a1_02_sky - Some basic physics The "Inverse Square Law of...

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The “Inverse Square Law of Light” (An object’s brightness or “Luminosity” fades as: 1/d 2 where d is the distance Some basic physics
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Stellar Magnitudes and Luminosity Hipparchus’ 1- 6 scale of apparent magnitudes (m) Magnitude change of 1 meant a brightness change of ~ 2.5 times Brightnesses of stars are generally expressed as magnitudes . For example: 1 st mag stars are bright, 2 nd are fainter, etc. Changes of 1 magnitude means a brightness difference by about 1/2.512 (i.e. 2.512 times fainter.)
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Stellar Magnitudes and Luminosity Really bright sources can have negative magnitudes (! ) o Sun: -26.7 mag, o Full Moon: -12 mag, o Venus: -4.5 mag o Jupiter: -2 o Saturn: 0 o Neptune: +8 o Pluto +14 Luminosity = total energy output/sec Magnitudes provides good approximation for stellar luminosities of one knows stellar distances.
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Magnitude scale is logarithmic . Brightness difference: 2.512 Δ mag Why choose 2.512 for the base? The actual base is exactly the 5 th root of 100, so adding 5 magnitudes multiplies brightness by exactly 1/100. Recall: greater mag numbers means fainter!) 2.512 x 2.512 x 2.512 x 2.512 x 2.512 = 100 Some Key Points about the Magnitude Scale
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The ratio of the intensities, I, of two stars or planets, A and B is given by: I A /I B = 2.512 (m B – m A ) This equation can be rearranged to be the magnitude difference of the two stars or planets: m A – m B = 2.5 Log (I B /I A ) Comparison of stellar or planetary brightnesses on the Magnitude Scale
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ancient astronomical roots. • There are three main divisions of time. – Day
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This note was uploaded on 05/05/2010 for the course ASTR 001 taught by Professor Robertfesen during the Spring '10 term at Dartmouth.

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a1_02_sky - Some basic physics The "Inverse Square Law of...

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