StudyGuide2 - Astronomy 309R Spring 2009 Please read Hawley Holcomb Part II Chapters 4 and 5 Please also read the slides of Lecture 5 on stars and

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1 Astronomy 309R – Spring 2009 Please read Hawley & Holcomb, Part II, Chapters 4 and 5. Please also read the slides of Lecture 5 on stars and Lecture 6 on the formation of stars and galaxies. Please also read the short excerpt, available in Course Documents and marked in red, from Arthur Berry’s A Short History of Astronomy (1899) that discusses the source of energy that powers the sun. Please understand the physical reasoning in the excerpt. Particularly general and important concepts are underlined. Give examples of sources of energy . Why are gravitational potential energy and nuclear potential (or nuclear binding) energy sources of energy? Please explain the logic behind the “source of energy” as a physical concept in view of the fact that energy cannot be created or destroyed, but simply changes form. Explain and give an example for the mass-energy duality. (What is antimatter? What are positrons? How much lighter are the electron and the positron compared to the proton? Explain what happens when a positron collides with an electron. What kind of photons is produced? At least how large is the energy of the two photons produced during electron- positron annihilation? Write down and explain the famous mathematical formula that quantifies mass-energy duality. What is the name of the proton’s “anti-particle,” with which it can annihilate to produce high-energy photons? What can happen when two photons with sufficiently high energies pass each other, or “collide”?) What is nuclear fusion? Explain the proton-proton chain. Why is the proton-proton chain possible only in very hot environments, such as in the centers of stars? (What is the approximate temperature at the center of the sun, in degrees Kelvin? What is approximate temperature at the surface of the sun, also in degrees Kelvin?) Understand how one estimates the amount of energy given to the environment during the proton- proton chain. (Hint: There are two steps to an estimate. In the first step, notice that the final product of the proton chain—which is the nucleus of what element?—has smaller mass than the mass of the four protons that fused together to produce it. In the second step, invoke the mass-energy duality, discovered by Einstein, to relate the discrepancy between the two masses to the energy that is liberated during the proton-proton chain. This is a very rough estimate, but good enough to see that the energy produced in the proton-proton chain is sufficient to power the sun.) For how long would the sun have shined only on the account of its gravitational potential energy? Describe in words and in a simple equation how one estimates the time that a star can shine, given a specific source of energy. (Hint: Think how far you can get on a full tank of gas, knowing the capacity of your tank in gallons, and the mileage per gallon for your car. We have the same idea here, except that instead of calculating distance, we are calculating time.) Describe in words how would one estimate the duration of time that sun can shine on the
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This note was uploaded on 09/14/2009 for the course AST 309 taught by Professor Johnlacy during the Spring '08 term at University of Texas at Austin.

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StudyGuide2 - Astronomy 309R Spring 2009 Please read Hawley Holcomb Part II Chapters 4 and 5 Please also read the slides of Lecture 5 on stars and

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