aphelion (farthest) will take place in January. Assuming no other changes, how might this change average summer temperatures for your location? What about average winter temperatures? What might the impact be on the biosphere and hydrosphere? (To aid your understanding of the effect of Earth’s orbital parameters on the seasons, you may want to review the section “Variations in Earth’s Orbit” in Chapter 6, page 191.) Answer: Because interplanetary space is essentially a vacuum, the difference between perihelion and aphelion of about 3 million miles has little influence on the quantity of radiation received by Earth. Considering that the primary cause of the seasons, the amount of solar energy received by any place on Earth on any day, and hence temperature, is due to the angle (intensity) of the Sun’s rays and the length of daylight and not Earth’s distance from the Sun, any changes in summer or winter temperatures would be minimal. Consequently, the overall impact on the biosphere and hydrosphere would be negligible. In what ways do the interactions between Earth and its Moon influence the Earth system? If Earth did not have a moon, how might the atmosphere, hydrosphere, geosphere, and biosphere be different? Answer: Perhaps the two most obvious interactions between the Earth and Moon are gravity and the light cast on Earth during the brighter phases of the Moon. Without the gravitational interaction, there would be no tidal effects on Earth. Ocean tides would not exist; the heat exchange between the ocean and atmosphere would be altered, and hence atmospheric heating and circulation would be modified; the tidal influence on the solid Earth would not exist; and because of the lack of ocean tides, ocean life, especially that in today’s tidal areas and shallow waters, would be different. Without a Moon and the light it casts, the lives of nocturnal animals would be altered as well as the migratory habits of some species. ADDITIONAL RESOURCES DVDs and Movies The Mechanical Universe…and Beyond. (1985) California Institute of Technology, 30 minutes each. All of the following are appropriate for this chapter and available for free streaming from http://www.learner.org/resources/series42.html • Episode 1: Introduction o Episode 8: The Apple and the Moon – Newton discovers forces between particles in the universe. o Episode 21: Kepler’s Three Laws Galileo’s Battle for the Heavens (2002) NOVA, PBS, 1 hour, 53 minutes. Galileo’s famous struggle with the church. Available on DVD or for free streaming from http://www.pbs.org/wgbh/nova/ ancient/galileo-battle-for-the-heavens.html The Founders of Modern Astronomy (2010) NOVA, PBS, 5 minutes. Discusses William and Caroline Herschel. Available for free streaming from http://www.pbs.org/wgbh/nova/pace/foundersastronomy.html Websites Astronomy Picture of the Day. http://apod.nasa.gov/apod/astropix.html Moon Exploration Photo Gallery. http://science.nationalgeographic.com/science/photos/moonexploration-gallery/ NASA. http://www.nasa.gov Phases of the Moon. From the U.S. Navy. Includes short video clip. http://aa.usno.navy.mil/faq/docs/ moon_phases.php Ptolemaic System Simulator. http://astro.unl.edu/naap/ssm/animations/ptolemaic.swf Astronomy Interactives. Includes simulation of orbital motion using Kepler’s laws and phases of the Moon. http://astro.unl.edu/interactives/ Chapter 22 Touring Our Solar System Touring Our Solar System opens with an investigation of the differences between the terrestrial and Jovian planets, followed by some general comments about the atmospheres of the planets. Included in the chapter is a detailed study of the Moon’s physical characteristics and history. An inventory of the solar system presents the prominent features and peculiarities of each planet (excluding Earth). The chapter closes with a discussion of the minor members of the solar system—asteroids, comets, and meteoroids. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 22.1 Describe the formation of the solar system according to the nebular theory. Compare and contrast the terrestrial and Jovian planets. 22.2 List and describe the major features of Earth’s Moon and explain how maria basins were produced. 22.3 Outline the principal characteristics of Mercury, Venus, and Mars. Describe their similarities to and differences from Earth. 22.4 Compare and contrast the four Jovian planets. 22.5 List and describe the principal characteristics of the small bodies that inhabit the solar system. TEACHING TOURING OUR SOLAR SYSTEM Give students a chart with the names of the planets and their distance from the Sun. Have students determine a scale that they can create. If you have the space and cooperative weather, you can give students meter sticks, measuring tapes, and sidewalk chalk. Have your class, working in groups, draw their scale models of distance on a piece of sidewalk outside, and then have each group explain their scales to the rest of the class. If you prefer, you can also have students construct their scale models inside using adding machine tape. This exercise is for scaling distances only; for an extra challenge you can have students construct a scale model of both distance and planetary diameters. Many students do not have a concept of just how large Jupiter is, or of planetary sizes relative to each other. Do the Play Doh planets scale model of the solar system exercise (see Additional Resources). It is a fun activity that drives home just how the planets differ in size from each other. You can demonstrate cratering with a pan of either mud or flour and balls of assorted sizes. You can roll modeling clay into different size balls, which will be bolides. Have your students predict what size of the crater each “bolide” will make, and then have the students drop the clay balls into the pan. Have your students discuss the sizes of the craters that the different size balls made. Have them relate this activity to cratering on solar system objects. Students sometimes do not understand how Jovian planets, Saturn, for example, can be of such low density that they could float on water despite their large size. You can prepare several objects of similar size but different masses to illustrate this. For example, you can have a ping pong ball, a similar-sized metal ball, and a clay ball also of similar size. Have the students pick up each object in turn. They will note the different weights of each; this can help them understand how different compositions can lead to different planetary densities. Often students take it personally that Pluto has lost planetary status. You can have your class, working in small groups; make a case either for restoring Pluto as a planet or leaving its demotion in place. Students need to come up with valid scientific arguments for their case. In order to do so, they will need to cement their understandings of what makes a planet a planet. Show students photographs of astronauts’ footprints on the Moon and explain that they are still there, more than 40 years later. Ask students how this can possibly be true. This can lead to a discussion of atmospheres and how weathering exists on Earth but the lack of atmosphere on Moon does not allow it. Bring samples of igneous rocks to class, especially basalts and granite. Ask students what rocks they would expect to see on the terrestrial planets. Have them recall the composition of various igneous rocks. Then ask why they think the gas giants are so named and relate their locations and compositions to where they formed in the early days of the solar system. Have students discuss the recent expeditions to Mars and research how long it took to get there (9 months). You might also discuss how long it is taking us to send a probe to Pluto. Discuss these time frames in the context of how far objects in our own solar system are and the difficulties of exploration even close to “home.” Any of the movies listed in Additional Resources are good ways to give students a visual perspective on the solar system. Often it is difficult for them to conceptualize these abstract ideas and showing them how the solar system works is a good way to help students understand its nature. CONCEPT CHECK ANSWERS Concept Check 22.1 Briefly outline the steps in the formation of our solar system, according to the nebular theory. Answer: The Sun and planets began to form in a rotating cloud of nebular gas and dust. These materials eventually clumped together with a hot protosun at the center and chunks of planetesimals rotating around it. Through repeated collisions, the planetesimals grew into protoplanets, and eventually the solar system came about. By what criteria are planets considered either terrestrial or Jovian? Answer: Location, size, and density. What accounts for the large density differences between the terrestrial and Jovian planets? Answer: Different chemical compositions. Explain why the terrestrial planets have meager atmospheres, as compared to the Jovian planets. Answer: Terrestrial planets were formed in a region where it was too hot for ice and gas to condense but the Jovian planets formed in colder locations. The terrestrial planets also are too small to exert the gravitational pull required to hold very light gases in their atmospheres. Why are impact craters more common on the Moon than on Earth, even though the Moon is a much smaller target and has a weaker gravitational field? Answer: Earth has a more substantial atmosphere than the Moon, causing more incoming bolides to burn up in the atmosphere before impact. When did the solar system experience the period of heaviest planetary impacts? Answer: In the early time of formation. Concept Check 22.2 Briefly describe the origin of the Moon. Answer: A bolide the size of Mars collided with a young, molten Earth, causing ejected debris to be thrown into orbit around the Earth. Eventually this material condensed into the Moon. Compare and contrast the Moon’s maria and highlands. Answer: Maria are smooth, basaltic plains. Highlands are elevated several kilometers above the maria and are breccias. The maria tend to appear dark and the highlands appear light from Earth. How are maria on the Moon similar to the Columbia Plateau in the Pacific Northwest? Answer: They are both made from fluid basaltic lavas that spread across a flat area. How is crater density used in the relative dating of surface features on the Moon? Answer: The greater the crater density, the older the feature is supposed to be. List the major stages in the development of the modern lunar surface. Answer: Formation of the original crust, excavation of the large impact basins, filling of maria basins, and formation of rayed craters. Compare and contrast the processes of weathering and erosion on Earth with the same processes on the Moon. Answer: The Moon lacks an atmosphere so erosion is largely from tiny particles from space bombarding the surface. There is no weathering as there is on Earth, which is a result of atmospheric processes. Concept Check 22.3 What body in our solar system is most like Mercury? Answer: Earth’s Moon. Why are the surface temperatures so much higher on Venus than on Earth? Answer: Venus has a thick atmosphere of carbon dioxide that causes an extreme greenhouse effect. Venus was once referred to as “Earth’s twin.” How are these two planets similar? How do they differ from one another? Answer: Venus and Earth are of very similar sizes. Both are terrestrial planets, with volcanic activity, surface topography, and mantle upwelling. However, the atmospheres of these two planets are very different, as are the surface temperatures, with Venus being much hotter. As a result, Venus has volcanoes that are shorter and wider. What surface features do Mars and Earth have in common? Answer: Volcanoes, polar ice caps, lava plains, and sand dunes. Why are the largest volcanoes on Earth so much smaller than the largest ones on Mars? Answer: Mars lacks plate tectonics; successive volcanic eruptions will accumulate in the same place rather than the volcanoes being relocated by way of plate motion. What evidence suggests that Mars had an active hydrologic cycle in the past? Answer: Stream-like features with teardrop shaped islands suggest flowing water, valleys that appear to have been made by catastrophic flooding, dendritic drainage networks, and minerals that form only in the presence of water are all found on Mars. Concept Check 22.4 What is the nature of Jupiter’s Great Red Spot? Answer: It is a giant cyclonic storm. Why are the Galilean satellites of Jupiter so named? Answer: They were discovered by Galileo. What is distinctive about Jupiter’s satellite Io? Answer: It is one of only three volcanically active bodies other than Earth known to exist in the solar system. Why are many of Jupiter’s small satellites thought to have been captured? Answer: They revolve in the opposite direction of the largest moons and have eccentric orbits. How are Jupiter and Saturn similar to one another? Answer: They both have dynamic atmospheres made of hydrogen and helium, both have rings, both have many satellites, and both are the two largest planets. What two roles do ring moons play in the nature of planetary ring systems? Answer: They exert gravitational pull on rings by altering their orbits and they also sweep up ring particles and subsequently eject them. How are Saturn’s satellite Titan and Neptune’s satellite Triton similar to one another? Answer: They are both the only satellites in the solar system known to have substantial atmospheres. Name three bodies in the solar system that exhibit active volcanism. Answer: Earth, Venus, Io. Concept Check 22.5 Where are most asteroids found? Answer: In the asteroid belt. Compare and contrast asteroids and comets. Answer: Asteroids are made of rocky and/or metallic material and are similar in composition to the terrestrial planets. Comets are less consolidated and made of ice, dust, and small rocky particles more typical of the outer portion of the solar system. What do you think would happen if Earth passed through the tail of a comet? Answer: There would be more particulate matter in the atmosphere and perhaps some meteor showers due to burning up of comet particles in the atmosphere. Where are comets thought to reside? What eventually becomes of comets that orbit close to the Sun? Answer: Comets are thought to reside in the Oort cloud and the Kuiper belt. Comets that orbit close to the Sun eventually vaporize. Differentiate among the following solar system bodies: meteoroid, meteor, and meteorite. Answer: A meteoroid is a small, solid particle that has entered Earth’s atmosphere from space. A meteor is the streak of light seen as the meteoroid burns in the atmosphere. A meteorite is the remains of a meteoroid that makes it all the way to Earth’s surface. What are the three main sources of meteoroids? Answer: Interplanetary debris missed by the gravitational sweep of the planets during the solar system formation, material that is ejected from the asteroid belt, and the rocky and/or metallic remains of comets that once passed through Earth’s orbit. Why was Pluto demoted from the ranks of the officially recognized planets? Answer: Pluto was reclassified as a dwarf planet. It was not large enough to clear its orbit of other debris, and other, larger Kuiper belt objects were discovered. Pluto also has an orbit that is dissimilar to the other 8 planets. GIVE IT SOME THOUGHT ANSWERS Assume that a solar system has been discovered in a nearby region of the Milky Way Galaxy. The accompanying table shows data that have been gathered about three of the planets orbiting the central star of this newly discovered solar system. Using Table 22.1 as a guide, classify each planet as either Jovian, terrestrial, or neither. Explain your reasoning. Answer: Planet 1 – terrestrial because of size and density. Planet 2 – Jovian because of size and density. Planet 3 – neither because of distance and orbital eccentricity. In order to conceptualize the size and scale of Earth and Moon as they relate to the solar system, complete the following: Approximately how many Moons (diameter 3475 kilometers [2160 miles]) would fit side-by-side between Earth and the Moon? Given that the Moon’s orbital radius is 384,798 kilometers, approximately how many Earths would fit side-by-side between Earth and the Moon? Approximately how many Earths would fit side-by-side across the Sun, whose diameter is about 1,390,000 kilometers? Approximately how many Suns would fit side-by-side between Earth and the Sun, a distance of about 150,000,000 kilometers? Answer: 3.7 Moons. 768,798 km/2 = 384,399/12,756 = 30. 109 Earths. 150,000,000 km/1,390,000 = 108 Suns. The accompanying graph shows the temperatures at various distances from the Sun during the formation of our solar system. Use it to complete the following: Which planets formed at locations where the temperature in the solar system was hotter than the boiling point of water? Which planets formed at locations where the temperature in the solar system was cooler than the freezing point of water? Answer: Mercury, Venus, Earth, Mars. Jupiter, Saturn, Uranus, Neptune. The accompanying sketch shows four primary craters (A, B, C, and D). The impact that produced crater A produced two secondary craters (labeled “a”) and three rays. Crater D has one secondary crater (labeled “d”). Rank the four primary craters from oldest to youngest and explain your ranking. Answer: Crater C is oldest, then crater D, then crater A, then crater B is the youngest. Crater C has secondary craters from A and D, so it must be older than them. Crater A has bright rays emanating from it, indicating that it is young. Crater B is superimposed on A, meaning B must be younger than A. The rays from A crosscut crater D, meaning D must predate A. The accompanying diagram shows two of Uranus’s moons, Ophelia and Cordelia, which act as shepherd moons for the Epsilon ring. Explain what would happen to the Epsilon ring if a large asteroid struck Ophelia, knocking it out of the Uranian system. Answer: The removal of Ophelia would allow for the Epsilon ring to fall apart and the particles that comprise the ring system would begin to scatter outward. It has been estimated that Halley’s Comet has a mass of 100 billion tons. Furthermore, it is estimated to lose about 100 million tons of material when its orbit brings it close to the Sun. With an orbital period of 76 years, calculate the maximum remaining life span of Halley’s Comet. Answer: 100,000,000,000/100,000,000 = 1000 × 76 years = 76,000 years remaining. The accompanying diagram shows a comet traveling toward the Sun at the first position where it has both an ion tail and a dust tail. Refer to this diagram to complete the following: For each of the three numbered sites, indicate whether the comet will have no tails, one tail, or two tails. If one tail or two tails are present, in what direction will they point? Would your answers to the preceding question change if the Sun’s energy output were to increase significantly? If so, how would they change? If the solar wind suddenly ceased, how would this affect the comet and its tails? Answer: 1) one tail – pointing up and to the right. 2) two tails – both pointing towards the left. 3) 1, maybe 2 tails pointing towards the lower left. If the Sun’s output were to increase significantly, the tails would become longer and perhaps the two tails at position 2 might merge into a single entity. If the solar wind ceased, the tails would decrease in size significantly, but the light from the Sun would still cause a tail to form. The comet would continue to exhibit a coma as it came close to the Sun. Assume that three irregularly shaped planet-like objects, each smaller than our Moon, have just been discovered orbiting the Sun at a distance of 35 AU. One of your friends argues that the objects should be classified as planets because they are large and orbit the Sun. Another friend argues that the objects should be classified as a dwarf planet, such as Pluto. State whether you agree or disagree with either or both of your friends. Explain your reasoning. Answer: Although the planets are quite small, they are orbiting in the region of the solar system that is occupied by the terrestrial planets. Mercury, the smallest terrestrial planet, is not much larger than our Moon so I would agree with the argument that the new bodies are actually planets because of their location. It would be difficult to classify the bodies as dwarf planets since Pluto and the other dwarf planets orbit well beyond the orbit of Neptune. It would be helpful to know the density of the new bodies to see if they are similar to that of the terrestrial planets (since Pluto is thought to have a low density), but the argument of them being planets makes more sense. EXAMINING THE EARTH SYSTEM ANSWERS On Earth the four major spheres (atmosphere, hydrosphere, geosphere, and biosphere) interact as a system with occasional influences from our near-space neighbors. Which of these spheres are absent, or nearly absent, on the Moon? Because the Moon lacks these spheres, list at least five processes that operate on Earth but are absent on the Moon. Answer: Of the four spheres, the atmosphere, hydrosphere, and biosphere are absent, or nearly absent (there is some indication of frozen water on the Moon) on the Moon. Because the Moon lacks these spheres, processes such as chemical weathering; erosion by wind, water, and ice; soil formation; weather in general; and sedimentation and lithification are all absent. Among the planets in our solar system, Earth is unique because water exists in all three states (solid, liquid, and gas) on and near its surface. In what state(s) of matter is water found on Mercury, Venus, and Mars? How would Earth’s hydrologic cycle be different if its orbit were inside the orbit of Venus? How would Earth’s hydrologic cycle be different if its orbit were outside the orbit of Mars? Answer: No water, in any state, has been detected on Mercury. Only scant water vapor has been detected in the Venusian atmosphere. The Martian atmosphere contains small amounts of water vapor, and the polar caps are made of water ice. If Earth’s orbit were inside the orbit of Venus, the liquid water currently found on Earth would be vaporized by the much higher temperatures and perhaps be driven away from the planet. Ice would not exist; and condensation, cloud formation, and precipitation would not likely take place. Without precipitation, runoff and infiltration would not occur. If Earth’s orbit were outside the orbit of Mars, the extreme cold would freeze all water, and only ice would exist. With only frozen water, there would be no precipitation, runoff, or infiltration. In essence, the hydrologic cycle would not exist. If a large meteorite were to strike Earth in the near future, what effect might this event have on the atmosphere (in particular, on average temperatures and climate)? If these conditions persisted for several years, how might the changes influence the biosphere? Answer: If a large meteorite were to strike Earth, the results could be disastrous. A large impact would add great amounts of dust to the upper atmosphere and significantly reduce the sunlight reaching the surface. If the condition persisted, global temperatures would fall, precipitation patterns would be altered, and climate in general would be significantly changed. The eventual effect on the biosphere would be to cause large- scale extinctions, such as those that occurred in conjunction with the extinction of the dinosaurs about 65 million years ago. ADDITIONAL RESOURCES DVDs and Movies How the Earth Was Made (2008) Narrated by Alec Baldwin. History Channel, 1 hour, 34 minutes. Available on DVD. Earth Revealed, Episode 1: Down to Earth (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Earth Revealed, Episode 2: The Restless Planet (1992) Annenberg Media, 30 minutes. Available on DVD or for free streaming video on demand from http://www.learner.org/resources/series78.html Planet Earth, Episode 4: Tales from Other Worlds (1986) WQED and National Academy of Science, 1 hour. Available for free streaming from http://www.learner.org/resources/series49.html The Planets. (1999) BBC, 49 minutes each. Includes eight episodes, including Different Worlds, Terra Firma, Giants, Moon, Star, Atmosphere, Life, and Destiny. Available on DVD. Origins: Earth is Born (2004) NOVA, PBS, 51 minutes. Available for free streaming from http://www.pbs.org/wgbh/nova/space/origins-series-overview.html#origins-earth-born The Pluto Files (2011) NOVA, PBS, 53 minutes. Discusses how Pluto lost planetary status. Available for free streaming from http://www.pbs.org/wgbh/nova/space/pluto-files.html Websites Evolution of Our Solar System – Framework for an interactive classroom activity where students learn the order of events that created the solar system. http://www.lpi.usra.edu/education/timeline/ activity/ Earth Science Picture of the Day – Various pictures that can open up discussion in the classroom http://epod.usra.edu Ideas for Creating Early Earth Teaching Activities. From Carleton University. http://serc.carleton.edu/ NAGTWorkshops/earlyearth/ideas.html Mars Exploration Rovers. From NASA. http://marsrover.nasa.gov/classroom/ Nine Planets. Detailed information on solar system objects. http://nineplanets.org Voyager approaching Jupiter short animation. http://www.planetary.org/multimedia/space-images/ jupiter/voyager-1-jupiter-approach-movie.html Chapter 23 Light, Astronomical Observations, and the Sun Light, Astronomical Observations, and the Sun begins with an examination of the nature of electromagnetic radiation and how it is used to gather information concerning the state of matter, composition, temperature, and motion of stars and other celestial objects. After an investigation of the nature of light, the focus shifts to astronomical tools and how they are used to intercept and study the energy emitted by distant objects in the universe. It includes discussions of optical telescopes, radio telescopes, and space-based observatories. The chapter concludes with descriptions of the structure of the Sun, some features that occur on the active Sun, and the source of the Sun’s energy. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 23.1 List and describe the various types of electromagnetic radiation. 23.2 Explain how the three types of spectra are generated and what they tell astronomers about the radiating body that produced them. 23.3 Compare and contrast refracting and reflecting telescopes. Explain why modern telescopes are built on mountaintops. 23.4 Explain the advantages of radio telescopes and orbiting observatories over optical telescopes. 23.5 Write a statement explaining why the Sun is important to the study of astronomy. Sketch the Sun’s structure and describe each of its four major layers. 23.6 List and describe the three types of explosive activity that occur at the Sun’s surface. 23.7 Summarize the process called the proton-proton chain reaction. TEACHING LIGHT, ASTRONOMICAL OBSERVATIONS, AND THE SUN If you have access to an infrared camera, using it in a demonstration can help students understand how we can see different parts of the spectrum using the right equipment. You can bring something that can be heated to class, along with an ice cube. You can use the ice cube to write a letter or number on skin and use the infrared camera to reveal what you have written. Relate this to different portions of the electromagnetic spectrum and how astronomers “see” with different tools. The Doppler shift is easy to demonstrate in a classroom and can be an effective way to illustrate this concept. If you have an object that can make a continuous sound, such as a toy fire engine with a siren, you can move this object towards and away from students to demonstrate the change in pitch of the sound waves. Explain that light waves behave in a similar fashion. When discussing refracting telescopes, it is easy to demonstrate refraction using a clear container half filled with water and a ruler. Show students that the ruler is straight, and then submerge half of it in the water. The ruler appears to bend, or refract, where it enters the water. If you have access to a planetarium, you can demonstrate the effects of light pollution. Pull up the night sky with no light pollution and have students observe how many stars they can see. Then add the effects of a city light glow and have students observe the difference. Discuss how light pollution is obstructing our view of the night sky and how this can impact decisions about telescope placement. Sunspotter devices or solar telescopes are a good way to have students make their own observations of the Sun. The Sun should never be looked at with the naked eye, but these tools are safe ways to look at the Sun. Have students sketch the locations of sunspots on different occasions. Have them note what, if anything has changed. Compare their sketches to other sunspot data. See if students can determine where in the sunspot cycle we are at the moment. Discuss the aurora borealis. Have students determine how and why solar flares can impact Earth’s atmosphere. Give students the opportunity to figure out this phenomenon before you teach it. Have students work in groups to discuss the advantages and disadvantages of both land- and spacebased telescopes. Tell them to consider all factors, including costs, to determine how scientists and funding agencies decide what to construct or fund. Show students images from the Hubble Space Telescope. Discuss whether or not these images would be able to be obtained from a mountaintop observatory. CONCEPT CHECK ANSWERS Concept Check 23.1 What term is used to describe the collection that includes gamma rays, x-rays, ultraviolet light, visible light, infrared radiation, microwaves, and radio waves? Answer: Electromagnetic radiation. Which color has the longest wavelength? The shortest? Answer: Red has the longest wavelength and violet has the shortest. How does the amount of energy contained in a photon relate to its wavelength? Answer: Shorter wavelengths have photons of higher energy. Concept Check 23.2 What is spectroscopy? Answer: It is the study of the properties of light that are wavelength dependent. Describe a continuous spectrum. Give an example of a natural phenomenon that exhibits a continuous spectrum. Answer: A continuous spectrum is a continuous band of wavelengths produced by an incandescent solid, liquid, or gas under high pressure. A star exhibits a continuous spectrum. What can a continuous spectrum tell astronomers about a star? Answer: It can tell about the energy emitted by the star and the surface temperature of the star. What can be learned about a star (or other celestial objects) from a dark-line (absorption) spectrum? Answer: It can tell about the composition of the star or celestial object. What produces emission lines (bright lines) in a spectrum? Answer: Hot gaseous materials at low pressure. Briefly describe the Doppler effect and describe how astronomers determine whether a star is moving toward or away from Earth. Answer: There is a shift in waves of all media when they are approaching an object that is different from when they are moving away from an object. For light from a star, its light appears redder than it actually is if it is moving away and its light appears bluer than it really is if it is moving closer. Concept Check 23.3 What is the main difference between reflecting and refracting telescopes? Answer: Refracting telescopes use lenses to collect and focus light; reflecting telescopes use a curved mirror to collect and focus light. Why do astronomers seek to design telescopes with larger and larger mirrors? Answer: They allow for collection of light from faint and distant objects. Why do all large optical telescopes use mirrors rather than lenses to collect light? Answer: Lenses allow for chromatic aberration, or a halo of colored light to form around the object being viewed. Mirrors avoid this problem. What are the advantages of charge-coupled devices (CCDs) over photographic film? Answer: They can detect more incoming light than film, and they are easily calibrated for variations in wavelength sensitivity. They can also collect light over several nights and synthesize it into one image. Why is the human eye an ineffective tool for astronomical observation? Answer: Different people perceive light intensity and faint color differently, and people can introduce personal bias into observed data. Provide two reasons why the largest telescopes are built on mountaintops away from large cities. Answer: This avoids as much of Earth’s turbulent atmosphere and light pollution as possible. Concept Check 23.4 Why are radio telescopes much larger than optical telescopes? Answer: Radio signals from space are weak so large telescope dishes are needed to intercept a signal strong enough for detection. What are some of the advantages of radio telescopes over optical telescopes? Answer: Much radiation from space objects cannot penetrate the atmosphere and be detected by optical telescopes but can be detected by radio telescopes; radio waves are much stronger than visible radiation. Explain why space makes a good site for an optical observatory. Answer: There is no Earth atmosphere to distort or not allow penetration of optical signals and there is no light pollution. What can astronomers learn about the universe by studying it at multiple wavelengths? Answer: Different parts of the universe are more obvious at some wavelengths. For example, infrared wavelengths can tell us about the temperature of celestial objects and x-rays have given us data about black holes, quasars, and high-temperature gases that are not available from just optical viewing. Concept Check 23.5 Why is the Sun significant to the study of astronomy? Answer: It is the only star close enough to Earth to allow easy study of its surface. Describe the Sun in relationship to other stars in the universe. Answer: It is average; some stars are larger, some are smaller, some are hotter, some are cooler, some are more red, and some are more blue. Describe the photosphere, chromosphere, and corona. Answer: Photosphere – it appears as the bright disk of the Sun and radiates most of the light we see; it is considered to be the Sun’s surface. Chromosphere – the region above the photosphere that is a relatively thin layer of hot, glowing gases. Corona – the outer part of the Sun’s atmosphere, and is visible only when the photosphere is blocked. It is a tenuous layer of ionized gases. Why are there no distinct boundaries between the layers of the Sun? Answer: The Sun is gaseous throughout its entirety. Why is the photosphere considered the Sun’s “surface”? Answer: It makes up the bulk of the bright disk that we see and is separate from the incandescent gases that surround it. Briefly describe the solar wind. Answer: This is streams of protons and electrons that emanate from the Sun’s corona. These ionized gases are moving fast enough to escape the Sun’s gravity. Concept Check 23.6 What did Galileo learn about the Sun from his observations of sunspots? Answer: He was able to determine that the Sun rotates on its axis about once a month. Briefly describe the 11-year sunspot cycle. Answer: The number of sunspots on the sun varies in an 11-year cycle; similar numbers of sunspots occur every 11 years. What are prominences? Answer: Prominences may be eruptive or quiescent. They are giant cloud-like gaseous structures that appear as bright arches extending into the corona. Eruptive prominences have a more explosive quality than quiescent ones. How do solar flares affect the solar wind? Answer: They cause the solar wind to noticeably intensify. Concept Check 23.7 What “fuel” does the Sun consume? Answer: Hydrogen. What happens to the matter that is consumed in the proton-proton chain reaction? Answer: It is converted to radiant energy and helium atoms. GIVE IT SOME THOUGHT ANSWERS Refer to Figure 23.2 to answer the following questions: Is the atmosphere mostly transparent or opaque to visible light? Is the atmosphere mostly transparent or opaque to radio waves with a wavelength of 1 meter (3 feet)? Is the atmosphere mostly transparent or opaque to gamma rays? Answer: transparent transparent opaque Imagine that the composition of Earth’s atmosphere were altered so that its ability to absorb visible and far infrared light were reversed. If you were outdoors when the Sun was at its highest point in the sky, how would the sky appear? Would there be an increase or a decrease in Earth’s average surface temperature? Answer: The sky would appear red in color. There would be an increase in Earth’s average surface temperatures. Suppose a well-known scientist claimed that stars consist primarily of helium rather than hydrogen. What type of object in the galaxy could you study to investigate whether stars consist primarily of helium or hydrogen? How could spectroscopy help you verify or disprove the scientist’s claim? Explain your reasoning. Answer: You could study nebulae. The spectra of most stars are the dark-line type. Spectroscopy would allow you to determine the composition of the stars by examining the relative intensities of the light in the dark lines. Imagine that you are responsible for funding the construction of observatories. After considering the four proposals listed below, state whether you would or would not recommend funding for each proposal and explain your reasoning. Proposal A: A ground-based x-ray telescope on top of a mountain in Arizona, designed to observe supernovae in distant galaxies. Proposal B: A space-based 3-meter reflecting infrared telescope designed to observe very distant galaxies. Proposal C: A ground-based 8-meter refracting optical telescope located on the top of Mauna Kea in Hawaii, designed to measure the spectra of binary stars in our galaxy. Proposal D: A ground-based 250-meter radio telescope array in New Mexico, designed to measure the distribution of hydrogen gas clouds in the disk of our galaxy. Answer: Proposal A – No. X-ray telescopes are typically orbiting, so a ground-based unit would not work. Proposal B – Yes. Infrared energy is blocked by our atmosphere, so a spaced-based unit would work. Proposal C – No. Large refracting telescopes are plagued by chromatic aberration and their weight causes technical issues as well. Proposal D – Yes. Large, ground-based, radio telescopes are good for detecting hydrogen. An important absorption line in the spectrum of stars occurs at a wavelength of 656 nm for stars not moving toward or away from Earth. Imagine that you observe four stars in our galaxy and discover that this absorption line is at the wavelength shown in the accompanying diagram. Using this data, complete the following questions. Explain the reasoning behind your answers. If you are unable to determine the answer to any of these questions from the given information, explain. a. Which of these stars is moving toward Earth the fastest? Which of these stars is closest to Earth? Which of these stars is moving away from Earth? Answer: Star D is moving the fastest towards Earth since it has the greatest shift towards shorter wavelengths. You cannot determine from the information given as to which star is closest to Earth. Star C is moving away as it is the only one where the shift is towards longer wavelengths. Consider the following discussion among three of your classmates regarding why telescopes are put in space. Support or refute each statement. Student 1: “I think it is because the atmosphere distorts and magnifies light, which causes objects to look larger than they actually are.” Student 2: “I thought it was because some of the wavelengths of light being sent out from the telescopes can be blocked by Earth’s atmosphere, so the telescopes need to be above the atmosphere.” Student 3: “Wait, I thought it was because by moving the telescope above the atmosphere, the telescope is closer to the objects, which makes them appear brighter.” Answer: Student 1: Support – the statement is essentially true. Student 2: Refute – the atmosphere does block certain wavelengths of electromagnetic radiation, but not visible light. Also, the light is coming from objects in outer space, not from the telescope itself. Student 3: Refute – the distance that space-based telescopes orbit above the Earth is insignificant compared to the distance to planets, stars, or galaxies. Placing telescopes in orbit does not get them any “closer” to the objects we are viewing. 7. Refer to the accompanying spectra, which represent four identical stars in our galaxy. One star is not moving, another is moving away from you, and two stars are moving toward you. Determine which star is which and explain how you reached your conclusion. Answer: Star B is not moving. Star A is moving away from you (overall shift towards the red end of the spectrum). Stars C and D are moving towards you (both are shifted towards the blue end of the spectrum). EXAMINING THE EARTH SYSTEM ANSWER 1. Of the two sources of energy that power the Earth system, the Sun is the main driver of Earth’s external processes. If the Sun increased its energy output by 10 percent, what would happen to global temperatures? What effect would this temperature change have on the percentage of water that exists as ice? What would be the impact on the position of the ocean shoreline? Speculate about whether the change in temperature might produce an increase or a decrease in the amount of surface vegetation. In turn, what impact might this change in vegetation have on the level of atmospheric carbon dioxide? How would such a change in the amount of carbon dioxide in the atmosphere affect global temperatures? Answer: If the Sun’s output increased by 10 percent, global temperatures would also increase. As a consequence, less water would exist as ice, and the position of the shoreline would move inland as sea level rose. Warmer temperatures would produce an increase in vegetation, which, in turn, would remove more carbon dioxide from the atmosphere during photosynthesis. Because carbon dioxide is a heat-absorbing gas in the atmosphere, a reduction in the amount of carbon dioxide would result in a decrease in global temperatures. ADDITIONAL RESOURCES DVDs and Movies Planet Earth, Episode 5: The Solar Sea (1986) WQED and National Academy of Science, 1 hour. Available for free streaming from http://www.learner.org/resources/series49.html Secrets of the Sun (2012) NOVA, PBS, 53 minutes. Available for free streaming from http://www.pbs.org/wgbh/nova/space/secret-sun.html Hunting the Edge of Space (2010) NOVA, PBS, 106 minutes. Discusses exploration using different telescopes. Available for free streaming from http://www.pb.org/wgbh/nova/space/hunting-edgespace.html Saving Hubble Update (2009) NOVA Science NOW, PBS, 11 minutes. Available for free streaming from http://www.pbs.org/wgbh/nova/space/saving-hubble-update.html The City Dark (2012) Edgeworx Studios, 60 minutes. Discusses light pollution. Available on DVD. Websites Hubble Space Telescope. http://hubblesite.org NASA Hubble Telescope Website. http://www.nasa.gov/mission_pages/hubble/main/#.Ux3HFvldWSo National Optical Astronomy Observatory. http://www.noao.edu The City Dark: The Effects of Light Pollution. http://www.pbs.org/pov/city_dark_take_action/take_ action.php National Radio Astronomy Observatory. http://www.nrao.edu/index.php/learn/radioastronomy/ Chandra X-Ray Observatory. http://chandra.harvard.edu Chapter 24 Beyond Our Solar System Beyond Our Solar System begins with an examination of the intrinsic properties of stars—distance, brightness, color, and temperature. Binary star systems, stellar mass, and the Hertzsprung–Russell diagram are discussed in detail. Also investigated are the various types of nebulae. Stellar evolution, from birth through protostar, main sequence, red giant, burnout, and death, is presented. Following stellar evolution are descriptions of the various stellar remnants—dwarf stars, neutron stars, and black holes. The chapter continues with a detailed discussion of the Milky Way Galaxy and a general description of types of galaxies, galactic clusters, and red shifts. FOCUS ON CONCEPTS After reading, studying, and discussing the chapter, students should be able to: 24.1 Define cosmology and describe Edwin Hubble’s most significant discovery about the universe. 24.2 Explain why interstellar matter is often referred to as a stellar nursery. Compare and contrast bright and dark nebulae. 24.3 Define main-sequence star. Explain the criteria used to classify stars as giants. 24.4 List and describe the stages in the evolution of a typical Sun-like star. 24.5 Compare and contrast the final state of Sun-like stars to the remnants of the most massive stars. 24.6 List the three major types of galaxies. Explain the formation of large elliptical galaxies. 24.7 Describe the Big Bang theory. Explain what it tells us about the universe. TEACHING BEYOND OUR SOLAR SYSTEM Use different colored light bulbs that have the same number of watts and transparent filters of matching colors. Have students view the light bulbs through the different colored filters. The blue light bulb will appear brightest when viewed through a blue film. The red light bulb will appear brightest when viewed through a red film. Have students take the transparent filters home and view stars at night through the films. See if they can determine which stars are blue, which are red, etc. Turn this into a discussion relating star color to temperature. The H-R diagram is an important plot that may be confusing to some students initially. You can introduce this diagram by showing students plots that may make more sense to them. For example, show a plot of human height vs. weight, and how most people fall along the “main sequence.” Have students discuss why high-mass stars have a different evolutionary pathway than medium-mass or low-mass stars. Have students discuss the effects of star size and gravitational forces on this evolutionary pathway. A common misconception is that the Big Bang event initiated the formation of the solar system. It is important to make a distinction between these two events and be sure students understand that they are very different. Many students think that there is little evidence to support the Big Bang theory. Point out that it is a theory, not a hypothesis; this is a good time to review the scientific method. Have students try to come up with some of the data scientists have to show evidence for this theory, such as the red shift and the cosmic microwave background. Sometimes students have concepts of black holes as being cosmic vacuum cleaners that suck up and destroy everything. Explain that this is not true, and objects can even be in orbit around black holes. It is true that things fall into them, but they are not plowing through space destroying everything in their paths. Many people think stars are eternal and never die. After students have a grasp of stellar nebulae, have them try to observe the Orion Nebula in the evening. If you have access to telescopes and can arrange a viewing as a class, this is often easier as students don’t always know where to look in the sky. Discuss the nebula as being a cloud of dust and gas, as it looks like, and also as a place for the birth of new stars in the future. A misconception about galaxies is that they are all easily viewable in the night sky with an unaided eye. Point out that most galaxies are so far away that they can only be viewed with telescopes. As you discuss the Milky Way galaxy, ask students if there are other galaxies contained in it. While the answer is no, a common misconception is that there are other galaxies within the Milky Way. Have students discuss the possibility of life beyond our solar system. Have them think about what we would define as life. Have students come up with ideas for how we might find life beyond Earth. Discuss ways we have already searched for extraterrestrial life. The Voyager spacecraft, which recently exited our solar system, contained messages to any other life forms that might find them. Discuss how likely this scenario is and what other civilizations might make of our message. Show students an image of the gold record that traveled on Voyager. CONCEPT CHECK ANSWERS Concept Check 24.1 What is cosmology? Answer: It is the study of the universe, including its properties, structure, and evolution. Explain how Edwin Hubble used Cepheid variables to change our view of the structure of the universe. Answer: These bright variable stars cycle through brightness at a known rate. Hubble examined these in fuzzy patches in the sky and determined that these fuzzy patches, now known as galaxies, lie outside the Milky Way. Thus astronomers came to realize that the universe was much larger than had been thought. When do cosmologist think the universe began? Answer: 13.7 billion years ago. Which two elements were the first to form? Answer: Hydrogen and helium. Concept Check 24.2 Why is the phrase “nursery of stars” an appropriate way of describing interstellar matter (nebulae)? Answer: Nebulae consist of the matter from which stars are created. Compare and contrast bright and dark nebulae. Answer: Bright nebulae are close to very hot stars and they glow. Dark nebulae are too far from bright stars to be illuminated. Why are reflection nebulae generally blue? Answer: Blue light is scattered more efficiently than red light in them. How are planetary nebulae different from other types of bright nebulae? Answer: They are less diffuse and originate from remnants of dying Sun-like stars. Concept Check 24.3 On an H-R diagram, where do stars spend most of their life span? Answer: In the main sequence. How does the Sun compare in size and brightness to other main-sequence stars? Answer: It is in the midpoint of the range for these stars and considered to be average. Describe how the H-R diagram is used to determine which stars are “giants.” Answer: These stars are compared with stars of known size that have the same surface temperature. If one red star is more luminous than another red star, it must be larger. Stars with large radiating surfaces are classified as giants and exist in the upper-right portion of the H-R diagram. Concept Check 24.4 What element is the fuel for main-sequence stars? Answer: Hydrogen. Describe how main-sequence stars become giants. Answer: The usable hydrogen is consumed and a helium-rich core is left. With no energy source, the core lacks gas pressure needed to support itself against the gravitational pull. The core contracts and gravitational energy is converted to thermal energy, some of which is radiated outward, generating more hydrogen fusion in the region around the core. This additional heat expands the star’s outer gaseous shell and it becomes a large red giant star. Why are less massive stars thought to age more slowly than more massive stars, despite the fact they have much less “fuel”? Answer: The less massive stars burn energy more slowly and never reach high enough temperature and pressures to fuse helium. List the steps that are thought to be involved in the evolution of Sun-like stars. Answer: Nebula, protostar, main-sequence star, red giant, planetary nebula, white dwarf. Concept Check 24.5 Describe degenerate matter. Answer: It is extremely high-density matter that originates after stars collapse into white dwarfs. Electrons in this type of matter are displaced inward from their regular orbits around their atoms’ nuclei. What is the final state of a medium-mass (Sun-like) star? Answer: White dwarf. How do the “lives” of the most massive stars end? What are the two possible products of this event? Answer: They end in a supernova. The two possible products of this event are a neutron star or a black hole. Explain how it is possible for the smallest white dwarfs to be the most massive. Answer: The atoms in these white dwarfs have been squeezed together so tightly that the electrons are pushed very close to the nucleus. Black holes are thought to be abundant, yet they are hard to find. Explain why. Answer: Their surface gravity is so immense that light cannot escape and so they disappear from sight. They are visible in the x-ray portion of the spectrum, but x-rays do not penetrate Earth’s atmosphere. Concept Check 24.6 Compare the three main types of galaxies. Answer: Spiral galaxies are flat and disk-shaped with a greater concentration of stars in the center. They have spiral arms extending from the center. Elliptical galaxies are ellipsoid to nearly spherical like spiral galaxies, but they lack the spiral arms. Irregular galaxies might once have been spiral or elliptical galaxies but were distorted by gravity. These galaxies have no symmetry. What type of galaxy is our Milky Way? Answer: Spiral. Describe a possible scenario for the formation of a large elliptical galaxy. Answer: Two smaller spiral galaxies converged in the same area and merged, forming a large elliptical galaxy. Concept Check 24.7 In your own words, explain how astronomers determined that the universe is expanding. Answer: They observed red shifts due to the Doppler effect as they observed galaxies. What did the Big Bang theory predict that was finally confirmed years after it was formulated? Answer: If the universe was originally hot beyond comparison, it should be possible to detect this radiation, which is called cosmic microwave background radiation. Which view of the fate of the universe is currently favored: the Big Crunch or the Big Chill? Answer: The Big Chill. What property does the universe possess that will determine its final state? Answer: Dark energy. GIVE IT SOME THOUGHT ANSWERS Assume that NASA is sending a space probe to each of the following locations: Polaris (the North Star) A comet near the outer edge of our solar system Jupiter The far edge of the Milky Way Galaxy The near side of the Andromeda Galaxy The Sun List the locations in order, from nearest to farthest. Answer: Sun, Jupiter, asteroid at the outer edge of the solar system, Polaris, far edge of the Milky Way, Andromeda. Use the information provided below about three main-sequence stars (A, B, and C) to complete the following and explain your reasoning: Star A has a main-sequence life span of 5 billion years. Star B has the same luminosity (absolute magnitude) as the Sun. Star C has a surface temperature of 5000K. Rank the mass of these stars from greatest to least. Rank the energy output of these stars from greatest to least. Rank the main-sequence life span of these stars from longest to shortest. Answer: Star A = greatest mass, then star B, star C has the lowest mass. Energy output is the same – A is the greatest and C is the least. C has the longest life span, then B, and A has the shortest. The masses of three clouds of gas and dust (nebulae) are provided below. Imagine that each cloud will collapse to form a single star. Use this information to complete the following and explain your reasoning. Cloud A is 60 times the mass of the Sun. Cloud B is 7 times the mass of the Sun. Cloud C is 2 times the mass of the Sun. Which cloud or clouds, if any, will evolve into a red main-sequence star? Which of the stars that will form from these clouds, if any, will reach the giant stage? Which of the stars that will form from these clouds, if any, will go through the supernova stage? Answer: None of the stars will evolve into a red main-sequence star. All three of the clouds have the potential to grow into the giant phase. Cloud A will probably go through a supernova event. The accompanying photo shows the Trifid Nebula, which can be easily observed with a small telescope. What unique properties does this nebula exhibit? Answer: The Trifid nebula exhibits glowing gases that are apparently excited from the light of growing stars within the cloud. Refer to the accompanying images (A, B, C, and D) to complete the following: Which of these nebulae, if any, is an emission nebula? Which of these nebulae, if any, formed near the end of a star’s lifetime? Which of these nebulae, if any, is a reflection nebula? Answer: Image B is an emission nebula. Image C is a planetary nebula, formed late in the life of a star. Image D appears to be a reflection nebula. How a star evolves is closely related to its mass as a main-sequence star. Complete the accompanying diagram by labeling the evolutionary stages for the three groups of main-sequence stars shown. Answer: Low mass stars – go straight to white dwarfs. Medium mass stars – red giant, planetary nebula, white dwarf. High mass stars – red supergiants, supernova event, then either neutron star or black hole. Refer to the accompanying photos of an elliptical galaxy and a spiral galaxy to complete the following: Which image (A or B) is an elliptical galaxy? Which of these galaxies appears to contain more young, hot, massive stars? How did you determine your answer? Answer: A = elliptical. Probably the elliptical galaxy as evidenced by the bright, whiter light at the center. Larger stars die out first. The spiral galaxy appears to be older based on the yellow colors in the nucleus of the galaxy. Consider these three characteristics of the universe: It does not have a center. It does not have edges. Its galaxies are all moving away from each other. Which of the three characteristics of the universe is/are depicted in the raisin bread dough analogy (see Figure 24.22)? Which of the three characteristics of the universe is/are not accurately depicted in Figure 24.22? Answer: The raisin bread does not have a center, like the universe and it also depicts the galaxies moving away from one another. The raisin bread does not accurately depict the lack of edges of the universe. EXAMINING THE EARTH SYSTEM ANSWERS Briefly describe how the atmosphere, hydrosphere, geosphere, and biosphere are each related to the death of stars that occurred billions of years ago. Answer: During the catastrophic death of stars, massive numbers of atoms of carbon, iron, and other elements are formed. It was from the debris of stellar deaths that the nebula that became our solar system evolved. Because the atmosphere, hydrosphere, solid Earth, and biosphere all contain these atoms, they are all related to the death of a star(s) that occurred billions of years ago. Scientists are continuously searching the Milky Way Galaxy for other stars that may have planets. What types of stars would most likely have a planet or planets suitable for life as we know it? Answer: Given that stars and planets form from the collapse of a nebula, and many young stars contain nebular discs of dust and gas around them, it appears likely that many different types of stars may have a planet or planets surrounding them. Assuming that only Sun-like stars have planets, there are a couple hundred million candidates in our own galaxy. Whether the planets are suitable for life depends on their compositions, temperatures, and other factors. However, the presence of planets and molecules like water or carbon dioxide does not imply life; these items are only the precursors to life. Based on your knowledge of the Earth system, the planets in our solar system, and the universe in general, speculate about the likelihood that extra-solar planets exist with atmospheres, hydrospheres, geospheres, and biospheres similar to Earth’s. Explain your speculation. Answer: Although answers are based on opinion and may vary, it appears that extrasolar planets similar to Earth are not very likely to exist. To produce such a planet would require such a unique set of circumstances, occurring in a very set pattern, that the odds of their happening are extremely small. ADDITIONAL RESOURCES DVDs and Movies Alien Planets Revealed (2014) NOVA, PBS, 53 minutes. Exploring the possibility of life elsewhere in the universe. Available for free streaming from http://www.pbs.org/wgbh/nova/space/alien-planetsrevealed.html Monster of the Milky Way (2006) NOVA, PBS, 53 minutes. Does a black hole exist in our galaxy? Available for free streaming from http://www.pbs.org/wgbh/nova/space/monster-milky-way.html Origins: Back to the Beginning (2004) NOVA, PBS, 53 minutes. Exploring the Big Bang. Available for free streaming from http://www.pbs.org/wgbh/nova/space/origins-series-overview.html#originsback-beginning The Mechanical Universe…and Beyond: Episode 51 – From Atoms to Quarks (1985) California Institute of Technology, 30 minutes. Available for free streaming from http://www.learner.org/ resources/series42.html Ultimate Space: Into the Cosmos (2007–2011) National Geographic, 5 hours, 15 minutes. Available on DVD. Journey to the Edge of the Universe (2008) National Geographic, 90 minutes. Available on DVD. Websites NASA Goddard Space Flight Center Supernovae. http://imagine.gsfc.nasa.gov/docs/science/know-l2/ supernovae.html Interactive Hertzsprung-Russell Diagram Explorer. http://astro.unl.edu/naap/hr/animations/hr.html Variable Star Photometry Analyzer. http://astro.unl.edu/naap/vsp/animations/ variableStarPhotometryAnalyzer.html Exoplanet 1.01 Orbits GJ 436 Animation. From JPL, Cal Tech, and National Geographic. http://video.nationalgeographic.com/video/exoplanet-gj436-flyby-vin Virtual field trip animations to black holes and neutron stars. NASA. http://apod.nasa.gov/htmltest/ rjn_bht.html Solution Manual for Earth Science Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa 9780321928092, 9780321934437
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