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Announcements: -Skipping section 12.4 Extra Credit opportunity!! Friday, April 17, 2007 Telescope Viewing at Evergreen Valley College for more information and directions: http://www.evc.edu You will need to get handout from class (or on class website) and complete it at the event in order to get credit!! 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Chapter 12 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Measuring the Properties of Stars (Chp. 12) • Distance from the Sun • Temperature • Luminosity (magnitudes) • Mass • Composition • Rotation • How we classify and categorize them 3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Astronomers can determine the composition of a star many light-years away by: a) sending probes to collect atmospheric material from the star and analyzing it with a mass spectrometer. b) Observing the spectrum of the star and applying Wien’s law. c) Observing the absorption line spectrum and matching up lines with the laboratory spectra of various elements. 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display How would you design a mission that could exceed the performance of the Hipparcos satellite? a) Use a larger baseline: the orbital diameter of Mars, for example. b) Use a larger baseline: select more distant stars, for example. c) Use a smaller baseline for higher accuracy. 5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Which unit of distance is larger, the light-year or the parsec? a) light-year b) parsec c) The parsec is a measure of angle, not distance d) They are the same. 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display READING QUIZ Stars are classified according to the types of absorption lines they have. The classification nomenclature is a) Based upon a numeric scheme running backwards from 25 to 1 (and missing some numbers). b) Based upon roman numerals one through ten (I, II, III, IV, V, VI, VII, VIII, IX, and X) c) Based upon an alphabetic scheme that does not include all of the letters and is not necessarily in alphabetic order. 7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Inverse-Square Law of Brightness 8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The Luminosity of a Star 9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Stefan-Boltzmann law Figure 12.5 L Energy emitted by 1 sq meter Number of sq meters on its surface = 10 T4 4 R 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Brightness: energy per unit area -intrinsic -apparent Luminosity: Total energy emitted -intrinsic -apparent 11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Star A has a radius R and a temperature T. Star B has the same temperature and is at the same distance from the Sun as star A. However, it is 4 times larger than star A. How many times more luminous is star B? a) 16 b) 4 c) both have the same luminosity d) not enough information to tell 12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Star A has a radius R and a temperature T. Star B is at the same distance from the Sun as star A. However, it is 4 times larger than star A and two times cooler. How many times more luminous is star B? a) 16 b) 4 c) both have the same luminosity d) not enough information to tell 13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Classification of Stellar Spectra 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Stellar spectra are not all the same! A B C What causes these differences? 15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Plotting a Spectrum Absorption lines 16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Annie Jump Cannon: • Hired as an assistant in 1896 by Edward Pickering of Harvard College Obervatory. • Classified stars by their spectra. • Looked at hundreds of thousands of spectra. • Published the Draper Catalogue. 17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Letters ordered by decreasing temperature: hot cold 18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display What causes such large differences in spectra? • 1920’s: Cecilia Payne • PhD thesis, Harvard College (1925) • Differences due to temperature, not composition! • All stars made mostly of hydrogen (very controversial!) • 19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display 24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display O: B: A: F: G: K: M: Ionized helium Neutral helium, stronger hydrogen Very strong hydrogen lines Absorption lines from other elements (metals) begin to appear (like ionized Calcium) Weaker hydrogen, neutral metals Some lines from molecules begin to appear Lots of lines from molecules like TiO, water Sun is a G2! 25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The HR Diagram 26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The Hertzsprung-Russell Diagram 27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Constructing an H-R diagram requires that you know the absolute (intrinsic) luminosity. You can compute this quantity by measuring the apparent luminosity and determining the distance to the star. 28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The H-R diagram Figure 12.16 29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Stellar luminosity classes I II III IV V 30 Supergiants Bright Giants (not widely used) Giants Subgiants Dwarfs (Main Sequence Stars) White Dwarfs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display