1. Outline of Ch 11b: The H-R Diagram
The Hertzprung-Russell (H-R) Diagram:
Surface Temperature vs Luminosity
Analogy: horsepower vs weight
Where Stars plot in the H-R diagram
Main Sequence: 90% of all stars
Why? stars spend 90% of their lives fusing hydrogen
Main sequence  Hydrogen fusion
Giants, Supergiants, White Dwarfs
Main Sequence Stars
(cont.)

2. Outline of Ch 11b: The H-R Diagram (cont.)
Main Sequence Stars
Stellar Masses and Densities along main sequence
Mass-Luminosity Relation (L~M3.5)
Lifetime on Main Sequence (TMS~ 1/M2.5)
Main sequence Thermostat
Star Clusters
What is so special about Star Clusters?
Open and Globular Clusters
Ages of Clusters

3. Homework online due Mon.

4. Review of Chapter 11a 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: binary stars, Doppler effect

5. What is a Hertzsprung-Russell (H-R) Diagram?

6. H-R diagram plots the luminosity vs. surface temperature of stars

7. Analogy with diagram that plots horsepower vs. weight of cars
Race cars
Sports cars
Most Cars
Horsepower
Economy cars
Weight

8. Hydrogen-fusion stars reside on the main sequence of the H-R diagram

9. Hydrogen-fusion stars reside on the main sequence of the H-R diagram
Main Sequence: 90% of all stars
Why? stars spend 90% of their lives fusing hydrogen
Main sequence  Hydrogen fusion

10. Remember Stellar sizes (radius)
Luminosity 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.

11. H-R Diagram: Radii of stars

12. Stars with low temperature and high luminosity must have ____ radius
SUPERGIANTS
Stars with low temperature and high luminosity must have ____ radius
GIANTS

13. SUPERGIANTS
Stars with low temperature and high luminosity must have a large radius: giants and supergiants
GIANTS

14. Stars with high temperature and low luminosity must have _____ radius

15. Stars with high temperature and low luminosity must have small radius: white dwarfs

16. Stellar Masses Mass-Luminosity Relation (L~M3.5)
For main sequence stars, the larger the mass the higher the luminosity
This mass-luminosity relation is valid only for the main sequence

17. Stellar Masses How do we know the masses of these stars?
For main sequence stars, the larger the mass the higher the luminosity
This mass-luminosity relation is valid only for the main sequence

18. Stellar Masses How do we know the masses of these stars?
The study of binary stars
For main sequence stars, the larger the mass the higher the luminosity
This mass-luminosity relation is valid only for the main sequence

19. Stellar Densities
Density = Mass/Volume
V= 4/3(R3)

20. Stellar Densities
Low
Same as water
High

21. Stellar Densities
Giants and Supergiants: same or lower density than air
M.S. same density as water
W.D. very dense

22. Stellar_mass_and_hr_diagr.swf

23. Lifetime on Main Sequence
TMS~ 1/M2.5
M in solar masses
T in units of Sun’s total lifetime on MS (10 billion years)

24. TemperatureColor, Spectral Type,
H-R diagram depicts:
TemperatureColor, Spectral Type,
Luminosity,
and Radius of stars
Luminosity
Temperature

25. TemperatureColor, Spectral Type,
H-R diagram depicts:
TemperatureColor, Spectral Type,
Luminosity,
and Radius of stars
(*Mass, *Lifespan,
of MS stars only)
Luminosity
Temperature

26. Outline of Ch 11b: The H-R Diagram (cont.)
Main Sequence Stars
Stellar Masses and Densities along main sequence
Mass-Luminosity Relation (L~M3.5)
Lifetime on Main Sequence (TMS~ 1/M2.5)
Main sequence Thermostat

27. Main Sequence Thermostat:
In the Sun, and in all main sequence stars gravity is balanced by outward pressure due to the outflow of energy.

28. Which star is the hottest?2
Which star is the hottest? 3 4
Luminosity 1
Temperature

29. Which star is the hottest?2
Which star is the hottest? 3 1 4
Luminosity 1
Temperature

30. Which star is the most luminous?2
Which star is the most luminous? 3 4
Luminosity 1
Temperature

31. Which star is the most luminous?2
Which star is the most luminous? 3 2 4
Luminosity 1
Temperature

32. 2
Which star is a main-sequence star? 3 4
Luminosity 1
Temperature

33. 2
Which star is a main-sequence star? 3 4 4
Luminosity 1
Temperature

34. 2
Which star has the largest radius? 3 4
Luminosity 1
Temperature

35. 2
Which star has the largest radius? 3 3 4
Luminosity 1
Temperature

36. Which star is most like our Sun?2
Which star is most like our Sun? 3 4
Luminosity 1
Temperature

37. Which star is most like our Sun?2
Which star is most like our Sun? 3 4 4
Luminosity 1
Temperature

38. 1
Which of these stars will have changed the least 10 billion years from now? 2 3
Luminosity 4
Temperature

39. 1
Which of these stars will have changed the least 10 billion years from now? 2 3
Luminosity 4 4
Temperature

40. 1
Which of these stars can be no more than a few million years old? 2
Luminosity 3 4
Temperature

41. 1
Which of these stars can be no more than a few million years old? 2
Luminosity 3 1 4
Temperature

42. What have we learned? • What is a Hertzsprung-Russell diagram?
An H–R diagram plots stars according to their surface temperatures and luminosities.

43. TemperatureColor, Spectral Type,
H-R diagram depicts:
TemperatureColor, Spectral Type,
Luminosity,
and Radius of stars
(*Mass, *Lifespan,
[*Age], of MS stars only)
Luminosity
Temperature

44. Outline of Ch 11b: The H-R Diagram
The Hertzprung-Russell (H-R) Diagram:
Surface Temperature vs Luminosity
Analogy: horsepower vs weight
Where Stars plot in the H-R diagram
Main Sequence: 90% of all stars
Why? stars spend 90% of their lives fusing hydrogen
Main sequence  Hydrogen fusion
Giants, Supergiants, White Dwarfs
Main Sequence Stars (cont.)

45. Outline of Ch 11 part II: The H-R Diagram (cont.)
Main Sequence Stars
Stellar Masses and Densities along main sequence
Mass-Luminosity Relation (L~M3.5)
Lifetime on Main Sequence (TMS~ 1/M2.5)
Main sequence Thermostat

46. Which of the following correctly fills in the blank
Which of the following correctly fills in the blank? A main-sequence star of spectral class B is _____ than a main-sequence star of spectral class G More massive 2. Hotter 3. Longer lived 4. More luminous The correct answer is A.     1 and 3 B.     2 and 3 C.     1, 2 and 4 D.     2, 3 and 4 E.      1, 2, 3 and 4

47. 2. Which of the following correctly fills in the blank
2.      Which of the following correctly fills in the blank? If a star is on the main-sequence and one knows its temperature, then one can estimate its ____. 1.            Spectral class 2.            Mass 3.            Luminosity 4.            Density 5.            Radial velocity   The correct answer is A.     1, 2, 3, 4 and 5 B.     1 and 5 C.     2 only D.    1, 3 and 5 E.     1, 2, 3 and 4

48. 3. Which of the following correctly fills in the blank
3.     Which of the following correctly fills in the blank? If a star of class O is on the main-sequence, that star must be ____. 1.            Hotter than most stars 2.            Very massive 3.            Much denser than water 4.            Very red 5.            Not very old   The correct answer is A.     2 and 3 B.     1, 2, 3 and 4 C.     1, 2, 3, 4 and 5 D.    1, 2 and 5 E.     4 and 5

49. 4. Which of the following correctly fills in the blank
  4. Which of the following correctly fills in the blank? If a star of class M is on the main-sequence, that star must be ____.   A.     Very hot B.     Very massive C.     Very blue D.    None of the other answers are correct

50. Outline of Ch 11b: The H-R Diagram (cont.)
Star Clusters: Confirmation of Stellar Evolution
Open and Globular Clusters
Ages of Clusters

51. Star Clusters: Confirmation of Stellar Evolution:
1. What is special about star clusters?
All stars formed at same time, so plotting clusters with different ages on H-R diagrams we can see how stars evolve
This confirms our theories of stellar evolution without having to wait billions of years observing how a single star evolves
2. Two types of clusters: Open and Globular
3. Ages of Clusters

52. Open cluster: A few thousand loosely packed stars

53. Useful movies to download:
Globular cluster visualization
Stellar collision simulation:
Globular cluster: Up to a million stars in a dense ball bound together by gravity

54. Two types of star clusters
Open clusters: young, contain up to several thousand stars and are found in the disk of the galaxy.
Globular clusters: old, contain hundreds of thousands of stars, all closely packed together. They are found mainly in the halo of the galaxy.

55. Our Galaxy

56. Which part of our galaxy is older?

57. How do we measure the age of a star cluster?

58. Theoretical Evolution of a star cluster

59. Massive blue stars die first, followed by white, yellow,
orange,
and
red stars
Star_cluster_evolving.swf

60. How do we know that this theoretical evolution is correct?

61. How do we know that this theoretical evolution is correct?
We plot observations of actual clusters on the H-R diagram

62. H-R Diagram of Young Stellar Cluster

63. H-R Diagram of Young Stellar Cluster
How do we know this cluster is Young?

64. H-R Diagram of Old Stellar Cluster

65. H-R Diagram of Old Stellar Cluster
How do we know this cluster is Old?

66. Pleiades cluster now has no stars with life expectancy less than around 100 million years
Main-sequence
turnoff

67. Main-sequence turnoff point of a cluster tells us its age

68. To determine accurate ages, we compare models of stellar evolution to the cluster data
Hr_diagr_and_age_of_cluster.swf

69. Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old
(The universe is about 13.7billion years old

70. What have we learned? How do we measure the age of a star cluster?
Because all of a cluster’s stars we born at the same time, we can measure a cluster’s age by finding the main sequence turnoff point on an H–R diagram of its stars. The cluster’s age is equal to the hydrogen-burning lifetime of the hottest, most luminous stars that remain on the main sequence.

71. Question 1
If the brightest main sequence star in cluster 1 is a B star and the brightest main sequence star in cluster 2 is an M star. What can we say about the age of these two clusters?

72. Question 1
If the brightest main sequence star in cluster 1 is a B star and the brightest main sequence star in cluster 2 is an M star. What can we say about the age of these two clusters?
Nothing, there is not enough information
Cluster 1 is older than cluster 2
Cluster 2 is older than cluster 1
None of the answers are correct

73. Chapter 12. Star Stuff Part I Birth of Stars
Birth of Stars from Interstellar Clouds
•Young stars near clouds of gas and dust
•Contraction and heating of clouds
• Hydrogen fusion stops collapse