Problem%20Set%203%20Solutions

Problem%20Set%203%20Solutions - Problem Set 3 Solution...

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- 1 - Problem Set 3 Solution Astronomy 202 Chapter 18: Review and Discussion: 1, 3, 9, 13, 15 1. Give a brief description of the interstellar medium. The interstellar medium is made up of gas and dust. Gas is much more abundant yet has a density of only about 1 atom per cubic centimeter. The density of dust is even lower, about 100 times less by mass. The composition is about 90% hydrogen, 9% helium, and 1 % heavier elements. Most of the heavier elements are found in the dust particles. Temperatures average around 100 K, although they can be much higher in emission nebulae and lower inside molecular clouds. 3. Why is interstellar dust so much more effective than interstellar gas at absorbing starlight? Interstellar gas is mostly transparent to radiation, absorbing only a few specific wavelengths. Dust particles can absorb many wavelengths and selectively reflect/scatter only the shortest wavelengths. Therefore, more of the spectrum is affected by dust than by gas. 9. Why are some spectral lines observed in emission nebulae not normally seen in laboratories on Earth? The so-called "forbidden" spectral lines occur only in a very low density environment that is impossible to reproduce in a laboratory. 13. What is 21-cm radiation? With what element is it associated? 21-cm radio radiation is emitted by cold hydrogen gas. This gas is found throughout the entire galaxy. This wavelength of radiation passes through dust clouds without being scattered; the entire galaxy is visible in 21-cm radiation. The temperature and density of the gas can be determined along with its motion. 21-cm radiation has been used to map out the entire galaxy. 15. How does a molecular cloud differ from other interstellar matter? Molecular clouds occur in the densest and darkest interstellar clouds. Here, molecules of hydrogen and other complex substances form. The dense dust cloud protects the molecules from destruction by starlight and may also provide a surface on which the molecules can form. That is, they are much denser and colder than the general interstellar medium. They are the centers of star forming regions. Problems: 1, 3, 6, 8, 10 1. The average density of interstellar gas within the “Local Bubble” is much lower than the value mentioned in the text—in fact, it is roughly 10 3 hydrogen atoms/m 3 . Given that the mass of a hydrogen atom is 1.7 10 -27 kg, calculate the total mass of interstellar matter contained within a Bubble volume equal in size to planet Earth. Each cubic meter will contain 10
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Problem%20Set%203%20Solutions - Problem Set 3 Solution...

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