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# IUPUI AST 105 - Star

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AST 105 1nd Edition Lecture 6 Outline of Last Lecture I. Stefan – Boltzmann LawII. Planck’s LawIII. Spectra IV. Spectroscopy a. Analysis of light V. Modern Theory of Atom a. Niels Bohr VI. Bohr’s LawVII. Doppler Shifa. Blue shifb. Red ShifOutline of Current Lecture Topic: StarsI. Local Stars a. Proxima Centauri b. Parallax i. How we know distance from starsii. Parallax formula c. ΘII. Temperatures of Starsa. Surface temp. of distant stars b. Two methods to find i. Photomomy ii. Spectroscopy III. Classifying Stars Current LectureStars- Our sun is one of 1022 stars in the visible universe - In our galaxy, milky way, there are about 1011 stars - Our sun is about 5 billion years into its 10 billion year life cycle where its converting hydrogen into helium These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Many of the stars we observe (out of the 1022) are at different points in their life cycle  We call life cycles of stars stellar evolution - During stellar evolution, the size, temp., color and luminosity of a star can vary Local Stars- Closest neighbor is Proxima Centauri  About 4x1016 meters away - Q: How do we know this distance?o A: Parallax - Parallax o As the earth orbits around the sun, a nearby star, shifs its location against the background of very distant stars Only works for nearby stars Employs geometry - Parallax Formula o Tanθ= opposite leg ÷ adjacent leg =x/ If/when θ is small tan θ=θ So, θ = x over d (x/d) AND/OR d = x over θ X = distance from sun to earth  1.58x10-5 LY [average distance]Q: How do we know surface temperatures of stars?A: analyze the light of the star2 methods to do that (photomemy and spectroscopy)Temperatures - find temp. by analyzibg the light from the distant star - 2 methods to do this  photomemy  spectroscopy o Photomemy  Focuses on colors Intensity (brightness) of star light at 3 different wavelengths is compared to the black body intensity curves  Then a “best fit” will give us the surface temperatureo Spectroscopy  “Stellar spectroscopy” uses both emission and absorption spectra techniques must have knowledge of atomic physics  spectra produced  uses computers and all of the EM spectrum (not just visible) analyzes all of the possible “n” transitions consider a 5,000 K starHere, most of the light is too weak to involve a n=2 electron (balmer line). So, no balmer series. consider a 10,000 K star at this temp., hydrogen easily provides balmer series lines (transition toanother, perhaps larger N)consider a 20,000 K starat this temp., hydrogen is ionized (electrons have developed)cannot produce any spectral line  no longer

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