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Chapter 11 Surveying the Stars

Chapter 11 Surveying the Stars. Outline of Chapter 11 Part I: Properties of Stars Not exactly like book. Parallax and distance. Luminosity and brightness Apparent Brightness (ignore “magnitude system” in book) Absolute Brightness or Luminosity Inverse-Square Law Stellar Temperatures

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Chapter 11 Surveying the Stars

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  1. Chapter 11Surveying the Stars

  2. Outline of Chapter 11 Part I: Properties of StarsNot exactly like book Parallax and distance. Luminosity and brightness Apparent Brightness(ignore “magnitude system” in book) Absolute Brightness or Luminosity Inverse-Square Law Stellar Temperatures Color, Spectral lines, Spectral Classification:OBAFGKM Stellar sizes (radius) Stellar Masses

  3. Properties of Stars • Our Goals for Learning • How far away are stars? • How luminous are stars? • How hot are stars? • How massive are stars? • How large (radius) are stars?

  4. I. Parallax and distance. p = parallax angle in arcseconds d (in parsecs) = 1/p 1parsec= 3.26 light years

  5. I. Parallax and distance. Nearest Star: Alpha Centauri d = 4.3 light years (since 1 parsec = 3.26 light years) distance in parsecs = 4.3/3.26 = 1.32 What is the parallax of this star? d=1/p hence p=1/d p for nearest star is

  6. I. Parallax and distance. Nearest Star: Alpha Centauri d = 4.3 light years (since 1 parsec = 3.26 light years) distance in parsecs = 4.3/3.26 = 1.32 What is the parallax of this star? d=1/p hence p=1/d p for nearest star is0.76 arcseconds All other stars will have a parallax angle smaller than 0.76 arcseconds

  7. Question 1 1.The distance of a star whose parallax is 0.25 arc seconds is

  8. Question 1 1.The distance of a star whose parallax is 0.25 arc seconds is 4 parsecs 40 light-years 100 astronomical units 0.25 parsec

  9. Question 1 1.The distance of a star whose parallax is 0.25 arc seconds is 4 parsecs 40 light-years 100 astronomical units 0.25 parsec

  10. II. Luminosity and Brightness Apparent Brightness (how bright it looks in the sky) Absolute Brightness or Luminosity (energy/sec) Inverse-Square Law

  11. Energy passing through each sphere is the same The further the observer the lower the apparent brightness proportional to 1/d2

  12. Energy passing through each sphere is the same The further the observer the lower the apparent brightness proportional to 1/d2 How many times fainter will the Sun seem from Jupiter (5AU) than from Earth?

  13. Energy passing through each sphere is the same The further the observer the lower the apparent brightness proportional to 1/d2 How many times fainter will the Sun seem from Jupiter (5AU) than from Earth? 25 times

  14. II. Luminosity and Brightness Apparent Brightness (how bright it looks in the sky) Absolute Brightness or Luminosity (energy/sec) Inverse-Square Law apparent brightness=(absolute brightness)/d2 Examples: light bulbs at different distances

  15. II. Luminosity and Brightness Apparent Brightness (how bright it looks in the sky) Absolute Brightness or Luminosity (energy/sec) Inverse-Square Law apparent brightness=(absolute brightness)/d2 Examples of absolute brightness and apparent brightness: light bulbs at different distances 10W, 1 meter away 100W, 10 meters away 20W, 2 meters away 90W, 3 meters away

  16. II. Luminosity and Brightness light bulbs at different distances 10W, 1 meter away 100W, 10 meters away 20W, 2 meters away 90W, 3 meters away Use formula: apparent brightness=(absolute brightness)/d2 Which one is faintest? Brightest?

  17. II. Luminosity and Brightness light bulbs at different distances 10W, 1 meter away 100W, 10 meters away 20W, 2 meters away 90W, 3 meters away Which one is faintest? b Brightest? a and d Watts is a unit of energy/sec (power)

  18. Review of distance, apparent brightness, absolute (intrinsic) brightness and luminosity Distance: If you know the parallax “p” (in arcseconds) you can calculate the distance “d” (in parsecs) d=1/p (1parsec= 3.26 lightyears) Apparent brightness: how bright a star looks in the sky The inverse-square Law: light from stars gets fainter as the inverse square of the distance (apparent brightness is proportional to 1/d2). If we know the apparent brightness and the distance to a star we can calculate its absolute (intrinsic) brightness: apparent brightness = (absolute brightness)/d2 Luminosity (energy/sec) is equivalent to absolute brightness (analogy with light bulbs: Watts)

  19. Question 2 1.Two stars have parallaxes of 0.1 arc seconds and 0.01 arc seconds, respectively, if the stars are equally luminous, how much brighter will the near one appear than the farther one?

  20. Question 2 1.Two stars have parallaxes of 0.1 arc seconds and 0.01 arc seconds, respectively, if the stars are equally luminous, how much brighter will the near one appear than the farther one? (Hint: calculate the distance first and then estimate the apparent brightness)

  21. Question 2 1.Two stars have parallaxes of 0.1 arc seconds and 0.01 arc seconds, respectively, if the stars are equally luminous, how much brighter will the near one appear than the farther one? (Hint: calculate the distance first and then estimate the apparent brightness) 100 1000 10,000 400

  22. Outline of Chapter 11 Part I Parallax and distance. Luminosity and brightness Apparent Brightness Absolute Brightness or Luminosity Inverse-Square Law Stellar Temperatures Color, Spectral lines, Spectral Classification:OBAFGKM Stellar sizes (radius) Stellar Masses

  23. How hot are stars?

  24. III.Stellar Temperatures Color ( hotter > bluer; cooler > redder) Spectral lines Spectral Classification: OBAFGKM (from hottest to coldest)

  25. Laws of Thermal Radiation hotter  brighter, cooler  dimmer hotter  bluer, cooler  redder (from Ch. 5)

  26. Hottest stars: blue Coolest stars: red (Sun’s surface is about 6,000 K)

  27. Lines in a star’s spectrum correspond to a spectral type that reveals its temperature: O B A F G K M (Hottest)(Coolest)

  28. Table 11.1

  29. IV.Stellar sizes (radius) Luminosity is proportional to surface area (how large) x temperature (how hot): L= 4R2T4 If we can measure the Luminosity and the temperature of a star we can tell how large its raduis is.

  30. IV.Stellar sizes (radius) Luminosity is proportional to surface area x temperature: L= 4R2T4 If we can measure the Luminosity and the temperature of a star we can tell how large its raduis is.

  31. Summary of Ch 11 Part I Distance: If you know the parallax “p” (in arcseconds) you can calculate the distance “d” (in parsecs) d=1/p (1parsec= 3.26 lightyears) Apparent brightness: how bright a star looks in the sky The inverse-square Law: light from stars gets fainter as the inverse square of the distance (brightness proportional to 1/d2). If we know the apparent brightness and the distance to a star we can calculate its absolute (intrinsic) brightness: apparent brightness = (absolute brightness)/d2 Luminosity (energy/sec) is equivalent to absolute brightness L= 4R2T4 If we can measure the luminosity and the temperature of a star we can tell how large it is. Binary stars allow us to determine stellar masses

  32. Summary of Ch 11 Part I Distance: If you know the parallax “p” (in arcseconds) you can calculate the distance “d” (in parsecs) d=1/p (1parsec= 3.26 lightyears) Apparent brightness: how bright a star looks in the sky The inverse-square Law: light from stars gets fainter as the inverse square of the distance (brightness proportional to 1/d2). If we know the apparent brightness and the distance to a star we can calculate its absolute (intrinsic) brightness: apparent brightness = (absolute brightness)/d2 Luminosity (energy/sec) is equivalent to absolute brightness L= 4R2T4 If we can measure the luminosity and the temperature of a star we can tell how large it is. Binary stars allow us to determine stellar masses

  33. Binary Stars • Definition • Three main types of Binary Stars • Visual • Spectroscopic • Eclipsing • Stellar Masses and Densities

  34. Binary Stars • Definition: • When two stars are in orbit around their center of mass • Three main types of Binary Stars • Visual: orbits • Spectroscopic: Review of Doppler effect, spectral lines, double and single lines • Eclipsing: masses and radii of stars • Stellar Masses and Densities

  35. Visual Binary

  36. Visual Binary

  37. Doppler Effect Radial Velocity Approaching stars: more energy, Receding stars: less energy,

  38. Radial Velocity Approaching stars: more energy, spectral lines undergo a blue shift Receding stars: less energy, spectral lines undergo a red shift / = v/c

  39. Spectroscopic Binary

  40. Spectroscopic Binary We determine the orbit by measuring Doppler shifts

  41. Eclipsing Binary We can measure periodic eclipses

  42. Eclipsing Binary: Masses and Radii

  43. Radii of Stars

  44. Stellar Masses

  45. Stellar Densities Low Same as water High

  46. Properties of Stars • Our Goals for Learning • How far away are stars? • How luminous are stars? • How hot are stars? • How massive are stars? • How large (radius) are stars?

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