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lecture4 - Lecture 4 From Light to Stellar Properties...

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Lecture 4: From Light to Stellar Properties
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Think/Pair/Share Planet Zork's star emits most of its radiation at a wavelength twice as long as the Sun's peak wavelength, about 1 micrometer. What can you conclude about the temperature of Zork's star? A. Twice the temperature of the Sun B. Same temperature as the Sun C. Half the temperature of the Sun D. Not enough information to tell
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Think/Pair/Share Which star is hotter? A. Star A B. They are the same temperature. C. Star C D. Not enough information to tell
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Think/Pair/Share Which star is hotter? A. Star A B. Star B C. They are the same temperature. D. Not enough information to tell
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Think/Pair/Share Which star is larger? A. Star A B. They are the same temperature. C. Not enough information to tell D. Star D
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Why not just measure the sizes of stars directly? O (observer) Star or nebula or galaxy distance width angle θ Good approximation for astronomy (because θ is typically so small): width = distance* θ
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Rough Angular Size of Stars (other than Sun) angle = diameter/distance diameter = 700,000 km or 7 x 10 8 m (assume Sun is typical) say distance = 100 lt-yr or 10 18 m angular size = 7 x 10 -10 radian or 4x10 -8 deg or 0.0001” (arcsec) cannot be resolved with current telescopes (0.1” is best we can do with Hubble Space Telescope (HST)) => For most stars, size can only be inferred from the Stefan-Boltzmann law!
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Think/Pair/Share Which star is larger? A. Star A B. Star B C. They are the same size. D. Not enough information to tell
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Rank from least to most luminous Temperature key: Sizes and temperatures: A. D,B=C=E,A B. D,B,C,E,A C. A,E,C,B,D D. B=D,C,A=E E. None of the above
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Stefan-Boltzman law relates: surface area luminosity temperature I f we know any 2 of these quantities, we can infer the third. Temperature can (in principle) be measured using ______ law. Can surface area and luminosity be measured?
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Inverse-Square Law
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Measuring Luminosity with the Inverse-Square Law f=L/4 π d 2 where: f is apparent brightness (power per unit area captured by your telescope), aka intensity or flux d is distance to the star We need to know the distance to the star to infer the luminosity!
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Example: Sun
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Sanity Check: the Sun if the Sun is a thermal emitter, we can also compute L sun a completely different way...
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