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Download Physics 127 Descriptive Astronomy Homework #20 Key
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Physics 127 Descriptive Astronomy Homework #20 Key (Chapter 14) Winter 2015 14-3. How did Edwin Hubble prove that the Andromeda “Nebula” is not a nebula within our Milky Way Galaxy? Hubble was able to detect Cepheid variable stars within that “Nebula.” Then by observing their light curves and using the known period- luminosity relation for Cepheids, he obtained and compared the absolute magnitudes of these Cepheids with his observed apparent magnitudes, yielding a distance for the Andromeda “Nebula” which put it far outside of our galaxy. 14-5. Which is more likely to have a blue color, a spiral galaxy or an elliptical galaxy? Explain why. A spiral galaxy. The blue light comes primarily from O- and B-type stars in a spiral galaxy. Elliptical galaxies contain little gas and dust and therefore little, if any, star formation occurs within them. Therefore they contain few or no blue O- and B-type stars. 14-6. Why are Type Ia supernovae useful for finding the distances to very remote galaxies? Can they be used to find the distance to any galaxy you might choose? Explain your answer. Because the peak luminosity of a Type Ia supernova is well known, as they are all nearly identical, and because they are extremely luminous, they are superb standard candles for determining the distances of remote galaxies. They can be used for virtually any galaxy. However, there is a downside. The appearance of a Type Ia supernova is completely unpredictable. An observer might have to wait decades, or even centuries for a Type Ia supernova to occur in a preselected galaxy. A wise observer takes advantage of such a supernova wherever and whenever it occurs, rather than choosing to wait for one in any particular galaxy. 14-9. What is the difference between a cluster and a supercluster? Are both clusters and superclusters held together by their gravity? A cluster is a group of associated galaxies, while a supercluster is a group of associated clusters. Both are held together by the gravity of either their member galaxies or their member clusters of galaxies.