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Chapter 5

# Chapter 5 - semimajor axis is 28,000 light-years We need to...

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Part II 31 semimajor axis is 28,000 light-years. We need to convert these to seconds and meters respectively: Using the fact that there are 9.46 10 12 km in a light-year, found in Appendix A: Now we can calculate the mass of the galaxy: 2.10 10 41 kg This number is probably easier to understand as multiples of the mass of the Sun, so let’s convert: Based on the given data, the mass of the galaxy is about 105 billion times the mass of the Sun. 2.10 * 10 41 kg * 1 solar mass 2 * 10 30 kg = 1.05 * 10 11 solar masses M galaxy = 4 p 2 a 6.67 * 10 - 11 m 3 kg * s 2 b A 7.25 * 10 15 s B 2 * A 2.65 * 10 20 m B 3 28,000 light-years * 9.46 * 10 12 km 1 light-year * 1000 m 1 km = 2.65 * 10 20 m 230,000,000 yr * 365 days 1 year * 24 hr 1 day * 60 min 1 hr * 60 s 1 min = 7.25 * 10 15 s Chapter 5. Light: The Cosmic Messenger This chapter focuses on the nature of light and matter, and also covers basic properties of telescopes. Note that throughout the book we use the term light as a synonym for electromagnetic radiation in general, as opposed to meaning only visible light. Thus, we are explicit in saying “visible light” when that is what we mean. As always, when you prepare to teach this chapter, be sure you are familiar with the relevant media resources (see the complete, section-by-section resource grid in Appendix 3 of this Instructor’s Guide) and the online quizzes and other study resources available on the MasteringAstronomy Web site. What’s New in the Fourth Edition That Will Affect My Lecture Notes? As everywhere in the book, we have revised to improve the text flow, added optional Cosmic Calculations boxes, improved art pieces, and added new illustrations. The art changes, in particular, will affect what you wish to show in lecture. We have not made any substantial content or organizational changes to this chapter. Teaching Notes (by Section) Section 5.1 Basic Properties of Light and Matter This section introduces several important concepts, including: wave properties of wavelength, frequency, and speed; wave-particle duality; the idea that light comes in the

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32 Chapter-by-Chapter Guide form of photons; the idea of light as an electromagnetic wave; and the basic terminology of atoms. Note that we never give the Bohr picture of the atom, instead discussing only the modern picture, albeit in rather vague terms (e.g., stating “electrons in atoms are ‘smeared out,’ forming a kind of cloud . . . ”). This reflects our belief that the Bohr model, while useful for purposes of calculation, tends only to reinforce misconceptions about atoms that most students bring with them to our courses— namely, the belief that electrons look and act like miniature planets orbiting a miniature Sun. A note on atomic terminology: Astronomers usually refer to the number of protons neutrons in an atom as its “atomic mass.” However, chemists use this term for the actual mass as a weighted average of isotopes found on Earth (i.e., the mass shown on the periodic table). Thus, the formal name for the number of protons neutrons is “atomic mass number.” We use this term so that students
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Chapter 5 - semimajor axis is 28,000 light-years We need to...

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