Fall 2005 ASTR 1120-001 Review for Midterm

Fall 2005 ASTR 1120-001 Homepage

Fall 2005 ASTR 1120-001 Review for Midterm Th Oct 6

The midterm will be mostly multiple-choice. The intention is that all material on the midterm (saving matters of logic and common sense) is referred to somewhere here.

The midterm will test everything covered in class so far, specifically, the material outlined in weekly summaries. Surely a couple of the clicker questions will be on the test. We have covered Ch 1, 4, 6, and 15-18 of Cosmic Perspective, although not exhaustively. The midterm will not test parts of the book that were not covered in class (for example, we did not do much on Ch 4, we did not do helioseismology, etc.).

Our place in the Universe. Solar system, Milky Way, Local Group, Local Supercluster, the observable Universe. What are they?
Estimation. How does order of magnitude estimation work? Think up an example of where you might apply order of magnitude estimation in astronomy.
Distances in Astronomy. Name two measures of distance commonly used by astronomers. What do these measures signify? Describe how trigonometric parallax works. Who first measured the Astronomical Unit, and how? Who measured the first distance to a star, and how?
Looking back. When we look deeper into space, we are looking back further in time. Explain. Be quantitative.
Escape velocity. What does gravitational escape velocity mean? What does the speed at which modern spacecraft go have to do with the gravitational escape velocity from Earth?
The Nature of Light. What is light? Is it a force? A form of electricity? A form of energy? What has light got to do with electricity and magnetism? Is light a particle or a wave? When does light behave like a wave, and when does it behave like a particle? What phenomena show that light is a wave? What phenomena show that light is a particle? What quantitative properties does light have? How are these quantitative properties related?
Types of Light. Radio waves are a kind of "light". What other kinds of "light" are there, and in what respects do they differ from each other? What kinds of light can you see? What kinds of light can you feel? What kinds of light can you hear? What kinds of light penetrate the Earth's atmosphere?
Spectrum. What does spectrum mean? What is the difference between emission lines and absorption lines? Draw pictures illustrating setups which might allow you to observe (a) emission lines, and (b) absorption lines from a substance.
Planck or Blackbody Spectrum. What is a Planck, or blackbody, spectrum (Bennett refers to radiation emitted with a blackbody spectrum as thermal radiation)? Under what circumstances would you expect something to have a Planck spectrum? How is the spectrum related to temperature? What is Wien's Law? How is the spectrum related to color? Is a red star hotter than a blue star or vice versa?
Energy levels of atoms. Why can electrons occupy only orbits with certain specific energies? Does the same principle apply to the solar system? Draw a schematic diagram of the energy levels of a hydrogen atom. How many levels are there? Draw processes corresponding to (a) emission, (b) absorption, (c) ionization, (d) recombination. Are the wavelengths/frequencies of emission lines the same as absorption lines? How is the frequency related to the energy levels of the emitting/absorbing atom?
Absolute Temperature. Review the worksheet on Temperature. What is absolute temperature? What is the conversion factor between energy and Kelvin? What is absolute zero? When can an object be described by a temperature? What is thermodynamic equilibrium? What is the spectrum of an object in thermodynamic equilibrium called? What is Wien's Law? What is the Stefan-Boltzmann Law?
Gravity. If a gravitating system loses energy, does it cool down (decrease temperature) or heat up (increase temperature)? Explain. Give one or more examples where this happens in astronomy.
Plasma. What is a plasma? Under what conditions does matter become a plasma?
Eddington. What profound new idea did Eddington have about the condition of matter inside the Sun that revolutionized our concept of the Sun? How did Eddington estimate the temperature at the center of the Sun? For a description of his ideas in his own words, read Eddington's 1920 Address (annotated by Prof. Dick McCray).
Hydrostatic Equilibrium. Bennett calls this "gravitational equilibrium". What is hydrostatic equilibrium? Is the Sun in hydrostatic equilibrium? Explain.
Layers of the Sun. The Sun has a core, a radiative layer, and a convective layer. What are they?
Outer regions of the Sun. What is the photosphere? The chromosphere? The corona? What role do magnetic fields play in shaping the appearance of the chromosphere and corona? Where do the magnetic fields come from?
Solar Energy. What is the source of the Sun's energy? Back at the beginning of the 20th century, a leading theory was Kelvin's suggestion that the Sun was powered by its own gravitational contraction? What was wrong with that theory? Has the Sun ever been powered by gravitational contraction?
Nuclear Fusion. Give an account of the fusion of hydrogen to helium in the Sun. Where does the energy come from?
Solar Neutrinos. How does the Sun produce neutrinos? Where do they go? What is the solar neutrino problem?
Solar Spectrum. Describe the Sun's spectrum. What are Fraunhofer (1814) lines? Explain why the Sun's spectrum looks the way it does. What is it possible to learn from the spectrum of the Sun?
Stellar Spectra. What are Williamina Fleming, Annie Jump Cannon, and Cecilia Payne-Gaposchkin famous for? What is spectral type? What has temperature got to do with spectral type? What is the sequence of spectral types?
Stellar Composition. Of what elements are stars composed, mainly? How do we know that?
Ha in stars. In what spectral type of star is the Ha line strongest? Is the line in absorption or emission? Draw an atomic energy level diagram of H showing the Ha, Hb, Hg ... lines. Sketch a graph showing how the strength of the Ha line varies with temperature. Explain physically why the Ha line varies with temperature the way it does.
Hertzsprung-Russell diagram. The Hertzsprung-Russell diagram was the observational key that unlocked our understanding of the stars at the beginning of the 20th century. You should have a thorough understanding of what an HR diagram is, where stars appear in it, and how they evolve in it. Don't forget Stefan-Boltzmann.
Star Clusters. Why are star clusters useful in studies of the HR diagram? The HR diagrams of different star clusters often look very different. Why? Describe the differences. What is the main sequence turn-off point of a cluster, and what can be deduced from it?
Main sequence. What is the main sequence? What is it a sequence of?
Stellar lifetimes. Which stars evolve fastest? Slowest? How do we know that?
So Simple a Thing as a Star. The structure and evolution of stars is simpler to understand scientifically than, for example, human beings. Why?
Stellar evolution. We focused on the evolution (a) of a star like the Sun, and (b) of a massive star (greater than about 8 solar masses). Draw the evolution of these stars on an HR diagram, and label the chief events in their lives.
Gravitational Energy. At the beginning of this course we discussed the great principle that when a gravitating system loses energy, it heats up, and we mentioned one example ž how a protostar gets hot enough to ignite hydrogen. We have now encountered several other examples. What are they, and what happens specifically in each case? For example, what role does gravitational energy play: (a) when a main sequence star exhausts hydrogen at its core? (b) when a core collapse supernova occurs? (c) in heating the material in an accretion disk up to x-ray emitting temperatures as it swirls on to a neutron star or black hole?
Quantum Mechanics. According to quantum mechanics, all particles are also waves, with a wavelength inversely proportional to their momentum, and a frequency proportional to their energy. Particles also have spin, a bit like a gyroscope, so they know about direction in space. Particles such as electrons satisfy an "exclusion principle": two electrons cannot occupy the same place simultaneously.
Electron Degeneracy. Electron degeneracy is a quantum mechanical effect that arises because electrons are waves, and they do not like to be packed closer than one wavelength. The fact that the outer electrons of solid metals are electron degenerate is what gives metals their characteristic metallic properties of high conductivity and reflectivity. Electron degeneracy provides the pressure support for white dwarfs and the cores of red giant stars (for stars less massive than about 3 suns). Electron degeneracy plays a crucial role in the Helium flash, and in supernovae of both Types. How does electron degeneracy solve Eddington's paradox: "If a star always grows hotter when it loses energy, how can it ever cool down?".
Helium flash. What is the Helium flash? When does it happen? What happens? Why does it occur? What does helium-burning produce that is crucial to the existence of life?
Red Giant. What is a red giant? What is it made of? What is happening at its center? Red giants typically have strong stellar winds; what does the wind do to the red giant? Are red giants necessarily more massive than main sequence stars? Are they necessarily older? Are they necessarily larger? Are they necessarily cooler?
White Dwarf. What is a white dwarf? What is the observational evidence for their existence? What pressure holds it up against gravity? What happens as the mass of a white dwarf increases, say because a binary companion is losing material on to the white dwarf?
Planetary Nebula. What is a planetary nebula? What is the connection between a red giant, a planetary nebula, and a white dwarf?
Chandrasekhar Limit. What is the Chandrasekhar Limit? What role does the Chandrasekhar Limit play in each of the two Types of Supernova?
Thermonuclear supernova (Type Ia). What observational evidence suggests that Type I (definition: spectrum does not contain H lines) supernovae represent the explosion of a white dwarf? [Note: actually it's just Type Ia supernovae that represent the explosion of a white dwarf; it is now recognized that some Type I supernovae, classified Type Ib and Type Ic, are core-collapse supernovae in which the massive star has lost its hydrogen envelope in a wind.] Why does the light curve (the brightness of the supernova as it changes with time) suggest that the light is powered by the radioactive decay of Nickel 56? What causes the white dwarf to explode? What is the source of the energy of the explosion? Does the supernova leave behind any part of the star?
Core-collapse supernova (Type II). What observational evidence suggests that Type II (definition: spectrum contains H lines) supernovae represent the explosion of massive stars? Describe the sequence of events inside a star more massive than about 8 solar masses which is thought to lead up to a supernova. What powers the explosion? The core of the star is made of iron just before it collapses. Why iron? What stops the collapse of the core? The electrons and protons of the iron core combine into neutrons, emitting in the process what kind of particle? What is the source of the energy of the explosion? Does the supernova leave behind any part of the star?

Van Gogh's Starry Night Fall 2005 ASTR 1120-001 Homepage

Updated 2005 Oct 4