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# dis8-slides - ASTR 100 Discussion 8 Oct 31 2011 1 Imagine...

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ASTR 100 Discussion 8 Oct. 31, 2011

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1. Imagine that concentric spheres are placed around the Sun at radii of 1, 2, and 3 units. (This is shown in the diagram below.) If the apparent brightness of the Sun at the patch on shell 1 is 16, then what is the apparent brightness of the Sun at the patch on shell 2 marked with an X? Give a numerical answer, but EXPLAIN YOUR REASONING IN WORDS.
Answer: This excellent answer was written by a student in the class: “The Inverse-Square Law gives the equation relating luminosity, apparent brightness, & distance. b = L/(4 d 2 ). Using this equation we see is a constant. So when d changes from 1 to 2, d 2 goes from 1 to 4, meaning the apparent brightness will decrease by a factor of 4 and become 4 from 16.”

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Grading checklist: Did you give a numerical answer? Did you discuss how distance affects brightness? Did you remember to discuss the fact that distance squared is important?
2. Imagine that concentric spheres are placed around the Sun at distances of 1, 2, and 3 units. If the apparent brightness of sunlight at the first patch (on shell 1) is 18, then what is the apparent brightness through the patch marked with an X on shell 3? Give a numerical answer, but explain your reasoning in words.

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Answer: This excellent answer was written by a student: “Using the Inverse-Square Law, we get that apparent brightness is inversely proportional to the distance squared. So if at distance 1, the sunlight has [an] apparent brightness of 18, then at shell 3, the apparent brightness decreases by a factor of 3 2 = 9, so on the patch marked X on shell 3, the apparent brightness is 18/9 = 2.”
Grading checklist: Did you give a numerical answer? Did you discuss how distance affects brightness? Did you remember to discuss the fact that distance squared is important?

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By observing the light emitted by a star, we can infer its composition from A. the lines in the star's spectrum. B.
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## This note was uploaded on 12/02/2011 for the course ASTR 100 taught by Professor Hayes-gehrke during the Fall '06 term at Maryland.

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dis8-slides - ASTR 100 Discussion 8 Oct 31 2011 1 Imagine...

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