1. Chapter 10 Measuring the Stars

2. Star Cluster NGC 3603 20,000 light-years away

3. Figure 10.1 Stellar Parallax

4. Stellar parallax Measure distance to nearest stars Baseline is earth’s orbital diameter of 2 A.U. Parallactic angle (or parallax) is half of total angle Star with parallax of 1” is 1 parsec distant 1 parsec (1 pc) is about 3.3 light-years Smaller parallax means more distant

5. Proxima Centauri Part of Alpha Centauri triple star system Closest star to earth, largest parallax 0.76” parallax (difficult to measure) 1.3 pc or 4.3 light-years away About 300,000X further away than sun

6. Figure 10.2 Sun’s Neighborhood

7. Nearest neighbor stars Less than 100 stars within 5 pc Several 1000 stars within 30 pc

8. Hipparcos satellite Can measure out to 200 pc Nearly a million stars Future satellites will measure to 25,000 pc

9. Stellar motion Radial velocity - along line of sight Measure using Doppler shift Transverse velocity - perpendicular to line of sight Monitor star’s position on sky

10. Figure 10.3 Real Space Motion a), b) Barnard’s Star - 22 years apart c) Alpha Centauri

11. Proper motion Annual movement of star across sky Corrected for parallax Barnard’s star moved 227” in 22 years 10.3” per year proper motion 1.8 pc distance - transverse velocity is 88 km/s

12. Brightness and distance Luminosity or absolute brightness (intrinsic) Apparent brightness is how bright star looks from earth Apparent brightness depends on luminosity and distance

13. Figure 10.4 Inverse-Square Law

14. Figure 10.5 Luminosity

15. Apparent brightness proportional to luminosity/distance 2

16. Magnitude scale Greek astronomer Hipparchus (2nd century BC) Ranked stars into six groups Brightest stars are 1st magnitude Next brightest stars are 2nd magnitude Faintest stars (to naked eye) are 6th magnitude Larger magnitude means fainter star

17. Apparent magnitude Expanded beyond stars visible to naked eye One magnitude difference is 2.5X in brightness A 1st magnitude star is 2.5X brighter than a 2nd magnitude star Full moon has an apparent magnitude of -12.5 Faintest objects visible by Hubble or Keck telescopes are apparent magnitude 30

18. Figure 10.6 Apparent Magnitude

19. Absolute magnitude It is the apparent magnitude of a star viewed from a distance of 10 pc Measure of absolute brightness or luminosity Our sun has absolute magnitude of 4.8 (If sun were 10 pc from us, its apparent magnitude would be 4.8, which is faint)

20. More Precisely 10.1 More on the Magnitude Scale

21. Figure 10.7 Star Colors

22. Stellar color Temperature of star determines blackbody curve Measurements at two wavelengths can determine blackbody curve temperature Use B (blue) and V (visual - green/yellow) filters Determines star’s color index or color

23. Figure 10.8 Blackbody Curves

24. Table 10-1 Stellar Colors and Temperatures

25. Stellar spectra Absorption spectrum Aborption lines determine elements Temperature determines strength of lines Hotter stars have more ionized atoms Coolest stars can have molecular lines

26. Figure 10.9 Stellar Spectra

27. Spectral classification In 1800’s letter classification used for stellar spectra Later rearranged into order of decreasing temperature

28. Table 10.2 Spectral Classes

29. O B A F G K M Highest to lowest temperature Mnemonic: Oh, Be A Fine Girl, Kiss Me Oh, Be A Fine Guy, Kiss Me Oh Beastly And Fearsome Gorilla, Kill Me (Make up your own)

30. Spectral class subdivisions Each letter has 10 subdivisions, 0 - 9 0 is hottest, 9 is coolest, within letter class Sun is G2 (cooler than G1, hotter than G3)

31. Direct size measurement Several stars are large, bright and close enough to measure their size directly

32. Figure 10.10 Betelgeuse

33. Indirect size measurement Most stars’ sizes can’t be measured directly Use luminosity  temperature 4 And luminosity  area (or radius 2 ) Indirectly determines radius

34. Figure 10.11 Stellar Sizes

35. Stellar sizes R  - radius of sun Giants - 10X to 100X R  Supergiants - up to 1000X R  Dwarf - comparable to or smaller than R 

36. Hertzsprung-Russell Diagram H-R diagram Each point represents a star Luminosity on vertical scale Temperature (decreasing) on horizontal scale

37. Figure 10.12 H-R Diagram of Well-Known Stars

38. Stellar radii and H-R diagram Radius-luminosity-temperature relationship gives radius Diagonal dashed lines on H-R diagrams represent constant radius

39. Figure 10.13 H-R Diagram of Nearby Stars

40. Analogy 10.1 People along a main sequence

41. Main Sequence Most stars in H-R diagram on main sequence Runs from top left (High luminosity and temp) To bottom right (low luminosity and temp) Runs from blue giants and supergiants to red dwarfs

42. Figure 10.14 H-R Diagram of Brightest Stars

43. More Precisely 10.2 Estimating Stellar Radii

44. Non-Main Sequence 90% of stars on main sequence 9% of stars are white dwarfs (bottom left) 1% of stars are red giants (upper right)

45. Figure 10.15 Hipparcos H-R Diagram

46. Figure 10.16 Stellar Distance

47. Analogy 10.2 Traffic lights further away are fainter

48. Main sequence or not? Spectral line widths affected by pressure and density Determines if main sequence or not

49. Table 10-3 Stellar Luminosity Classes

50. Figure 10.17 Stellar Luminosities

51. Table 10.4 Variation in Stellar Properties within a Spectral Class

52. Sun Spectral class G Subdivision 2 Luminosity class V G2V

53. Binary stars Most stars are members of multiple-star systems - Binary-star systems (2) most common Visual binaries (see 2 stars) Spectroscopic binaries (detect Doppler shift from one or both orbiting stars) Eclipsing binaries (one passes in front of other, varying light output)

54. Figure 10.18 Binary Stars

55. Determining stellar masses Measure binary properties Use orbital radii and period Universal law of gravitation

56. Figure 10.19 Stellar Masses

57. More Precisely 10.3 Measuring Stellar Masses in Binary Stars

58. Figure 10.20 Stellar Mass Distribution

59. Table 10.5 Measuring the Stars

60. Stellar lifetime Depends on mass (how much fuel) and Luminosity (how fast fuel is consumed) A B2V star lives 90 million years A G2V star lives 10,000 million years (our sun) An M5V star lives 16,000,000 million years

61. Table 10.6 Key Properties of Some Well-Known Main-Sequence Stars

62. Figure 10.21 Stellar Radii and Luminosities