lecture_tan19 - Key Concepts: Lecture 19: Measuring the...

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Key Concepts: Lecture 19: Measuring the properties of stars Temperature, T: from peak of spectrum and spectral lines Composition: from spectral lines Apparent brightness (or energy flux, F): from telescope detectors Distance, d: from parallax Luminosity: L = 4 ! d 2 F Spectrum of an Object Intensity Classifying the Spectra of Stars • Late in the 19 th century photography became good enough to record the spectrum of a star • Annie Jump Cannon (1863-1941) – Harvard college observatory – Found that the spectra of stars fell into natural categories – She took spectra of nearly 400,000 stars The Spectral Classes of Stars: OBAFGKM • Stars given a letter class based on which absorption lines are present. • We now know this corresponds to different temperatures - from hot to cold the sequence is OBAFGKM – O stars hottest, M stars coolest – Each letter class divided into 10 subclasses from 0 to 9 – within each letter class the spectra are similar, arranged so the classes merge smoothly into each other • B9 is more similar to A0 than B0 • Sun is a G2 star with T=5,800K
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lecture_tan19 - Key Concepts: Lecture 19: Measuring the...

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