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

2. 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

3. Open cluster: A few thousand loosely packed stars

4. 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

5. 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.

6. Our Galaxy

7. Which part of our galaxy is older?

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

9. Theoretical Evolution of a star cluster

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

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

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

13. H-R Diagram of Young Stellar Cluster

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

15. H-R Diagram of Old Stellar Cluster

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

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

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

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

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

21. 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.

22. 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?

23. 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

24. Chapter 12. Star Stuff (mostly different from book)
Birth of Stars from Interstellar Clouds
•Young stars near clouds of gas and dust
•Contraction and heating of clouds into protostars
• Hydrogen fusion stops collapse
II. Leaving the Main Sequence: Hydrogen fusion stops
1. Low mass stars (M < 0.4 solar masses)
Not enough mass to ever fuse any element heavier than Hydrogen → white dwarf
2.Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun)
He fusion, red giant, ejects outer layers → white dwarf
3.High mass Stars (M > 4 solar masses)
Fusion of He,C,O,…..but not Fe (Iron) fusion
Faster and faster → Core collapses → Supernova
blows up and produces all elements heavier than Fe

25. 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

26. 12.1 Star Birth Our Goals for Learning • How do stars form?
• How massive are newborn stars?

27. We are “star stuff” because the elements necessary for life were made in stars

28. How do stars form?

29. I. Birth of Stars and Interstellar Clouds
•Young stars are always found near clouds of gas and dust
● The gas and dust between the stars is called the interstellar medium.
•Stars are born in intesrtellar molecular clouds consisting mostly of hydrogen molecules and dust
• Stars form in places where gravity can make a cloud collapse

31. Orion Nebula is one of the closest star-forming clouds
Infrared light from Orion

32. Summary of Star Birth
Stars are born in cold, relatively dense molecular clouds.
Gravity causes gas cloud to shrink
Core of shrinking cloud collapses under gravity and heats up, it becomes a protostar surrounded by a spinning disk of gas.
When core gets hot enough (10 million K), fusion of hydrogen begins and stops the shrinking
New star achieves long-lasting state of balance (main sequence thermostat)

33. Hubble Space Telescope Image of an edge-on protostar and its jets

34. Protostar to Main Sequence (in book)
Protostar contracts and heats until core temperature is sufficient for hydrogen fusion.
Contraction ends when energy released by hydrogen fusion balances the gravity
Takes less time for more massive stars to reach the Main Sequence (more massive stars evolve faster)

35. I. Birth of Stars and Interstellar Clouds
• Protostar in the H-R diagram

36. I. Birth of Stars and Interstellar Clouds
• Protostar in the H-R diagram
This is the track of a collapsing and heating protostar but we do not see most of them because they are inside dense clouds of gas and dust

37. I. Birth of Stars and Interstellar Clouds
• Protostar’s T-Tauri phase: a very active phase of protostars that clears the gas and dust from the surrounding disk

38. Question 2
What happens after an interstellar cloud of gas and dust is compressed and collapses?

39. Question 2
What happens after an interstellar cloud of gas and dust is compressed and collapses?
It will heat and contract
If its core gets hot enough (10 million K) it can produce energy through hydrogen fusion
It can produce main sequence stars
All of the answers are correct

40. Main Sequence ( Hydrogen Fusion)
Main sequence Thermostat : very stable phase

41. How massive are newborn stars?

42. A cluster of many stars can form out of a single cloud.

43. Very massive stars are rare
Low-mass stars are common.
Minimum mass needed to become a star: 0.08 solar masses
Luminosity
Temperature

44. • How massive are newborn stars?
Low mass stars are more numerous than high mass stars
Newborn stars come in a range of masses, but cannot be less massive than 0.08MSun.
Below this mass, pressure in the core is not enough (10 million K) for hydrogen fusion, and the object becomes a “failed star” known as a brown dwarf.

45. Equilibrium inside M.S. stars

46. Question
What happens when a star can no longer fuse hydrogen to helium in its core?
A. Core cools off
B. Core shrinks and heats up
C. Core stays at same temperature
D. Helium fusion immediately begins

47. Question
What happens when a star can no longer fuse hydrogen to helium in its core?
A. Core cools off
B. Core shrinks and heats up
C. Core stays at same temperature
D. Helium fusion immediately begins

48. Ch. 12 Part II (not like book)
Ch. 12 Part II (not like book). Leaving the Main Sequence: Hydrogen fusion stops
1. Low mass stars (M < 0.4 solar masses)
Not enough mass to ever fuse any element heavier than Hydrogen  white dwarf
2.Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun)
He fusion, red giant, ejects outer layers  white dwarf
3.High mass Stars (M > 4 solar masses)
Fusion of He,C,O,…..but not Fe (Iron) fusion
Faster and faster  Core collapses  Supernova
 Blows up and produces all elements heavier than Fe

49. Outline of Chapter 12 Part II Evolution and Death of Stars
Leaving the Main Sequence:
BEWARE THAT THE BOOK DOES NOT USE THE SAME DEFINITIONS OF LOW, INTERMEDIATE AND HIGH MASS STARS.
AS MENTIONED, THE EXAM WILL BE BASED ON THE LECTURES AND NOT ON THE BOOK

50. Remember: Stellar Masses

51. Composition inside M.S. stars
Eventually the core fills up with helium and hydrogen fusion stops

52. Leaving the Main Sequence: Hydrogen fusion stops
1. Low mass stars (M < 0.4 solar masses)
Not enough mass to ever fuse any element heavier than Hydrogen  white dwarf
White Dwarfs

53. I. Leaving the Main Sequence: Hydrogen fusion stops
2. Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun)
He fusion, red giant, ejects outer layers  white dwarf

54. Helium fusion requires much higher temperatures than hydrogen fusion because larger charge leads to greater repulsion

55. Stars like our Sun become Red Giants after they leave the M. S
Stars like our Sun become Red Giants after they leave the M.S. and eventually White Dwarfs

57. Most red giants stars eject their outer layers

58. Only a white dwarf is left behind
A star like our sun dies by puffing off its outer layers, creating a planetary nebula.
Only a white dwarf is left behind
Great planetary-nebula movies can be downloaded from Press Release (Helix Nebula) of the Space Telescope Science Institute (see hubble.stsci.edu)

59. A star like our sun dies by puffing off its outer layers, creating a planetary nebula.
Only a white dwarf is left behind

60. A star like our sun dies by puffing off its outer layers, creating a planetary nebula.
Only a white dwarf is left behind

61. A star like our sun dies by puffing off its outer layers, creating a planetary nebula.
Only a white dwarf is left behind

62. II. Leaving the Main Sequence: Hydrogen fusion stops
3.High mass Stars (M > 4 solar masses)
Fusion of He,C,O,…..but not Fe (Iron) fusion
Faster and faster  Core collapses  Supernova
 Produces all elements heavier than Fe and blows up

63. Supernovas 3. High mass star (M > 4 solar masses)
Fusion of He,C,O,…..but not Fe (Iron) fusion
Faster and faster  Core collapses  Supernova
Produces all elements heavier than Fe and blows envelope apart ejecting to interstellar space most of its mass
Supernova Remnants:
Crab nebula and others

64. An evolved massive star (M > 4 Msun)

65. An evolved massive star (M > 4 Msun)

66. before
after
Supernova 1987A in a nearby galaxy is the nearest supernova observed in the last 400 years

67. Crab Nebula: Remnant of a supernova observed in 1054 A.D.

68. Pulsar (a kind if neutron star) at center of Crab nebula

69. Older Supernova Remnant