|Astronomy 1 (Lecture Class) Information|
Preparing an Answer for Essay Questions 2 - 15
The following is a detailed breakdown of each of the essay questions, other than question 1, which is covered elsewhere. Parts of the breakdown which are in parentheses and/or include a question mark represent material which is directly related to the question, but is not specifically asked for by the wording of the question. In general, such topics might be briefly covered by an exceptional answer, but would not be considered as important as those specifically asked for.
Question 2: Describe the apparent motions of stars in different parts of the sky, as seen at different latitudes. Discuss how the Sun's motion differs from that of the stars, and how its changing position causes the seasons. Discuss how the seasons differ at different latitudes, and discuss other temperature/weather variations which depend upon location, such as climatic zone and geographical effects. Explain how the tilts of the planets' axes of rotation affect the intensity of their seasons at different latitudes. Compare/contrast orbital effects with effects caused by the rotations of the planets.
Describe how the rotation of the Earth produces an apparent rotation of the sky (that is, an apparent motion of the stars around/across the sky). Describe what the motion of the stars looks like, as seen at the Equator, the Poles, and our latitude (discuss how the altitude of the Pole is related to latitude?). In doing so, explain how the motion is similar, or different, at each of those latitudes. Use simple diagrams to illustrate the motions, and enough discussion of those diagrams to make it clear that you understand what they are supposed to show.
Describe how the Earth's motion around the Sun produces the Sun's apparent motion around the sky, during the course of the year (discuss the difference between the rotation period of the Earth, and the length of the day, caused by our motion? discuss how the motion causes the seasonal stars?). Discuss how the fact that our axis is tilted causes the Sun to move north and south in the sky, and explain how this causes the seasons — by increasing or decreasing its height, and the amount of time that it is up or down. Again, use diagrams to show the way this works, either as seen from the surface of the Earth, or from space, or both. Be sure to thoroughly explain the diagrams.
Show how the Sun, by mimicking the motion of the stars at different latitudes, creates extreme seasons near the Poles, nearly nonexistent seasons at the Equator, and in-between seasons at our latitude. Show how, if we were not tilted, the Sun would not move north and south during the year, and we would not have seasons, but that would still have differences in weather in the various climatic zones (tropic, temperate, and arctic) because of the different slant that its light has at different latitudes. (Discuss the odd fact that between the tilt of our axis, and the greater amount of water in the southern hemisphere, the Earth is actually warmest when furthest from the Sun?)
Explain how, for planets such as Mercury, which have no tilt, there would be no seasons, as would be the case for the Earth, if it were not tilted. Explain how, for planets such as Mars, which are tilted similarly to the Earth, the seasons must be similar to the Earth. Explain how, for planets such as Uranus, which are nearly tilted on their "side", seasons are extreme everywhere on the planet, not just near the poles.
Point out that the surface temperatures of the planets should depend upon their distances from the Sun, and that other than Venus, every planet is colder than the next planet towards the Sun. Also explain that if the distance changes substantially (that is, if the eccentricity is not close to zero), then the planet's temperature should change during an orbit. Give examples of how this works, using the semi-major axes and eccentricities of various planets. Discuss how, in the case of the Earth, the eccentricity is so small (less than 2%) that there is no observable effect on our weather or temperature (even geographical differences are more significant), but that for Mercury, Mars, and Pluto, there are observable temperature changes, and for Mars and Pluto, some minor changes in the weather. (Although even for these planets, only Mercury has eccentricity effects greater than seasonal effects, partly because it has no seasons, in the first place.)
Question 3: For each of the planets, discuss the mass, diameter, density and surface gravity, and briefly explain how, for any given planet, these quantities are related to each other. Discuss each planet's internal structure and composition in as much detail as possible. Discuss the surface features and magnetic fields of each planet, and how these can be used to infer their internal temperatures. For planets whose surface features cannot be used for this purpose, discuss the reason for that, and how we do infer their internal temperatures.
Discuss the values asked for, for each planet. To save time (especially for me, since I have hundreds of essays to grade), you should use a table similar to that in the Planetary Data Table to list the values for each planet. Do not waste your time learning the values to better than one or two non-zero-digit accuracy, as I cannot think of any reason for you to do so, and will not give you extra credit for extra digits.
Why you need to know these values, but do not need to know them to better than one or two non-zero-digit accuracy: You should be able to discuss how being larger or smaller, heavier or not so heavy, dense or not so dense, etc, affects (or tells us something about) the structure and properties of a planet. As an example, you should be able to discuss how Jupiter, at more than 300 Earth masses, is by far the largest of the planets, being over three times the mass of Saturn, which has less than 100 Earth masses, and over twice the mass of all the other planets put together, and how this tremendous mass causes Jupiter to be far denser than we might otherwise expect, for an object that is almost entirely made of hydrogen gas; and that as a result, it is not made of hydrogen "gas", but of a "liquid" formed by compressing that gas to several times the normal density of liquid hydrogen, and in fact so compressed a "liquid" that most of it behaves like a metal, and is therefore called metallic hydrogen. In this example, the fact that Jupiter has 317.8 Earth masses, instead of merely "more than 300" Earth masses, is of no importance. You need to know the information asked for accurately enough to intelligently discuss the planets' characteristics; but any additional accuracy has no value, and you should not waste your time trying to learn the information to several digit's accuracy, as you will only waste your time, and make things far more difficult, and far more frustrating.)
Discussion of appropriate units: In the Planetary Data Table, most values used in this question/answer are given in comparison to the Earth. You may use other units, if they make more sense to you (e.g., miles or kilometers for the diameters of the planets), so long as they are reasonably correct. For instance, most people learn the densities of the planets in comparison to water, instead of in comparison to the Earth (this is why both units are used in the Table). Just be sure that, for a given quantity (e.g., density) you use the same units for every planet. Don't give the density of some planets in comparison to the Earth, and of other planets in comparison to water, as that makes it appear that some of your values are off by a ratio of five (the difference between the two units).
Discuss the structures and compositions of the planets in as much detail as possible. You can use the Planetary Data Table as a "quick and dirty" summary of the planets' composition, but as with all parts of all questions in this class, if you just put down a minimal answer you will receive a minimal grade. To receive a high grade, you need to use diagrams and detailed discussion to explain how the planets are similar to or different from each other. When there are uncertainties in the internal structure, you need to show what we can be reasonably certain of, and where things might turn out slightly different when we know more (in the future). For the Jovian planets, when discussing the "ice" which lies above the rocky core, be sure that you indicate that because of the high temperatures inside these planets, it is not ice in the normal sense of the word, and that depending upon the pressure and temperature, it is more likely to be dense liquids such as water, rather than pseudo-solid versions of those liquids. For the Terrestrial planets, be sure to discuss the difference in the sizes of their cores, and the reasons for believing that their cores are of different sizes.
For the Jovian planets, explain the nature of their "surfaces", and explain that, since they have no surface in the normal sense, they don't have surface features. For Pluto, explain that since we haven't yet obtained pictures of the surface, we don't know what it looks like. For EACH of the Terrestrial planets, including the Earth, give a summary of the important types of features observed, and discuss the ways in which these features are similar to those on other planets, or unique to the particular planet under discussion. Explain that if, as in the case of the Earth, there is considerable internal heat driving motions within the mantle, then those motions will affect the surface features, producing a surface which is "young", whereas if, as in the case of Mercury, there is little internal heat, so that there are no such motions, then the surface can be "old". Discuss (briefly) the bombardment which the planetary surfaces have suffered (particularly when they were forming, but even, to a lesser extent, since then), and how extensive cratering implies an "old" surface, and therefore very little internal motion (and heat), while a lack of cratering would imply the opposite. It would also be a good idea to point out that when there are substantial effects of weathering and erosion (as on Earth), then the surface could appear much less cratered simply because of those effects.
Discuss the magnetic fields of the planets in general, and of the Earth and Jupiter, in particular (the Earth, because we live here, and Jupiter, because it has the most impressive field, and for both of them, because they are discussed in detail in the book). Discuss the nature of the magnetic field, its interaction with the solar wind, and the radiation belts trapped within the field. Discuss the liquid dynamo theory — how convective motions within the molten metallic core of the Earth, and similar regions within other planets, are responsible for their magnetic fields. Explain the nature of the magnetic field for each planet known to have a magnetic field, and for each planet, whether it has a field or not, what we think that implies about the structure of the planet (e.g., whether the core is molten or not, what kind of metallic liquid must produce the field, etc.).
Question 4: For each of the planets, discuss the amount and composition of its atmosphere, and its "normal" range of surface temperature. Discuss the structure of the Earth's atmosphere and the ways in which it is similar to or different from other planetary atmospheres. Discuss how the atmosphere of a planet can affect its surface temperature. Discuss how a planet's gravity affects the kind of atmosphere it can have, and the origin and evolution of planetary atmospheres.
For each planet, discuss the amount and composition of its atmosphere, compared to the Earth, or each other. For Mercury and Pluto, where there is essentially no normal atmosphere, discuss the nature of their "temporary" atmospheres. For the Jovian planets, discuss how the lack of a real surface makes it difficult to state exactly how thick their atmospheres actually are, either in terms of the distance or the weight of the atmosphere.
Use diagrams as well as discussion to show the temperature and density changes which occur as you go from the surface of the Earth to outer space, and how temperature variations are used to define that various layers of the Earth's atmosphere. Explain how the pressure variations required to offset the weight compressing the atmosphere require either or both the density and temperature to decrease as you go upwards, and discuss the actual changes which occur. Discuss the heat sources which allow different parts of the atmosphere to be relatively warm, and explain why the heat source in the middle of the Earth's atmosphere doesn't exist in the atmospheres of other planets, so that they have relatively cold middle atmospheres. Compare the Earth's atmospheric composition and structure to those of the other planets.
List the temperatures for each planet, giving a range of values where appropriate, and discuss the various ways in which the atmosphere can affect the temperatures. Note that you do NOT need to discuss the ways in which other factors, such as distance from the Sun, seasons, day/night variations and such, affect the temperature. Consider those other variations in listing the range of "normal" temperatures, but only discuss the way in which the ATMOSPHERE affects temperatures.
Discuss theories of the formation and evolution of planetary atmospheres, especially the theory of volcanic outgassing. Explain how the absorption of ultraviolet radiation broke down hydrogen compounds in the atmosphere, leading to the loss of hydrogen, and the depletion of those compounds, and how the higher temperatures on Venus kept all such compounds as vapors, and allowed them to be destroyed, whereas the lower temperatures on the Earth and Mars would have liquefied water, and kept it at least somewhat "safe" from harm. Explain how the absorption of carbon dioxide by water, and the subsequent precipitation of carbonate rocks, removed almost all of the carbon dioxide from our atmosphere, and how a similar situation on Mars may have considerably reduced its atmosphere, leading to a runaway "refrigerator" effect there.
Discuss the way in which gases can escape from the outer atmosphere of a planet, and the factors (escape velocity and temperature of the exosphere) which affect whether gases could escape. Give examples of planets in various size ranges (four ranges were discussed in class) for which different kinds of gases can or cannot be retained by the planets' gravity, so that those gases are or are not found in those planets' atmospheres. For the size range in which both size (gravity) AND temperature (related to distance from the Sun) are important, give examples of objects which are too small or too hot to hold onto gases, and contrast those with otherwise similar objects which are big enough, and cool enough, to hold onto gases.
Question 5: For each planet, list the names of its larger satellites, and discuss their most interesting physical and/or orbital characteristics. Be especially thorough in discussing our own Moon. For our Moon, also discuss theories of its origin and the origin of its surface features.
List each planet, and for each one which has moons, list the names of a few of the moons of that planet (make sure that you don't list them with the wrong planet!). For each of the moons which you discuss, write a sentence or two for moons of minor importance, a short paragraph for moons of major importance, and several paragraphs for our own Moon (since the question specifies that you should be especially thorough, with it).
For Mercury and Venus, even though they don't have moons, you should mention that fact, so that it is clear that you know it, rather than having simply forgotten it. For planets with only one or two known moons, such as Mars
and the Earth
, discuss all known moons. For planets with a large number of moons, the following list seems reasonable, although you could discuss slightly fewer, if you are especially thorough with the ones which you discuss, or more of them, if you feel that your discussion might lack something, in terms of depth:
, and Callisto
(the four large, Galilean moons).
, and three or four other moons of your choice. A good way of choosing them would be to pick ones that you find especially memorable, for some reason — that way, you are more likely to remember them now, and on the day of the Final exam.
and two or three others of your choice.
, and one or two others of your choice.
In deciding what to write about each moon, you may choose those topics which you find most unusual or interesting. Since there are so many moons, and they are so varied in their characteristics, I am more interested in seeing that you have made a reasonable effort to learn about the moons as interesting objects, than that you learned some specific fact which might not be of any interest to you, at all. However, for our Moon, since certain topics (its origin
, and the evolution of its surface features) are specifically asked for, you should be sure to cover those topics, in addition to whatever other topics are of greatest interest to you.
Question 6: Discuss the physical and orbital characteristics of asteroids, and compare/contrast them with those of comets. Discuss the characteristics of meteorites, and describe how they are related to the asteroids. Discuss why the planets formed as single large bodies, while the asteroids remained as many small bodies.
Discuss the numbers of asteroids of different sizes (point out that although most asteroids are small, most of the mass is in the few large ones), and their physical properties (as estimated either from direct observation, or from comparisons with meteorites, since those are closely related to asteroids), and their typical orbits, in as much detail as possible. Briefly discuss the physical properties of comets, and their typical orbits, to show that you realize how different they are from asteroids (a typical answer would have more than a page about asteroids, and only a paragraph about comets).
Discuss the different types of meteorites. Discuss how the study of meteors shows that there are two types of meteoroids: light, fluffy things with orbits like comets, and dense, compact things with orbits like asteroids. Explain how, although most meteoroids are bits of comets, meteorites, being dense, must be bits of asteroids, and therefore help us know what asteroids are like.
Explain how, during early stages of the formation of the Solar System, the large numbers of small bodies caused frequent collisions and fast growth of the planetesimals; whereas later, once there were fewer objects, collisions were less frequent, and things grew more slowly. Where there were very few objects, as in the present asteroid belt, collisions slowed down so much that the process of growth is still unfinished. Where there were a lot of objects, as in the outer solar system, growth hardly slowed at all, until the protoplanetary bodies were much larger than asteroids. Explain how, since they were bigger than the asteroids, the planets were better able to sweep up the rubble left from their formation, and better able to hold onto the fragments which results from high-velocity collisions, so they could continue to grow, even after there were very few objects in a given region, until everything was in a single large object.
Question 7: Discuss the physical and orbital characteristics of comets, and compare/contrast them with those of asteroids. Discuss meteors and meteor showers, and explain how they are related to the comets. Discuss how comets change within a given orbit and over long periods of time. Discuss the origin of short and long-period comets, and the nature and probable origin of the Kuiper Disk and Oort Cloud.
Discuss the appearance of comets, and their typical orbits, in as much detail as possible. As mentioned below, be sure to discuss the way in which, as a comet approaches the Sun, the nucleus develops a head, a gas tail, and a dust tail. Briefly discuss the physical properties of asteroids, and their typical orbits, to show that you realize how different they are from comets (a typical answer would have more than a page about comets, and only a paragraph about asteroids).
Discuss the appearance of meteors, and how they are produced by meteoroids running into our atmosphere at high speed. Discuss how the gradual loss of material by comets produces most of the meteoroids which the Earth encounters, and how meteor showers result from our running into large numbers of cometary meteoroids within a short period of time.
Discuss the fact that the loss of material by comets must gradually cause them to decrease in mass, until eventually they become "dead" comets, unable to produce a head or tail. Discuss the theory of the Oort Cloud, and how perturbations of cometary nuclei in the cloud could cause them to change their orbits in such a way as to "replace" comets which had run out of material.
Discuss how the sweeping up of material by the Jovian planets in general, and Jupiter in particular, probably created a large, flattened disk of icy debris, which we call the Kuiper Disk, and how interactions of nearby stars within the cluster of stars that the Sun formed in would have gradually transformed that disk to a much larger, more spherical region, which we call the Oort Cloud.
Question 8: Describe the planetesimal accretion theory of the formation of the planets out of the gas and dust of the Solar Nebula. Discuss how the compositions and sizes of the planets are related to their position within the Nebula. Discuss the melting of the planets, their subsequent differentiation, and other events which occurred during the early history of the Solar System. Discuss the regularities of the planets' motions, and how these are thought to be related to the origin and evolution of the Solar System.
Describe how, during the formation of the planets (and the Sun), a large, flattened disk of gas and dust formed around the Sun, and how, through collisions, dust grains built up into larger objects, called planetesimals, and eventually, through both collisions and gravitational attraction (especially important for the Jovian planets, which had to gravitationally attract the large amounts of hydrogen gas which make up most of their mass), into planets.
Explain how the different temperatures at different distances from the Sun caused the composition of dust grains in different parts of the Nebula to differ, leading to objects of different composition, and, because different materials had different abundances, of different sizes.
Explain how, during early stages of the process, the large numbers of small bodies caused frequent collisions and fast growth of the planetesimals; whereas later, once there were fewer objects, collisions were less frequent, and things grew more slowly. Where there were very few objects, as in the present asteroid belt, collisions slowed down so much that the process of growth is still unfinished. Where there were a lot of objects, as in the outer solar system, growth hardly slowed at all, so that large sizes were reached before the Sun blew away the gases of the solar nebula, allowing the gravitational attraction of large amounts of material. Where there was an in-between amount of material, as in the inner solar system, growth slowed enough so that the gases were all blown away before the planets were large enough to hold onto them, but they were still able to sweep up material quickly enough to finish forming as individual planets.
Discuss the heat sources which caused the (Terrestrial) planets to melt, how that melting led to their differentiation (layered structure), and how the gradual reduction of the heating by radioactive materials led to their resolidification.
Discuss how the sweeping up of rubble left over after the origin of the planets would have scarred their surfaces, and the surfaces of any other bodies with solid surfaces, during the time that was required to complete that sweeping up. Discuss how the dating of rocks brought back from the Moon establishes the fact that this sweeping up must have been completed well before 4 billion years ago, so that any surface which shows evidence of the heavy bombardment caused by the sweeping up must be well over 4 billion years old.
Discuss the regularities of the planetary motions, and explain how these are thought to be related either to the way the Solar Nebula was rotating when the planets formed, or to gravitational interactions between the planets at that, or later, times. (Note: This is briefly covered in the online text, under Orbital Regularities
Question 9: Describe the appearance of the Sun, and of the various regions in the solar atmosphere; discuss the conditions within each part of the atmosphere, and the nature of the Sun's "surface." Describe the cycle of solar activity, discuss the changing appearance of the Sun during that cycle, and explain how these changes are related to changes in the Sun's magnetic field.
For each of the three layers (photosphere, chromosphere, and corona), describe what the region looks like, and how we observe it; discuss how thick it is (in miles or kilometers), its density and temperature range, and features (such as granulation and spicules) which normally occur within a given region. Discuss why the photosphere is brighter than the upper layers in the atmosphere, despite the fact that it is cooler than they are. Explain why, though the Sun is made of gases throughout its entire body, so that it has no real surface, it appears to have a surface.
Discuss the 11-year cycle of solar activity. Discuss the features, such as sunspots, prominences, filaments, and flares, which are associated with the cycle. Discuss how the number of sunspots, and their positions, change over time. Discuss how the differential rotation of the Sun gradually winds up the solar magnetic field, and how the intensified field causes the sunspots and other "active-Sun" phenomena.
Question 10: Describe the Sun's internal structure. Discuss how density, temperature, pressure and other quantities change as we go from the surface to the center. Describe the creation of energy in the solar core, the way in which the Sun's energy gets from the center to the surface, and the changes which occur in that energy while passing through the solar interior.
Discuss how these quantities change. Point out that, since the Sun is entirely gaseous, for it to be stable, pressure must increase as you go down into the Sun, so that it balances the increasing weight of the overlying layers; and, since pressure depends upon density and temperature, both those quantities must also increase. Give examples of the values for these quantities at various depths inside the Sun. Discuss the conversion of hydrogen to helium through nuclear fusion, and the energy produced through this process.
Discuss how the energy created in the center gets to the surface. Discuss how difficult it is for the energy to pass through the dense gases, and how long it takes to make the trip. Discuss how, in struggling through each layer of gas, the energy is changed to black-body radiation corresponding to the temperature of the gas, so that when it finally leaves the Sun, it is black-body radiation corresponding to the temperature of the photosphere. Discuss the convection and radiation zones, and explain, insofar as possible, what they are like and why they exist.
Question 11: Discuss how we can determine the distances, motions, apparent and real brightnesses, masses, diameters, densities, and temperatures of stars, and discuss the normal range of values for each quantity. Show how we can use a Hertzsprung-Russell Diagram to discuss the characteristics of different kinds of stars, to determine stellar distances, and to study stellar evolution.
Discuss the concept of parallax, and show how we can use the parallax caused by our orbital motion to determine the distances of stars; explain why this only works for some stars, and discuss alternate methods of determining distance for stars which are too far away to measure parallax. In particular, since it is specifically asked for, show how we can compare the Hertzsprung-Russell Diagram of a cluster of stars of unknown distance to the HR Diagram for stars of known distance to determine the distance of the cluster. Also discuss how, if we can estimate the "position" of a star in the HR Diagram, we can use that position, and the apparent brightness of the star, to estimate its distance.
Discuss the proper motions of stars, and explain how they are related to the actual motions of the stars; be sure to discuss how proper motion is affected by distance, and how, for very distant objects, the proper motion may be too small to measure. Discuss how, using the Doppler effect on a star's spectrum, we can determine its radial velocity. Discuss the fact that, although parallax and proper motion are affected by the distance of a star, radial velocity is not, so that we can measure the radial velocity of stars or galaxies no matter how far away they happen to be.
Discuss the apparent magnitudes of the stars, and explain how we can discuss different stellar brightnesses using the magnitude scale. Show how the apparent magnitude is affected by the star's distance, and how we can compensate for that by using the absolute magnitude. Explain how the distance modulus relates the apparent magnitude, absolute magnitude, and distance of a star. Discuss the concept of stellar luminosity, and explain how it is similar to, and how it is different from, the concept of absolute magnitude.
Explain how the spectrum of a black-body changes as its temperature increases, and discuss how this causes the colors of stars to change according to their temperature. Discuss the concept of color magnitudes, and show how, by comparing the B and V color magnitudes, we can create the color index, B-V, and relate it to the temperature of a star. Explain the advantages and disadvantages of the color index method of measuring stellar temperature.
Discuss the change in the spectra of stars due to their differing temperatures, and how that can be used to define spectral classes. For each spectral class, give the information specified in the question. Explain the way in which increasing temperature causes the changes in the spectrum, and (since this is sometimes a source of confusion) be sure to explain that these changes are due to temperature, rather than to a difference in stellar composition. Explain the advantages and disadvantages of the spectral class method of measuring stellar temperature; point out that, particularly when using the finer numerical subdivisions, each spectral class represents a much more accurate link to temperature than does color index.
Draw an HR Diagram, and explain how it displays the characteristics of stars of different brightnesses and temperatures, and discuss how it allows us to compare stars with different characteristics.
Show how we can use the orbital motions of binary star systems, and Kepler's Laws of Planetary Motion, to find the masses of the stars. Discuss the range of stellar masses which we have determined from such calculations. Discuss the problems that we can encounter trying to estimate the size of an orbit, and how they can give us an erroneous value for the mass. Explain how we can estimate the masses of stars that are not in binary systems using comparative methods such as the Mass-Luminosity Diagram discussed below.
Explain how we can use the eclipses of stars in eclipsing binary systems to measure their sizes, and discuss the range of stellar sizes which we have found to exist. Point out that such direct measurements are only possible in rare circumstances, so that for most stars we need to use some different kind of estimate. Discuss the black-body radiation laws, and show how, using Stefan's Law, we can estimate the size of a star from a knowledge of its brightness and temperature. Be sure to mention that, although there are very few stars whose sizes can be directly measured, they provide the proof that using Stefan's Law will work. (You may also want to discuss the technique of stellar interferometry for measuring stellar sizes, but since it is even more limited than eclipsing binaries, and not as accurate, if you are running short of time you may want to leave it out of your discussion.)
Explain how, once we know the masses and sizes of stars, we can calculate their densities, and discuss the range of densities that stars have been found to have.
Discuss the typical values of each quantity for stars in various parts of the HR Diagram. If you didn't already discuss how we can use the HR Diagram to estimate stellar sizes from Stefan's Law, be sure to cover it here. Discuss the Mass-Luminosity relationship that has been discovered for Main Sequence stars, and, if you didn't already explain how it could be used to estimate stellar masses in the discussion above, discuss that here.
Question 12: Discuss the formation of stars from clouds of gas and dust in the interstellar medium. Discuss the various stages of stellar formation in as much detail as possible, and explain why massive stars form very quickly, and low-mass stars form very slowly. Describe how the stars reach the Main Sequence, and why Main Sequence stars hardly change over long periods of time. Discuss how long different mass stars remain Main Sequence stars.
Discuss the conditions inside interstellar clouds, and explain why they are normally stable. Explain how, under some circumstances, clouds can collapse or be forced to collapse to much smaller sizes, leading to the formation of dark globules that are destined to become stars. Explain how conditions change inside the collapsing cloud, accelerating the initial collapse at first, then halting it when the clouds become opaque. Discuss the slow contraction which follows the initial collapse, and explain why, during this contraction, massive objects become very hot (in comparison to their size) and contract relatively quickly, while less massive objects become less hot, and contract relatively slowly. Explain how, because of this, massive stars are destined to become much larger and brighter than less massive stars, and to form and live out their lives in much shorter times. If you have time, discuss the ionization of hydrogen, and how it punctuates and separates the proto-stellar cloud, infrared object stage of formation from the red giant star, visible object stage of formation.
Whether or not you have time to discuss the ionization of hydrogen, discuss how, after the proto-stellar clouds become red giants and subgiants, they gradually contract to the Main Sequence. Be sure to point out the differences in the brightness, evolutionary paths and formation times for stars of different mass (be sure to point out that low mass stars become convective throughout, whereas high-mass stars don't; this will be useful in the next question). Discuss how, as internal temperatures climb, nuclear reactions can begin, and how, as the energy of those reactions begins to replace the heat being lost at the surface, the contraction of the stars begins to slow. It is a very common error in over-simplified discussions of stellar formation to suggest that when the nuclear reactions inside stars begin, they suddenly stop contracting, and suddenly become hot, bright upper Main Sequence stars. Since this is NOT correct, be sure to point out how, by stopping the contraction of the star, nuclear reactions stop the proto-stars from changing, so that whatever they are like when the nuclear reactions start up, that's the way they will remain until the nuclear reactions run out of fuel. Discuss how, since massive stars are very bright, even though they have lots of fuel, they will run through it very quickly and live very short lifetimes, while low-mass stars will husband their fuel and live virtually forever. Give examples of the lifetimes of stars of various masses.
(Also refer to the discussion of question 12 in Brief Additional Notes About Essay Questions 12 and 13)
Question 13: Discuss the ways in which stars of various masses age and die, starting with low-mass stars which never make it to the Main Sequence, and ending with massive stars which blow themselves to bits in supernova explosions. Discuss the structures and characteristics of white dwarfs, neutron stars and black holes. Explain why these dead stars are usually difficult to observe, and describe how they can (sometimes) be observed.
This question is a continuation of the previous one. Because of this, students often try to run them together. DO NOT DO THIS, as it will make it more difficult for me to grade the individual questions, and may hurt your grade. Where necessary, as pointed out below, BRIEFLY refer to previously discussed topics; but don't just run the two questions together.
Start by briefly discussing (or referring back to the previous questions' discussion of) the fact that, during the formation of stars, massive stars become relatively hot at a given size, and low mass stars are relatively cool at the same size, so that, to initiate nuclear burning, smaller mass stars must become much smaller and denser than more massive stars (if possible, give examples of the sizes and densities of stars of various masses, to emphasize the high density and small size of the lower-mass stars). Discuss how, if this continues down to stars of very low mass, they might become so dense that they would become electron-degenerate (since you are supposed to discuss white dwarfs later on in the question anyway, it might help save time to discuss what white dwarfs are like here, instead). Explain how, if such low-mass stars tried to form, they would presumably become quasi-liquid white-dwarf-like objects before becoming hot enough to burn nuclear fuel, and without any further energy of gravitational contraction, be forced to cool off and become "brown" or "black" dwarfs. Discuss what we mean by these terms, and discuss the possibility (or impossibility) of observing objects of this sort.
Next, discuss the stars of lowest mass on the Main Sequence. Explain how, since they are thoroughly mixed, these stars can burn all of their fuel while on the Main Sequence, so that they have none left to burn when they run out of fuel (if you forgot to discuss the convective structure of stars of low mass in the previous question, be sure to do so here). Discuss how, without any fuel to burn, they would shrink to become "red" white dwarfs, then cool off to become brown or black dwarfs; but be sure to point out that, since these stars have lifetimes of trillions of years, none can have actually died already, so that if any brown or black dwarfs already exist, they must be the very-low-mass stars previously discussed.
Next, discuss stars which are not thoroughly convectively mixed, like the Sun. Discuss how, as they run out of fuel in the core, they will have to adjust their structures to try to make up for this, and how this will lead to gradual increases in brightness while still Main Sequence stars, and to a rapid swelling to red giants once their fuel runs out. Discuss how, while the stars are red giants, they could try to burn various types of fuel, and what happens if they succeed in doing so. Discuss how in most such stars, like the Sun, electron degeneracy eventually stops the contraction of the core, causing a collapse of the outer layers of the star, ejection of a planetary nebula, and the formation of a white dwarf.
Finally, discuss the death of stars which are too massive to become white dwarfs. Explain how (and why) they never have to contend with electron degeneracy, and as a result will instead suffer some kind of catastrophic collapse, such as the iron-burning catastrophe discussed in class, leading (in many cases) to a supernova explosion, and to the formation of a neutron star or black hole.
(As mentioned above, white dwarfs might be best discussed when covering the death of stars too small to make it to the Main Sequence.) Discuss the nature of each kind of dead star. Discuss their sizes, masses, and densities, and the kind of material that they are made of. Be sure to discuss the peculiar fact that more massive white dwarfs and neutron stars are smaller than less massive ones, and that there is a limiting mass for such objects, so that very heavy dead stars can't be either of these kinds of objects. Point out that although white dwarfs are theoretically observable, their small size makes them relatively inconspicuous, so that we may not be aware of many of them, even when fairly close. Explain why it is practically impossible to observe neutron stars or black holes at interstellar distances. Discuss the peculiar sorts of objects, such as pulsars, X-ray bursters, and the like, which we think require neutron stars or black holes to explain. Discuss how the mass limit for neutron stars suggests that if we can ascribe such events to objects of substantial mass, they must, presumably, be black holes.
(Also refer to the discussion of question 13 in Brief Additional Notes About Essay Questions 12 and 13)
Question 14: Describe the structure of our Galaxy, indicating the positions of the Sun, the nucleus, the disk, the halo, globular clusters, and spiral arms. Describe the structures of other types of galaxies, and the differences between the various types of galaxies. Discuss "dark" matter in galaxies.
Use diagrams and discussion to show that you know what the structure of our galaxy is like. Be sure to show the distances and sizes of the various regions in either LY or pc.
Discuss the three types of "normal" galaxies. For each type, discuss what the structure is like, what the galaxy looks like, and the typical characteristics of such galaxies. Be sure to point out that, although ellipticals have little gas, and therefore contain almost exclusively old stars, and the other types still have gas out of which new stars are still forming, all of the galaxies appear to have actually started forming many billions of years ago, so that none of them are actually "young" in the normal sense of the word.
Discuss the fact that the masses of galaxies, and clusters of galaxies, as measured by the motions of the galaxies and the stars inside them, are much larger than the masses that would be estimated by looking at the visible stars that they contain. Explain how this leads to the concept of "dark" matter, much more abundant than the visible stars, to explain the mass difference. Be sure to make it clear that the dark matter discussed in this context MUST actually exist, and that the only question is, what exactly is it? Point out that although most discussions in recent years have concentrated on types of matter that are completely different from normal matter, and may or may not actually exist, it is quite possible, if not even probable, that the dark matter is simply ordinary stuff in a form that is difficult to observe (e.g., dead stars of various sorts, or superheated gases that are too thin and too hot to easily observe).
Question 15: Describe the structure of the Universe, and how we know that it is expanding. Describe the Open and Closed Big Bang theories of the Universe, and the Inflationary model, and the differences between them. Discuss the current version of the Big Bang theory, and its implications for the future of the Universe. Discuss the theory of "dark" matter as it relates to the structure of the Universe.
Very briefly discuss the distribution of galaxies in space, and how, although there is a lot of structure on a small scale, things look pretty uniform on a very large scale. Discuss the Hubble Expansion graph, and the observations which led to its discovery. Discuss how the relationship between the distances of galaxies and their recession speeds implies that the Universe is in some way expanding, or getting larger. Discuss the concept of Universal expansion, and explain how, if this is correct, the Universe will be much larger in the future, and would have been much smaller in the past. Discuss how this leads to the concept of the Big Bang, and how, in that theory, the Universe must have looked much different in the past than it does now. Discuss the observations which suggest that it did indeed look different in the past, so that the Big Bang must have actually occurred in some way at least similar to the theories discussed in the texts.
Discuss how the expansion of the Universe is affected by its mass, and how the expansion could continue forever, if the mass is small, or stop and reverse itself if the mass is large. Discuss the various versions of the Big Bang theory which result from these possibilities.
Discuss the smoothness of the microwave background, and how that leads to concerns about the structure of the early Universe. Discuss how the Inflationary Universe model of the Big Bang explains the smoothness of the microwave background, and how the theory suggests that the mass of the Universe is just on the borderline between the Closed and Open Big Bang models, so that the mass of the Universe should be much larger than the total mass of all the galaxies and clusters of galaxies. Discuss how this implies the existence of some kind of dark matter that is completely different from normal matter. Be sure to point out that, although the dark matter in galaxies is often thought to be the same kind of stuff as this new kind of dark matter, the two terms are not necessarily interchangeable, as the dark matter in galaxies could be normal matter in a form that is just difficult to observe. Also point out that, although the Inflationary model is very attractive, there is no proof that it is actually correct, so its prediction of huge amounts of "weird" dark matter is also not necessarily correct. Discuss how, in fact, recent observations of the Hubble constant cast considerable doubt on how massive the Universe can be, and on whether "hot" or "cold" dark matter can actually exist, so that alternative versions of the Inflationary model are now being proposed which do not include the concept of dark matter as previously understood.
Discuss how the most recent of those models, involving a more and more rapidly expanding Universe, which will expand forever and ever, faster and faster, because the mass ("normal" and, if it exists, "dark" matter) is no more than 1/3 of the critical mass, if that. Briefly discuss how this is actually a prediction of Einstein's Theory of General Relativity, which treats gravity as a positive curvature of space-time, and expansion as a negative curvature of space-time.