# lec18 - Lecture 18 Binary Stars and Stellar Atmosphere...

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Unformatted text preview: Lecture 18 Binary Stars and Stellar Atmosphere Purdue University, Astronomy 364 1 Lecture 18 Binary Stars Lecture 17 Purdue University, Astronomy 364 2 Kepler’s third law revisited: P 2 = 4 π 2 G ( M A + M B ) a 3 Total mass of the system is then given by M A + M B = 4 π 2 G a 3 P 2 Many stars are in a binary system. a = a A + a B Visual Binaries Lecture 17 Purdue University, Astronomy 364 3 Sirius consists of two stars: Sirius A and Sirius B, in an elliptical orbit with period P=50.05 yrs and semimajor axis a=19.78 AU. Total Mass: M A + M B = a AU 3 P yr 2 = (19.78) 3 (50.05) 2 = 3.09 M Center of Mass: M A /M B = a B /a A =2.2 M A = 2.12 M , M B = 0.97 M Lecture 18 Purdue University, Astronomy 364 4 Spectroscopic Binaries Lecture 17 Purdue University, Astronomy 364 5 The measurable quantities are: orbital period ( P ), line-of- sight velocities ( v A sin i, v B sin i ). ( M A + M B )sin 3 i = 4 π 2 GP 2 ( a A sin i + a B sin i ) 3 = P 2 π G ( v A sin i + v B sin i ) 3 For eclipsing binaries, sin(i) ~ 1 In general, the inclination angle of a binary system is difficult to obtain. binary system is difficult to obtain....
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## This note was uploaded on 04/23/2011 for the course ASTR 364 taught by Professor Cui,weik. during the Spring '11 term at Purdue.

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lec18 - Lecture 18 Binary Stars and Stellar Atmosphere...

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