INTRO ASTRONOMY: EXAM 2 STUDY GUIDE
INTRO ASTRONOMY: EXAM 2 STUDY GUIDE 10273
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This 12 page Study Guide was uploaded by Nicole Salem on Thursday February 18, 2016. The Study Guide belongs to 10273 at Texas Christian University taught by Dr. Ingram in Winter 2016. Since its upload, it has received 104 views. For similar materials see Intro Astronomy: Earth&Planets in Physics 2 at Texas Christian University.
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Date Created: 02/18/16
Astronomy Guide Exam 2 (52) Briefly explain the capture hypothesis for the origin of the solar system. Name and briefly explain three arguments that tend to contradict this hypothesis. The capture hypothesis is that the sun forms by itself and then gravitationally captures passing planets. Contradictions: Collisions are too rare, space is too big, and planets are fairly common. Process of capture is physically difficult because to be able to capture the planet must slow down with friction but space is empty. Orbits would be randomly aligned and more elliptical. All orbits are the same. (53) Explain two assumptions scientists made about the nature of the universe and space itself that made the capture hypothesis seem plausible at the time. They underestimated the scale of the galaxy/universe—they thought the galaxy /universe was not that big and that therefore collisions would happen often but in fact the galaxy/universe is so big that they rarely happen. Assumed universe was filled with “air” (aether) which would slow down passing planets. (54) From chapter 8.2 of your textbook, explain why the temperature of the solar nebula increased as the cloud collapsed. Also explain why the initially slowly spinning cloud (especially the central, densest part the protosun) spun faster as the cloud collapsed. Heating: The temperature of the solar nebula increased as it collapsed. Such heating represents energy conservation action. As the cloud shrank, its gravitational potential energy was converted to kinetic energy of individual gas particles falling inward. These particles crashed into one another, converting the kinetic energy of their inward fall to the random motions of thermal energy. The sun was formed into the center, where temperatures and densities were highest. Spinning: The solar nebula rotated faster and faster as it shrank in radius. The rotation of the cloud may have been imperceptibly slow before its collapse began, but the clouds shrinkage made fast rotation inevitable. The rapid rotation helped ensure that not all the material in the solar nebula collapsed into the center: The greater the angular momentum of a rotating cloud, the more spread out it will be. Angular momentum is conserved. (55) From chapter 8.2, explain how we believe the Sun slowed down from its initially fast rotation. What observational evidence supports this model? Charged particles in the nebula cause a drag on the sun’s magnetic field that cause the sun’s rotation to slow down over time. Evidence that supports this is that young stars typically rotate rapidly and have strong magnetic fields and strong winds, but older stars have slowed down rotations like the sun so some force must have caused them to drag and slow down as the stars grew older. (56) From chapter 8.3 of your textbook, briefly describe the evidence in meteorites (related to Xenon gas) that support the idea that the collapse of our solar nebula initially was caused by a supernova explosion. Radioactive elements are made in the violent stellar explosions called supernovae, so the presence of them in meteorites and on earth underscores the fact that out solar system is made from remnants of past generations of stars. Some meteorites contain the rare isotope Xenon129, which is gaseous even at extremely low temperatures, so it could not have condensed and become trapped in the meteorites as they condensed in the solar nebula. Instead, it must be a product of radioactive decay, and laboratory studies show that its parent isotope is iodine129 (its halflife is only 17 million years). Therefore, we conclude that xenon129 in meteorites today, is the product of radioactive decay of iodine129produced in a supernova that occurred no more than a few tens of million sof years before the collapse of the solar nebula began. (57) Briefly explain the catastrophe hypothesis for the origin of the solar system. Explain two misconceptions we had about the nature of the universe that led us to believe this theory was originally plausible. The catastrophe hypothesis says that the sun formed by itself and it had a collision with another star. The resulting debris formed into planets. Misconceptions: Underestimated the size of the universe, and overestimated the frequency of collisions Assumed that planets are made of same stuff as the sun. Earth is rocky minerals and the sun is 99% helium. (58) Name and briefly explain two arguments that contradict the catastrophe hypothesis for the origin of the solar system. Contradictions: Collisions are too rare and space is too big Volatiles on Earth would not exist if material that makes up Earth was once inside the sun because volatiles are easily destroyed by heat. Ex water (59) According to the International Astronomical Union (IAU), what is the official definition of a planet now and why does this definition exclude Pluto from having the status of a planet? A planet is an object that orbits a star, is large enough to have settled into a round shape and, crucially “has cleared the neighborhood around its orbit”. Pluto is considered as a large asteroid. (60) It was once thought our solar system had over 20 planets before the definition of planet was last modified in 1852. Explain what all of these extra planets were and why they were demoted from planet status. The extra planets turned out to be asteroids and where demoted because in the telescope hey appeared as little spots of light, whereas planes showed up as little disks. (61) The authors propose an improvement of the definition of planets based on a factor represented by the greek letter mu. What is "mu"? The ratio between the mass of a body and the mass of all other bodies that share its orbital zone. (62) Some astronomers argue that planets should be defined only by their intrinsic properties (such as size, shape or composition) rather than by their location or surroundings. What is the authors' counterargument to this idea? The author argues that it overlooks the fact that astronomers classify all objects that orbit planets as “moons” although two of them are larger than Mercury and many are captured asteroids and comets. (63) Explain why the large, roughly spherical solar nebula collapsed into a disk shape rather than just a smaller sphere. The gravity from a the solar nebula pushes in all directions but there is a centrifugal force that is pushing the nebula outward in only certain directions (the directions parallel to the spinning motion) and there is a conflict of forces that causes it to collapse into disk shape. (64) Explain how evidence found in meteorites tends to confirm our ideas about the accumulation phase of the origin of the solar system. A meteorite up close looks like a collection of particles stuck together. From here we can agree that random collisions inside the nebula built up over time to form meteorites. (65) Name and briefly explain three reasons why Jupiter is so much larger than the Earth. As part of your answer, be sure to explain both ways in which ice particles make it easier for larger planets to form. 1) Jupiter has a larger feeding zone—this explains why outer planets are bigger than the inner planets. 2) Ice advantage: ice is sticky, collisions and accumulation becomes easier. Also, ice is 2x more abundant than rocks and metal combined so more solid materials to form with. 3) Jupiter is massive enough and cold enough to capture and hold hydrogen and helium gases. There is a large abundance of Hydrogen and Helium gases. (66) Name and explain two reasons why the Earth cannot accumulate Hydrogen gas while Jupiter can. One has to do with temperature, the other with escape velocity. Earth is not cold enough, it has a higher temperature than Jupiter which means that the gases in earth move more quickly. The quicker the gas moves, the more likely it would be able to escape the atmosphere. High temperatures allow helium and hydrogen gases to reach escape velocity Escape velocity: the speed the molecule must travel in order to escape an objects atmosphere. Earths escape velocity is 25,000 mph and Jupiter’s is 120,000 mph then it is much more difficult for helium and hydrogen to escape because Jupiter has a much higher escape velocity. (67) From chapter 10.3 of your textbook (mathematical insight 10.2), explain why it is possible for some gases to escape a planet's atmosphere even though the average gas velocity is a little bit less than the escape velocity. Use a graph like the book's in order to help explain your answer. Some gas particles can escape even if the peak thermal velocity is much lower. This atoms can escape if they are moving in the right direction (upward) and do not collide with other particles on their way out( once these atoms escape, ongoing collisions redistribute the speed on the remaining atoms, so that the shape of the distribution remains the same and the same small fraction always exceed escape velocity). (68) From chapter 13.1 of your textbook, describe the astrometric technique for detecting extrasolar planets (what do we measure and how). Explain how the astrometric technique differs from the Doppler wobble technique. Explain why this technique can only work for stars that are among the closest stars to the Earth and not for more distant stars, assuming both kinds of stars as easy to see and measure. Astronomers use two distinct observation methods to search for this type of orbital motion by stars. The first, called the astrometric method (astrometry) means the measurement of the stars), uses very precise measurements of stellar positions in the sky to look for the slight motion caused by orbiting planets. The second, called the Doppler method, takes advantage of the Doppler effect, which allows us to measure changes in a stars velocity towards or away from us that are caused by orbiting planets. One constraint that limit the number of planetary systems that can be studied with the astrometric method is that the farther away the star is, the smaller its sidetoside movement would appear. (69) From http://www.pbs.org/wgbh/nova/education/activities/3113_origins_06.html, answer the following: Which gas, oxygen or ozone, is probably a better indicator of the existence of life on a planet and why? Ozone is a better indicator of the existence of life on a planet. The ozone layer protects the earth from the sun’s harmful ultraviolet radiation. Presence of ozone is also a indicator that normal oxygen is present. Without oxygen, ozone could not exist. (70) Explain the difference between primary and secondary eclipse in a system which contains a parent star and a transiting planet. Explain how an exoplanet atmosphere can be studied by looking at a system's spectrum just before and just after a secondary eclipse. When a planet passes in front of its star, it presents its nightside to the observer. At other times, it shows at least part of its dayside, and just before the planet goes behind the star, the dayside is facing Earth. Although the star is far, far brighter, the planet itself also glows, mostly in the infrared. That glow vanishes abruptly, however, when the planet moves behind the star; its contribution to the combined light of planet and star vanishes. If astrophysicists can do a beforeandafter comparison, they can deduce what the planet alone would look like (71) One of the Astronomers featured in the article, Dave Charbonneau, is looking for planets around dim "M dwarf" stars instead of sunlike stars. Briefly describe four reasons why it may be easier to find Earthlike planets in the star's habitable zone this way. First, an Earthsize planet would block a relatively greater percentage of the small star's light. Such a planet would also exert a relatively greater gravitational pull on the star, making it easier to gauge the planet's mass and thus its density. The habitable zone for a small, cool star would also be much closer in than it is for a hot, sunlike star, which makes transits more likely to be spotted (because the orbit of a closein planet does not have to be so precisely aligned for it to pass in front of the star). Finally, there are vastly more M dwarfs in the Milky Way than there are sunlike stars—about 250 of the former lie within 30 or so lightyears of Earth, compared with only 20 of the latter. (72) Why does the Earth have a hard time capturing Hydrogen while easily maintaining an abundance of gases like Carbon Dioxide? As part of your answer, explain why gas velocity is higher from lighter gases. All gases in an atmosphere have the same average temperature and same average kinetic energy so the temperature of C02 is the same as the temperature of H, then hydrogen has a small mass so it moves 4 times faster than the velocity of oxygen so Hydrogen escapes much easier. ½ mv^2 =1/2mv^2 (73) In the nebula theory of planetary formation, explain why Neptune is smaller than Jupiter despite being further away from the Sun where, presumably, it has a larger feeding zone and even colder temperatures. When Neptune was formed, the density of Nebula was lower so Neptune did not have enough time to capture the gases. (74) Know the rules of Doppler shifting. Be able to explain with a simple diagram the difference between radial and transverse velocity. 1. A) Objects moving towards you appear blue shifted (shorter wavelength) B) Objects moving away from you appear red shifted (longer wavelength) 2. Amount of the shift is proportional to the radial velocity Radial velocity is along the line of sight and can be measured by Doppler shift Transverse velocity is across the line of sight and speed cant be determined. (75) Suppose we see a star wobbling back and forth and can measure its Doppler shift changing over time. What two properties of the star’s companion planet can we deduce from this information? Explain briefly how each is deduced. Mass—the mass can be determined by the amplitude of the stars wobble of the companion planet. The bigger the wobble, the more the mass. The period shows us the distance of the planet from the star due to how long it takes to orbit the star. (76) Name and briefly explain two reasons why Earthsize planetary transits must be observed from space rather than from groundbased telescopes. The motions in the atmosphere are constantly bending the rays of light from each star into different directions. To detect a planetary transit as short as 2 hours out of a year requires measuring the brightness of the stars continuously. There is no part of the sky that can be continuously monitored throughout the year. Also bad weather make it inefficient. (77) How can the Kepler mission tell the difference between a variation in brightness caused by a planetary transit compared to a variation caused by, for example, a sunspot moving across the star or a cycle of high/low stellar activity (such as the "Maunder minimum" cycle observed on our Sun)? The stars do vary in brightness all the time. In fact it is almost impossible to make a perfectly constant source of light. Fortunately, the stars we are most interested in are stars like our Sun. They vary less than the change in brightness caused by an Earth size planetary transit on the same time scale as a transit (a few hours). Our Sun varies over many time scales: There are Maunder minimums, which do not occur for many centuries or longer and have caused "mini ice ages" even as recently as during the 17th century. There is an elevenyear "solar cycle" of minimum and maximum activity. The largest shortterm variations are caused by "sun spots" that appear and fade, and rise and set as the Sun rotates with a period of four weeks. Planetary transits have durations of a few hours to less than a day. The measured solar variability on this time scale is 1 part 100,000 (10 ppm) as compared to an Earthsize transit of 1 part in 12,000 (80 ppm). Even then, most of the variability is in the UV, which is excluded from the measurements by the Kepler Mission (78) Describe the two competing Jupiterformation scenarios from the article: core accretion and disk instability. Rocky worlds, being relatively small, are easy to assemble in a bottomup process called core accretion, where colliding rocks gradually glom together over as much as 100 million years. Giant planets could also form much like stars do in a topdown process called disk instability. In this scenario, something like Jupiter would achieve planethood through the direct, rapid collapse of a cold, overdense clump of gas and dust in the outer region of a circumstellar disk. (79) The astronomers in the article hope to observe very young Jupitersized exoplanets to determine how they formed. Explain how and why Jupiters formed from each method will appear differently to the observers. (80) By carefully observing our own planet Jupiter today, could we potentially determine which method was involved in its formation? Why or why not? It is almost impossible to distinguish between these two scenarios for Jupiter today because essentially all the evidence is literally buried below the giant planet’s dense, thick atmosphere. (81) Explain the roles of adaptive optics and the coronagraph in the attempt to image an extrasolar planet. Adaptive optics are computercontrolled deformable mirrors that change their shape hundreds or even thousands of times a second to combat atmospheric distortions, allowing astronomers to capture images of celestial objects that rival those available from space telescopes. To reveal the star's known planet, Macintosh engages another device, a coronagraph, that strips out most of the starlight: the light encounters a series of masks that filter out 99 percent of the photons. The ones that make it through are focused and aimed at a mirror with a central hole polished to atomicscale smoothness. (82) How do we know that water has been present on the Earth since shortly after its formation? Earth is 4.6 billion yrs old and the oldest sedimentary rocks (formation requires water) are 3,9 billion yrs old. Also, the composition of microns (formed 4.4 billion yr ago) show that water was present at that time. (83) Briefly explain three possible ways early Earth's water could've been "lost" and not become a permanent part of Earth's oceans. 3 possible ways 1. Ice Clumps collided with Earth during the initial formation of the planet, putting large quantities of water into the atmosphere in the form of steam. Then much of the water was lost back into the atmosphere and the molecules destroyed by ultraviolet radiation even though some survived and condensed into oceans. 2. Meteorites that contain 0.1 percent water by weight bombard the Earth so technically the Earth would be composed of at least 4 times the amount of water that is held today. Perhaps half an ocean of moisture became trapped within minerals of the mantle 3. Water may have also taken up residence in Earth’s dense iron core, which also contains some relatively light elements, like Hydrogen (84) Explain the evidence from recentlyobserved comets (such as Halley) that implies most of Earth's water probably did not come from comets. Comets contain a high concentration of deuterium and is twice as abundant in comets as it is in seawater. Seawater just does not have as much deuterium as it should if the water came from the comets. (85) What is the Faint Sun paradox and how has it been "resolved" by our atmosphere? The disparity between this prediction and the fossil evidence has been turns the faint sun paradox. The faint sun paradox is that when the sun first became stable, it was 30% dimmer. With this the Earth’s oceans would have been completely frozen until about 2 billion years ago. However, fossils have proved that liquid water and life became present about 3.8 billion years ago. This is possible because the Earth’s atmosphere contained more carbon dioxide and more methane. This would have increased the greenhouse effect and kept the Earth warmer. (86) From chapter 8.3 in your textbook, your book discusses how we can use our understanding of geological and chemical processes to deduce the original composition of a rock for the purposes of radioactive (or radiometric) dating. One example discussed in the book is with potassium and argon. Explain how we know that the original abundance of Argon 40 in a rock is zero and how we know no Argon 40 has escaped from the rock. Argon40 is a gas that does not combine with other elements and did not condense in the solar nebula. Therefore, if you find argon40 gas trapped inside minerals, it must have come from radioactive decay of potassium40. (87) What are the three ingredients necessary for life? What is a possible source for each ingredient? Energy—most of the energy comes from the sun. Water comes from rivers and oceans. Organic molecules come from space (88) On the 5km highway of time representing the history of Earth, what length accurately reflects all of recorded human history? The last few millimeters represent all of recorded human history. (89) Critics of evolution wonder how a process based on random chance could result in extremely complex organisms. How do biologists respond to this argument? Random chance is directed by natural selection. If it is advantageous, that trait would stick around. The variations that have survived are the ones that have promoted survival. (90) What discovery did the Magellan spacecraft make about the recent geological history of Venus? Venus underwent a global catastrophic melting. Venus is hot enough to melt lead and it is the most volcanic reactive planet after Earth. (91) What recent evidence discovered in Antarctica implies that life may have once existed on Mars? In Antartica, there were found meteorites that came from Mars and inside them they were little fossilized organisms. (92) What is the habitable zone? Explain how recent discoveries of life in extreme environments (e.g. black smokers, worms in methane ice, etc.) has affected our view of a habitable zone. What is a "gravitational" habitable zone? A habitable zone is a region around a star where life is possible. It has the same 3 ingredients (energy, water, organic molecules) but water has to be liquid. A gravitational habitable zone is a zone where gravity from a close planet can provide heat. (93) If we were to find fossil evidence of ancient, extinct singlecelled organisms on Mars, explain why that would be significant to our understanding of evolution on Earth. It would let us know that life is inevitable if the three ingredients are present, meaning that life is common in other places of the universe. (94) Explain why searches for other planetary systems so far have been biased in favor of discovering high mass planetary companions as well as planetary companions that are very close to their parent stars. The high mass planetary companions make stars have a bigger wobble so it is easier to observe it. The bigger the wobble, the easier it is to detect it with technology. Planets closer to the sun have shorter orbital periods and it allows for us to observe 3 orbits in a shorter time so we have not been looking enough into the planets that are far away from their parent stars. (95) Explain why we cannot necessarily believe that indirect observations of planetary companions via Doppler wobbling are truly planetary companions. Are our assumed companion planet masses higher or lower than true planet masses in tilted systems? Explain. We measure radial velocity via Doppler wobbling but Doppler wobbling cannot measure transverse velocity so two stars that have the same radial velocity will appear to produce the same wobble and therefore appear to have same mass. Scientists therefore assume that the companion planet masses are lower than the mass of the planets if it is located in a tilted system. (96) Explain the details and significance of the discovery of an eclipsing (or transiting) extrasolar planetary system. Why was this kind of system so important to find in light of the uncertainty in Doppler wobble measurements? A transitory extrasolar planetary system is when a planet briefly blocks a small fraction of light from its parent star in edge on system. Scientists are able to measure the time that the light Is blocked, and therefore be used to discover the planets true velocity and mass accurately. This helps to prove that the wobble seen is caused by a planet and not by a binary star system. This will help us to discover in the future if there is light. (97) What does it mean to say that we are communicating with extraterrestrials already? What do our signals sound like at various distances from the Earth? Explain. We are broadcasting signals leaving Earth at the speed of light. The further from the earth you are, the older the transmissions that you are hearing. (98) Why do radio wavelengths work as the best way to communicate with potential extraterrestrials? Radio wavelengths of the spectrum are the quietest, the others are noisy. They can carry words, pictures cheaply at the velocity of light. (99) What is the Fermi paradox (Fermi's question), and what is one possible response? The Fermi paradox is “since aliens haven’t visited, they do not exist”. If alien life forms were advanced they would have visited us by now. One possible response is “absence of evidence is not evidence of absence”. Also, another response is that they do not communicate. (100)What is an example of an observation Galileo made to show that objects in the Universe are not static like paintings on the inside of cathedrals? When he looked at the sun he saw sunspots. In Florence the sunspots moved across and were like rotating balls of plasma. (101)What arguments can you make that evolution is very unlikely to result in intelligence? Of all of the species that have evolved on earth, only one species have evolved intelligence. (102) How and why does the lifetime of a civilization relate to the probability that we are not alone? The longer the time line, the most likely it is to over lapse.
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