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1 point	PROBLEM 1 Why does the Earth seem to be at the center of the Hubble expansion? The Earth appears at first glance to be the center of the universe's expansion because all galaxies are moving away from us. However, because the speed at which those galaxies are receding from us is proportional to their distance, the same would be true for any other galaxy, as well. Or, put another way, if an astronomer in any other galaxy were to measure the redshift of galaxies that he could see, he would see the same relationship between recession velocity and distance that we see here. This is similar to the balloon expansion demo done in class.
	PROBLEM 2 If the universe is closed, describe the reults that some future scientist like Edwin Hubble would obtain when he or she looked through a telescope during the period of contraction. Would other galaxies be visible? Would they display a redshift?
1/2 point	If a future scientist were to study galaxies during the contraction, he or she (or it, hey, they don't have to be human) would obtain results that are the opposite of those Hubble discovered. All of the galaxies would be moving TOWARD the Earth. Their velocity would still be proportional to their distance, but this time the DIRECTION would be the opposite of the Hubble case.
1/4 point	The other galaxies might or might not be visible, depending on how much time has elapsed between now and the observations. If the universe is only just barely closed, or it only has very slightly over the amount of mass necessary to cause the universe to contract, then a lot of time, hundreds of billions of years at least, will pass between now and when the universe turns around and starts to contract again. If this is the case, then the universe might already be well on the way to heat death by the time the contraction really gets going, with all of the stars reduced to black dwarfs and invisible. However, if there is ample mass to close the universe, then the path to the big crunch will begin relatively soon and the galaxies will be visible to a future observer.
1/4 point	The galaxies will not be redshifted, they will be blueshifted, or shifted up in frequency (down in wavelength) because of their motion toward the observer.
1/10 point each	PROBLEM 3 Outline the major events for the current big bang model of the evolution of the universe. Life of a Universe I. The Big Bang a. Before 10-43s: all 4 fundamental forces unified. b. 10-43s: gravity splits off from the other forces. c. 10-35s 1. Strong nuclear force splits from electroweak force. 2. Matter forms preferentially over antimatter in the form of quarks and electrons. 3. Inflation? d. 10-10s: nuclear weak and electromagnetic forces split. e. 10-5s: quarks form nucleons (protons and neutrons). f. 1.8 * 102s: nucleons form atomic nuclei. g. 2 * 1013s: recombination 1. Electrons and nuclei form atoms 2. Matter and radiation are decoupled, universe becomes transparent. II. Current universe a. Galaxies form -- we're not quite sure just when this happens, we know its after recombination, but as we keep looking further and further away, we keep seeing older and older galaxies. The edge, along with the first forming galaxies, has yet to be discovered. b. 1.2 * 1010 years: present day.
1/3 point each	PROBLEM 4 What are the fates of an open, closed and flat universe? Open universe: not enough mass to halt expansion, expands forever. Flat universe: just enough mass to halt expansion, but not enough to initiate contraction -- heat death. Closed universe: more than enough mass to halt expansion, universe contracts toward big crunch.
	PROBLEM 5: If a galaxy is 500 Mpc (megaparsecs) away, how fast will it be receding from us? This one's a plug-and-chug into the Hubble relation, equation 1 above. v = Hod v = 75(km/s)/Mpc * 500Mpc v = 37500 km/s
1 point	3.75 * 104 km/s
1/2 each	PROBLEM 6 How can we talk about the evolution of stars over billions of years when human beings have been observing stars for only a few thousand years? There are two ways the evolution of stars over such huge timescales can be studied over a much shorter time period. The first is theoretical: once stars' power sources were discovered it became possible to create analytical and computational models of stars and their evolution over time. These models are based, however, on observations. Although we can't follow a single star through its entire lifetime, we CAN observe many separate stars, each at a different phase of their evolution. From the proto-planetary disks that HST discovered in the Orion nebula, through T-Tauri stars and other phases of star formation, to main sequence stars like the sun, to red giants like Betelguese, and finally to white dwarfs like Sirius B and supernovae like SN1987A, stars can be found that can be pieced together to form a coherent picture of what a single star's entire lifetime might look like.
1/3 point for each italicized aspect	PROBLEM 7 What are some differences between a larger star and a smaller star? This is a poorly worded question, it should read "what are some differences betwen a MORE MASSIVE star and a LESS MASSIVE star?". More massive stars are hotter, brighter(both while on the main sequence only), and live less long than less massive stars. If a star has enough mass, it will eventually fuse elements heavier than Hydrogen, and if it has more than about 8 solar masses it will go supernova after creating iron in its core.
1/2 each	PROBLEM 8 What is meant by "hydrogen burning" in the Sun, and why does it have to take place in the center of the sun? "Hydrogen burning" refers to the nuclear reaction of many protons combining to form Helium (called fusion), which in the process liberating energy. Fusion requires very high temperatures and pressures, and only occurs in the center of the sun because the center is the only location at which these contditions exist.
1/9 for each phase, interior, or surface	PROBLEM 9 Outline the evolution of a star similar to the Sun from its main-sequence phase to its white dwarf stage. For each significant step, briefly describe and distinguish the physical conditions in the interior of the star, and those properties that are observed on the surface of the star. Life of a 1 Solar Mass Star Phase Timescale Interior Surface main sequence 1010 years Hydrogen fuses to Helium yellow(5800K), 1.3*106km across red giant few 106 years Helium fuses to Carbon, temp higher than for main sequence red (~3000K), about 1AU in radius white dwarf few 1010 years no fusion, exterior held up by degeneracy pressure white (~10,000K), about 10,000km in diameter, cools slowly over the rest of the age of the universe
	PROBLEM 10 If the Sun shines for 11 billion years at its present energy output, how much total energy will the star send into space during its lifetime? Here we'll just rearrange equation 2 and get the total energy by multiplying the power times the time. Be careful to convert 11 billion years into seconds, since a Watt is a Joule/s. P = E/t E = Pt E = 4.24*1023kW * 1.1*1011years E = 4.24*1026J/s * 1.1*1011years * 3.15*107s/year Remember to convert kW to W, too.
1 point	E = 1.5 * 1045 Joules