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ASTRONOMY 373 INTRODUCTION TO ASTRONOMY – Stars, Galaxies, & Universe Spring 2015

ASTRONOMY 373 INTRODUCTION TO ASTRONOMY – Stars, Galaxies, & Universe Spring 2015 Sachiko Tsuruta. Lec 1. I. INTRODUCTION FK (= Freedman, Geller & Kaufmann 10 th Edition) Ch. 1) II. INTRODUCTION TO CLASSICAL ASTRONOMY

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ASTRONOMY 373 INTRODUCTION TO ASTRONOMY – Stars, Galaxies, & Universe Spring 2015

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  1. ASTRONOMY 373 INTRODUCTION TO ASTRONOMY – Stars, Galaxies, & Universe Spring 2015 Sachiko Tsuruta

  2. Lec 1 I. INTRODUCTION FK (= Freedman, Geller & Kaufmann 10th Edition) Ch. 1) II. INTRODUCTION TO CLASSICAL ASTRONOMY II-1. Stellar Distance and Stellar Motion (Main Ref.: Lecture notes; FK Sec.17-1)

  3. II-1a. Stellar Distance Stellar Parallax: = Apparent motion of a star due to Earth’s annual motion = Angular size of semi- major axis of the orbit of Earth around Sun Fig. II-1: Parallax

  4. Fig. II-2: Stellar Parallax

  5. Units of Distance: Use mks system: length=meter, mass =kgm, time=sec Astronomical Unit (AU): Distance from the earth to the sun = semi-major axis of the orbit of Earth around Sun 1 AU = d(sun) = 1.5 x 1011 m Parsec (PC): Distance at which 1 AU subtends Angle of 1 second 1 pc (parsec) =206625 AU = 3.086 x 1016 m = 3.262 ly Light Year (ly): Distance light travels in 1 year 1 light year (ly) = 63240 AU = 9.46 x 1015 m

  6. DISTANCE d (pc) = 1 / p(sec.) Eqn (1) • Distances to the nearer stars can be determined by parallax, the apparent shift of a star against the background stars observed as the Earth moves along its orbit ***************************************************** EX 1: Alpha Centauri • p = 0.76 sec • d = 1 / p = 1 / 0.76 = 1.32 pc = 4.29 lys See class notes for details

  7. EX 2: Barnard’s Star Barnard’s star has a parallax of 0.547 arcsec See class notes for details

  8. II-1b Stellar Motion Fig. II-3: Stellar Velocity

  9. V

  10. vr Doppler shift: see class notes and FK Sec. 5-9, and Box 5-6 vT d

  11. RADIAL VELOCITYvr vr / c = ( – 0) / 0 =  / 0Eqn(2a) Non-relativistic (see FK 5-9) • TRANSVERSE VELOCITYvT vT = 4.74  / p Eqn (2b) • vT in km/s;  in arc second/year; p in arc second • SPACE VELOCITYv v2 = vr2 + vT2 Eqn(2c) Study Examples in FK Box 17-1 (Non-science majors optional)

  12.  for

  13. II-2. Stellar Brightness, Magnitude, and Luminosity (Main Ref.: Lecture notes; FK Sec.17-2, 17-3) II-2a. Brightness and Luminosity (Main Ref.: Lecture notes; FK Sec.17-2, Box 17-2) Definitions: Luminosity: L = energy/sec = Power Output (Watts = W) Brightness: b = Luminosity/surface area (W/m2) Area: A = 4  d2 Eqn (3) d = distance

  14. Inverse Square Law b = L / A = L / (4  d2) 1/d2Eqn (4) ************************************* Fig. II-4a: The Inverse-Square Law

  15. EX 3: Candle at 10 m and 100 m Ans: At 10m 100 times brighter See class notes for details EX 4: Sun L(sun) = 3.86 x 1026 W ; d(sun) = 1.5 x 1011 m; Use Eqn (4), and get Ans: b(sun) = 1370 W/m2 See class notes for details ******************************************************** From Eqn (4) L = 4  d2 b Eqn (5a)

  16. Divide Eqn(5a) for star by that for sun  L / L(sun) = (d / d(sun))2 (b / b(sun)) Eqn (5b) Do the same for Star *1 and Star *2 L1 / L2 = (d1 / d2 )2(b1 / b2) Eqn (5c) 2 *2 1 Fig. II-4b: The Inverse-Square Law (conti.) d1 d2

  17. EX 5: Sirius A: d = 8.61 ly; L = 26.1 L(sun); What is brightness b? Ans: 8.79 x 10-11 brightness of Sun (See class notes for details.) ********************************* EX 6: Star *1 and Star *2 (same brightness: b1 = b2 = b) Star 1: L1 = 1 L(sun); Star 2: L2 = 9 L(sun) How far is Star 2 compared with Star 1? Ans: 3 times further away. (See class notes for details.) Study more examples in FK Box 17-2.

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