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Analyzing Starlight

Analyzing Starlight. Chapter 11. Analyzing Starlight. Starlight is only information we have Take picture Measure brightness Determine color Measure position Take spectrum Measure spectral lines. Stellar Positions. Stars move (slowly) Proper Motion (sideways motion)

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Analyzing Starlight

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  1. Analyzing Starlight Chapter 11

  2. Analyzing Starlight • Starlight is only information we have • Take picture • Measure brightness • Determine color • Measure position • Take spectrum • Measure spectral lines

  3. Stellar Positions • Stars move (slowly) • Proper Motion (sideways motion) • Determined by measuring position

  4. Brightness of Stars • Luminosity • amount of energy emitted per second • not the same as how much we observe! • We observe a star’s apparent brightness • Depends on: • luminosity • distance • Brightness decreases as 1/r2(as distance r increases) • other dimming effects • dust between us & star

  5. Measuring Brightness: History • Photometry - the measure of brightness • Hipparchus (150 B.C.) • catalog of 1000 stars • included position & brightness • Magnitude = brightness • brightest stars were first magnitude • faintest stars were sixth magnitude • estimated by eye (logarithmic response)

  6. Modern Magnitude System • Refined scale of Hipparchus • 5 magnitude difference = brightness factor of 100 • 6th mag. star is 100x fainter than 1st mag. star • 1 magnitude difference is 2.5 times brighter/fainter

  7. Star Colors • Measure brightness at different wavelengths • U=ultraviolet magnitude • B=blue magnitude • V=visual (yellow) magnitude • Color = magnitude difference • (B-V) or (U-B) • Measures stellar temperature (recall blackbody)

  8. Stellar Spectra • Cool atmosphere in front of star

  9. Spectra of Stars • Fraunhofer measured solar spectrum • Spectra of other stars not same • hydrogen (and other) lines: • Sometimes strong • Sometimes weak • not always present • because stars have different temperatures

  10. Stellar Spectra Hot Cool

  11. Stellar Spectra Hot Star Cool Star

  12. Classification of Stars • Classified by absorption lines in their spectra • Annie Jump Cannon at Harvard (1863-1941) • classified large numbers of stars (over 400,000!) • HD catalog (still used today) • From hottest to coolest stars: O, B, A, F, G, K, M Cool Hot Oh, brother, astronomers frequently give killer midterms! Oh be afine guy/girl kiss me!

  13. What We Learn from Spectra • Stellar Temperature • Which lines are present and their strength Hot Cool

  14. What We Learn from Spectra • Abundances of elements • From strength of absorption lines • Radial Velocity • From Doppler shift • velocity toward (blueshift) or away (redshift) from us • Stellar Size • indirectly from atmospheric pressure • Low pressure: lines are narrow • giant & supergiant stars • High pressure: lines are wide • normal stars • Luminosity Class: I, II, …, V • I = narrow lines = supergiant = large radius • V = broad lines = normal star = small radius

  15. Homework for today was WORKBOOK EXERCISES: “Analyzing Spectra” (pages 43-46 in workbook)

  16. Parallax and Distances • Parallax • angle by which object shifts when viewed from different places • Triangulation • calculating distance from parallax angle

  17. Distances to Stars • Use Earth’s orbit as baseline • measure parallax of nearby stars • triangulation gives distance • Parallax angle • extremely small (stars very far)

  18. The Parsec • If star at distance of 206,265 AU • parallax = 1 arc second (1/3600 degrees) • defines new distance unit, the parsec(pc) • 1 pc = 206,265 AU = 3.26 light-years • distance of star distance = 1/parallax (distance in pc; parallax in arcseconds)

  19. Which of the following stars is closest to us? • Procyon (parallax angle = 0.29”) • Ross 780 (parallax angle = 0.21”) • Regulus (parallax angle = 0.04”) • Sirius (parallax angle = 0.38”)

  20. Below are two star photos taken six months apart and laid atop one another so the background stars (circles) line up. There are two nearby stars also shown. Which of these nearby stars is closer?

  21. Brightness • Two kinds: • Absolute (or intrinsic) brightness • Same as luminosity • Apparent brightness • depends on luminosity and distance • Magnitudes measure brightness • Apparent magnitude • Measures apparent brightness • Absolute magnitude • Measures luminosity • When observer at standard distance (10 pc) from star: apparent magnitude = absolute magnitude Recall:Apparent Bright ~ L/d2

  22. WORKBOOK EXERCISES: “Apparent and Absolute Magnitudes of Stars” (pages 67-68 in workbook)

  23. Results for Luminosities • Luminous stars are rare • More low luminosity stars than high • Most stars less luminous than sun Sun

  24. The H-R Diagram Bright • Plot of Luminosity vs. Temperature • by Einar Hertzsprung and Henry Norris Russell (hence H-R diagram) • Reveals relationships between stellar parameters • THE fundamental tool of stellar astronomy Faint Hot Cool

  25. H-R Diagram

  26. Measuring Stellar Size • Indirect from Luminosity and Temperature • L ~ R2T4 Large Small

  27. HOMEWORK WORKBOOK EXERCISE: “H-R Diagram” (pages 77-78 in workbook)

  28. Stellar Masses • Mass is the primary characteristic • determines luminosity, temperature, lifetime • How to measure mass? • Use binary stars (orbiting pair) • Measure orbital period, P, and orbital size, a • Kepler’s Law gives sum of masses (M1+M2)P2 = a3

  29. Binary Stars • First binary star discovered in 1650 • Mizar (in handle of Big Dipper) • Double Star(not same as binary) • pair of stars (close together in sky), but not orbiting each other • Visual Binary • both stars seen in telescope • stars orbit each other • Spectroscopic Binary • only one star seen in telescope • other inferred from spectroscopy • Doppler shift measures orbital motion Visual Binary

  30. Binary Star Orbits • Stars orbit around each other • each star orbits around Center of Mass

  31. Spectroscopic Binaries • As each star orbits, spectral lines Doppler-shifted back and forth

  32. Range of Stellar Masses • Smallest mass • about 0.08 Msun • smaller objects never hot enough to fuse hydrogen • Largest mass • about 100 Msun (very rare) • larger stars unable to maintain hydrostatic equilibrium • would blow themselves apart!

  33. H-R Diagram High Mass Low Mass

  34. Brightness and Distances • If we: • know absolute magnitude (luminosity) • measure apparent magnitude  can calculate distance. • need“STANDARD CANDLES” • objects whose luminosity is known

  35. Distances from Spectral Types • Find luminosity class (from spectrum) • place on H-R diagram • gives luminosity • Luminosity + apparent magnitude gives distance • Method called spectroscopic parallax

  36. Pulsating Variables • Stars that pulsate (expand and contract) • Two special types: • Cepheid variables • RR Lyrae variables • Pulsation period depends on luminosity • Can be used as standard candles

  37. Period-Luminosity Relation • Discovered by Henrietta Leavitt • Measure period of brightness variation • place on plot at right • find luminosity • Luminosity + apparent magnitude gives distance

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