StudyGuide4 - Astronomy 309R Spring 2009 Study Guide to...

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1 Astronomy 309R – Spring 2009 Study Guide to help prepare for Exam 4 Please read chapters 9, 10, and 11 in Hawley & Holcomb. Chapter 11 is more mathematically advanced than we need for this course, but please do read it carefully nonetheless. Also, please do review the lecture slides on black holes and the expansion of the universe in the Blackboard. Particularly general and important concepts are underlined. Black Holes Explain the event horizon of a black hole. Is the event horizon a material surface, like that of a star or a planet? (The event horizon is not a material surface, but an imaginary, geometrical surface around the center of the black hole.) Write down the formula that relates the Schwarzschild radius to the mass of a black hole. The Schwarzschild radius equals the radius of the event horizon when the black hole does not rotate; the event horizon of a rotating black hole is slightly smaller than that of a non-rotating black hole. How can we relate the Schwarzschild radius to the escape velocity? [The Schwarzschild radius coincides with the radius where the escape velocity equals the speed of light. Square of the escape velocity at distance R from a body of mass M equals ( V escape ) 2 = 2 G M / R. When R equals the Schwarzschild radius, the escape velocity equals the speed of light.] What happens to the “strength of gravity,” as we have defined in lecture, as one approaches the event horizon of a black hole? Imagine that from a distance, we are watching a light source fall into a black hole. As the source approaches the event horizon, would it appear redder of bluer than it really is? (Please consider the gravitational redshift or blueshift form the lecture on general relativity.) Consider a clock approaching the event horizon of a black hole. Does the clock appear fast or slow? How could an observer inside a rocket notice that they are falling into a black hole? (The tidal gravitational field is strong; the observer’s body will be stretched head from toe.) Consider a person falling into a black hole. At the moment that the person passes the event horizon, the person’s entire future (all the events that can be causally influenced by the person) is contained inside the black hole. The observer’s entire past (all events that that can causally influence the person) is contained outside the black hole. At the moment of passing the event horizon, the person sees only the world outside the black hole, but not the world inside the black hole. Consider a spaceship that uses its rocket engines to fly around a black hole just outside the event horizon. What can be said about the velocity of the spaceship? (The closer to the black hole that a rocket tries to orbit, the higher the rocket’s velocity must be. Just outside the black hole’s horizon, the rocket’s velocity must be close to the speed of light.) What are the two sizes in which black holes appear in the universe? (Astronomical black holes have been found with masses between about 5 and 15 solar masses—these are the “stellar-sized”
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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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StudyGuide4 - Astronomy 309R Spring 2009 Study Guide to...

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