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Course Code
ASTR 111

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Astronomy Unit 5 -many star names start with Al which is an Arabic article, -The brightest stars in the constellation are usually given Greek letter names in the order of their brightness -most of the image structure of stars seen in photographs is caused by the telescope and film system -star magnitudes: -that bright stars have small or negative numbers as their magnitude and faint stars have large or positive numbers -stars are put into their spectral classes (to within 0.1 of a spectral class) by the relative strengths of their absorption lines -the frequency of absorbed and emitted light from an atom depends upon the difference in the initial and final energy levels of the electron, not upon the actual level numbers of the initial and final state -that each spectral class covers a range of temperatures but that the actual classification depends upon the spectral lines present -the Saha equation allows us to determine these lines from the temperature and pressure at the star’s surface -the temperature of the gases determines the relative absorption line strengths of the various atoms present in the stellar atmosphere -Objective brightness refers to what an instrument (such as photographic fi lm) would measure if calibrated so its reading is proportional to the energy received -1st magnitude-average of selected bright -faintest stars most people can see are about 6th magnitude -The brightest stars may have negative magnitudes -classification is subjective because it is based upon the eye of the observer -Photographic, photometric, and CCD cameras directly measure the amount of light, so magnitudes determined by them can always be uniquely converted to amount of light. -Magnitude was based on an estimate of the eye’s response (subjective brightness) -Objective brightness is a measure of the amount of light -It was assumed to take about 2 1/2 times as much change in objective brightness to make a factor of 2 change in subjectiv
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