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# Problem%20Set%205%20Solution - Problem Set 5 Solutions...

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- 1 - Problem Set 5 Solutions Astronomy 202 Assigned: 03 April 2006 Due: 17 April 2006 Chapter 23: Review and Discussion: 1. What evidence do we have that we live in a disk galaxy? A wide band of light completely surrounds us and is known as the Milky Way. Perpendicular to this band are relatively few stars. Our perspective is thus from within a large disk of stars meaning we are in a spiral galaxy. 2. Why is it difficult to map out our Galaxy from our vantage point on Earth? Because we are inside the disk of the Galaxy, it is hard to determine what it looks like. Dust clouds obscure most features that are distant especially towards the galactic center. In many directions, the disk looks pretty much the same. 5. How are Cepheid variables used in determining distances? Cepheids have the unique property of a relationship between their period of pulsation and their luminosity. By observing the Cepheid and determining its period of variation, its true luminosity is known. Comparing the true luminosity with the apparent luminosity allows the distance to be determined. 9. Contrast the motions of disk and halo stars. The stars in the Galactic disk move in roughly circular orbits around the center of the Galaxy. The orbits all lie in the plane of the disk. The halo stars have approximately the same velocities as the disk stars but are moving in various directions, relative to the disk, forming a spherical halo around the Galaxy. These stars pass in and out of the disk, towards the center of the Galaxy and back out again into the halo. When seen locally, they appear to have high velocities relative to the Sun and other disk stars and the direction of their motion is at an angle to the plane of the disk. 10. How do we know that the Milky Way Galaxy has spiral arms? Radio observations of objects in the Galaxy reveal its spiral shape. Radio waves travel unhindered through dust clouds and so the entire Galaxy can be mapped. 13. What is self-propagating star formation? Imagine a group of newly formed massive stars somewhere in the galactic disk. When these stars form, the H II regions, or emission nebulae, that appear around them send shock waves through the surrounding gas. These waves can trigger new star formation. Similarly, when the stars explode in supernovae, more shock waves are formed. The formation of one group of stars provides the mechanism for the formation of more stars. 15. What does the rotation curve of our Galaxy tell us about its total mass?

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- 2 - Different parts of the Galaxy rotate at different rates. The rotation is actually the motions of the stars and gas around the Galaxy. Because the Galaxy is not a solid body, like the Earth, different parts of it move at different rates. The orbital motions are determined strictly by the amount of mass interior to the orbit. The velocity of the orbits at differing distances gives us the rotation curve of the Galaxy. By looking at the outermost orbits, the total mass of the Galaxy can be determined. 16. What evidence is there for dark matter in the Galaxy
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Problem%20Set%205%20Solution - Problem Set 5 Solutions...

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