Astronomy 300 homework #7 Spring ’16 Due date: Thursday, March 17
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| Astronomy 300 HOMEWORK #7 Spring ’16
Due date: Thursday, March 17 1. Order-of-magnitude estimates (no calculators!!) Show all your steps in detail. How fast would you need to travel to make a red stoplight appear green? In what direction would you need to travel relative to the stoplight? Compare this the speed of the space shuttle around Earth. Is the space shuttle going fast enough to make red stop lights look green, if there were any up in low Earth orbit? Calculators may now be used. Watch those significant figures (reread handout!) 2. Hydrogen atom. (i) Compute the energy of the photon that would be emitted if an electron dropped from the n=7 to n=3 level. (ii) What wavelength would this photon have? (iii) Which “series” is this photon part of (Lyman, Balmer, …) 3. Relative motion of the Andromeda galaxy (M31) and the Milky Way The rest wavelength of the H line of hydrogen is at λ0 6562.8 Å. Suppose you observe this line in the spectrum of M31, but find that it is at λ 6560.2 Å. (i) What can you conclude about the relative motion of the Milky Way and the Andromeda galaxy? Are they moving together or apart? At what speed? (ii) At this rate, about how long might it be until the two galaxies collide? Express your answer in seconds and in years. (iii) How well do your results in (i) and (ii) compare to the statements on p.8 of Croswell? Make a quantitative comparison. (iv) Can you think of any reason the two galaxies might not actually collide, at least not on the “next pass”? Explain. (v) To check your result, you decide to look at the H line, for which λ0 4861.3 Å. At what wavelength do you expect to observe this line in the spectrum of Andromeda? 4. Spectral lines. Imagine that you have obtained a spectrum of a star and find that it has seven prominent absorption lines. You observe the lines at the following wavelengths: 3930.7 Å, 3964.7 Å, 4098.6 Å, 4336.4 Å, 4380.3 Å, 4856.9 Å, 6557.5 Å
in various ionization states, can you deduce what lines these are? What element, in what ionization state, is responsible for each one? Justify your answer in detail.
your analysis? Verify that each of the lines gives a consistent result. (iii) Considering Croswell’s discussion of star motions in Ch. 4, is this star more likely to be part of the Galactic disk (like the Sun) or part of the halo (whose stars do not orbit the Galaxy rapidly)? 5. Rotation of galaxies Suppose you observe the H line (rest wavelength = 6563 Angstroms) from an edge-on spiral galaxy. You find that at the center of the galaxy the line is at 6565 Angstroms. At a distance 50,000 ly from the center, the H line is at 6560 Angstroms and 6570 Angstroms on the left and right sides, respectively. (i) Is the galaxy moving toward or away from the Milky Way? How fast? (ii) How fast is the galaxy spinning? (iii) What can you infer about the mass of the galaxy? (Hint: assume the lines come from stars that are in circular orbits around the center of the galaxy.) 6. Essay (1 page, in your own words). Please type, hand in on separate sheet with name.
Edward Pickering that the faint companion to the nearby star Omicron Eridani was of spectral type A (see Croswell, p. 55). In the context of what you have learned about thermal (“blackbody”) radiation, explain in your own words why he would have been so surprised, and why this implied that a new and very strange type of star had been discovered. Download 10.26 Kb. Share with your friends:
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