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p. 216

Birth of Stars. p. 216. Properties of stars: Mass Luminosity (Intrinsic brightness) Surface temperature Size (radius) Composition Distance from us is NOT a property of the star. Luminosity : the brightness of a star as we see it depends on the

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p. 216

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  1. Birth of Stars p. 216

  2. Properties of stars: Mass Luminosity (Intrinsic brightness) Surface temperature Size (radius) Composition Distance from us is NOT a property of the star. Luminosity: the brightness of a star as we see it depends on the intrinsic brightness (luminosity measured as energy per unit time) and distance from us. Luminosity (L) = 4πR²×brightness. L = 4πR²σT4. Where R is radius of the star and T is temperature in Kelvin. σ is a constant. The luminosity of a star is normally expressed in units of the luminosity of the Sun.

  3. Stars are born – protostars Middle age – main sequence stars Death – White dwarf or Supernova Life cycle of a star

  4. The brightness (magnitude), as seen by us, of a star depends on the star luminosity (intrinsic brightness) and the distance from us. For all practical purposes, stars (except the Sun) are so far away that all we observe is a point source. Brightness of a star

  5. Continuous spectra of stars λmax(nm)= 2,900,000/T (ºK) Fig. 11-2, p.218

  6. Emission spectra of Hydrogen Fig. 11-3, p.219

  7. Spectral types Fig. 11-5, p.220

  8. Type Temperature O > 30,000°K B 10,000°K – 30,000°K A 7,000°K – 10,000°K F 6,200°K – 7,000°K G 4,800°K – 6,200°K Sun5,800°K K3,500°K – 4,800°K M < 5,500°K Spectral Type

  9. Distance (d) to a star by parallax method d = 1 / p If p in arc sec Then d is in parsec 1 parsec = 3.26ly Fig. 11-7, p.221

  10. Brightness as seen by us Intensity α 1/R² Fig. 11-9, p.223

  11. Luminosity L = 4πR²σT4. Size and temperature! I.e. If temperature is twice the L is 16 times higher. Absolute Magnitude M same as apparent magnitude m at 10 parsec = 32.6 ly. m – M = -5 + 5log 10 (d/3.26). D in ly. If two of the quantities are known, the third can be computed. Sun: m = -27, M = 5. Sirius: m = -1.5, M = +1.5. d = 9 ly. Spica: m = +1, M = - 4. d = 262 ly. Star Luminosity and absolute magnitude

  12. H R Diagram Bright Temperature and Intrinsic brightness are related Dim Hot Cool Fig. 11-13, p.227

  13. Stellar abundance

  14. Star masses determined from double stars. About half the stars are in groups of two or more stars. Stellar sizes measured directly from eclipsing binaries. There are about 2,000 eclipsing binaries. Stellar size can also be computed from luminosity. Star Mass and Size

  15. Double star Albireo, also known as  Cygni, contains a B star and a K star. Beautiful pair because of their contrasting colors. Fig. 11-17, p.230

  16. Two stars revolving around each other And moving in space. MA + MB = R³/ P² Where M is in solar mass units, P in years and R in AU Fig. 11-22, p.233

  17. Doppler shift due to motion Fig. 11-16, p.230

  18. Star mass determination m1 + m2 = R³/ P² and m 1 R 1 = m 2 R 2 p.231

  19. Eclipsing binary measure mass and diameter About 2,000 eclipsing binaries Fig. 11-20, p.232

  20. Plots of luminosity, radius,temperature vs. mass for main sequence stars

  21. Variable starsused to determine absolute (intrinsic) brightness • Light intensity varies with time. • RR Lyra: Period 6 to 12 hours. Absolute magnitude about 0.5. • Cepheid variable stars: Period from 1 to 50 days. Absolute magnitude related to period.

  22. Cepheid variable star Fig. 11-26, p.236

  23. Period determines the absolute magnitude M. Apparent magnitude (m) is measured (how bright it looks to us). m – M = -5 + 5log 10 (d/3.26). D in ly. Distance d is computed as two of the three variables are known. In a cluster of stars, or even in a galaxy, only one variable star observation is needed to calculate the distance. Variable stars to determine distance

  24. Table, p.237

  25. Open cluster of stars in the constellation Carina. Fig. 11-29, p.238

  26. Open cluster, surrounded by the Rosette Nebula Fig. 11-30, p.238

  27. Globular cluster M15 in the constellation Pegasus. Has a massive Black hole in the Center. Fig. 11-31, p.238

  28. The vast majority of stars are main sequence. The most important quantity is mass. The larger the mass; The higher the surface temperature. The larger the size (diameter). The brighter the star. The shorter the lifetime. Main Sequence Stars

  29. Temperature: 3,000ºK to 50,000ºK; Sun 5,800ºK Mass: 0.08 to 80 times mass of Sun. Size: 0.01 to 100 times diameter of Sun. Luminosity: 10-4 to 10+6 times the Sun. Composition (by number): 92% hydrogen, 8% helium and <<1% other heavier elements. Main sequence stars

  30. H-R Diagram Fig. 11-32, p.239

  31. Double cluster in Perseus, Fig. 11-33, p.240

  32. Fig. 11-34, p.241

  33. M<0.08 .08<M<0.4 0.4<M<1.4 1.4<M<~4 M>~4 P R O T O S T A R | M a i n S e q u e n c e | R E D G I A N T | | | Planetary Supernova | | | Nebula | | W h i t e D w a r f | B r o w n D w a rf Neutron Star OR Black Hole Stellar Evolution M A I N S E Q U E N C E R E D G I A N T W H I T E D W A R F B R O W N D W A R F

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