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| The Milky Way |
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1. | The hydrogen atom becomes unbound if we supply 13.6 eV of energy to it. In that case the electron has sufficient energy to escape the hydrogen nucleus. This process is called ionization. Calculate the energy in ergs to ionize hydrogen. This energy is called the ionization potential of hydrogen. |
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2. | Calculate the wavelength of light necessary to ionize hydrogen. |
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3. | Compare the average densities of (1) the particles in the solar wind near the Earth, (2) emission nebulae, and (3) the best vacuums that can be produced in Earth laboratories. |
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4. | The nebulosity surrounding the Pleiades Cluster produces a faint reflection nebula. Why do you think this is not an emission nebula instead, since there are hot blue stars in the cluster? |
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5. | Consider the orbits for individual stars in our galaxy. The stars with the most elongated orbits also have low metal content. Why do you think the metal content could cause the orbit of a star to be elongated rather than circular? |
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6. | There are several "tracers" of spiral structure in spiral galaxies. What do all of them have in common? |
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7. | Approximately how many trips has the Sun made around the galaxy since its birth? |
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8. | The hot coronal gas of the interstellar medium is a small component by mass, but a large component (perhaps the largest) by volume. Explain. |
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9. | Why must one correct for the effect of interstellar dust to construct an accurate HR diagram? |
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10. | By how much will the intensity of visible light be diminished if it passes through 8000 parsecs of interstellar medium comparable to that near the Sun? Does this explain the difficulty of seeing the central region of the galaxy in visible light? |
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11. | Suppose you see an absorption line in a stellar spectrum. What is one way to test whether the absorption happened in the surface of the star or whether it might have been due to a gas in the interstellar medium between you and the star. Hint: how does gas density influence the spectrum? |
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12. | In the vicinity of the Sun, how much does visible light decrease in intensity because of dust absorption over 1000 parsecs? |
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13. | How could you test the idea that the absence of galaxies in the sky in the "zone of avoidance" near the Milky Way isn't a real lack of galaxies but just that the dust of our galaxy's disk is hiding them from our view? |
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14. | The companion of the strong black hole candidate, Cygnus X-1, is a blue supergiant star called HDE 226868. It is apparent magnitude 9, so it is easily visible through a small telescope if one knows where to look (the companion, not the black hole!). However, the Observer's Handbook says, "in telescopes with sufficient aperture to cause colors to appear, the star appears orange". Why, if it is a blue supergiant? How do we know that it is a blue supergiant if it is orange? Hint: does the Milky Way run through Cygnus? |
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15. | For dust density typical for that near the Sun, how much will the magnitude of a Cepheid variable be overestimated because of obscuration by dust for a thousand parsecs of distance? How much error would this cause in the distance if not corrected? |
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16. | If Pop II stars were the first stars to form in the galaxy we would expect them to have low metal content. Their metal content is low, but not zero. From where could the metals have come? |