The document discusses star clusters and compares the H-R diagrams of 10 open clusters. It finds that while the main sequences of the clusters are generally similar, some clusters have differences like stars distributed to the right of the main sequence (NGC 6791) or outside of it (NGC4755, M6, M103). These similarities and differences can be explained by the different properties and ages of the clusters, ranging from 14 million years (NGC4755) to 8 billion years (NGC 6791).

1. Study and Comparison of H-R
Diagrams of Open Clusters

2. What is a star cluster/star clouds?
• A group of stars
• Can be distinguished into two types:
-Globular Cluster
Open Cluster

3. What is an open cluster?
• A group of few hundred to few thousand stars
• Formed by the same molecular cloud
• Loosely bound by mutual gravitational attraction
• Generally survive for few hundred million years
• Shapes:
-Spiral
-Irregular
• Similar Ages
• Similar Chemical Composition

4. Comparison of Star Clusters
Parameter Open Cluster Globular Cluster
Form Loose Tight
Age Young Old
Amount ~100 ~100,000
Gravitational
bound
Weak, easily disrupted by
gravitational influence of
giant molecular cloud
Strong, stars are tightly
bound together
Shape Spiral or irregular spherical
Density Low High
Location Disk of Milky Way Halo of a galaxy

5. Historical Observation
• Prominent clusters have been recognized since antiquity
• Expected to be a single star, not a cluster
• 1609, Italian Scientist Galileo Galilei firstly used telescope
• Discovered that they are a group of stars
• 1774-1781, Charles Messier published a catalogue of
celestial objects
• 26 open clusters were included
• 1790s,English astronomer William Herschel began
extensive study on nebulous celestial objects
• Stars are initially scattered, later become clustered due to
gravitational attraction

6. Historical Observation
• 1888-1905 Danish-Irish astronomer J.L.E.
Dreyer published New general Catalogue for
open cluster
• Proper motion of different open cluster is
measured in succession by Dutch-American
astronomer Adriaan van Maanen
• 1911, Ejnar Hertzsprung published the first
color magnitude diagram of open cluster

7. Formation
Collapse of part of
giant molecular
cloud
Undergo hierarchical
fragmentation into
infrared dark clouds
New star formed
inside the collapsing
infrared cloud

8. Formation
Newly formed stars
emit intense
ultraviolet radiation
The surrounding gas
of giant molecular
cloud is ionized into
a HII Region
Stellar Winds and
radiation pressure
drive away hot
ionized gas

9. Formation
Cluster will
experience its first
core-collapse
supernovae
After these process
the cluster of gas
will be stripped
no further star
formation takes
place and the
cluster is finally
formed

10. Numbers and distribution
• Known open clusters: ~1000
• Spiral galaxy: Mostly found in spiral arms where gas
densities are highest
• Close to galactic plane
• Irregular galaxy: May be found throughout the galaxy,
and concentrated in places with high gas densities
• Elliptical galaxy: Cannot be found as star formation has
ceased many millions of years ago. The original open
clusters have already dispersed
• Milky Way: older clusters are further from galactic
center, at a substantial distances above or below the
galactic plane

11. Morphology and Composition
• Generally, core of an open cluster is 3-4 light
years across
• The density of the center of the cluster is typically
1.5 stars per cubic light years
• Young clusters contain hot blue stars, while old
clusters have more yellow stars
• Some open clusters contain hot blue stars
younger than others called blue stragglers
• Few white dwarfs can be found as they are
ejected out of the cluster

12. Ending
• Dispersion: as the cluster have small mass that
the escape velocity of the system is lower than
the constituent stars
• Evaporation of cluster members: close
encounters of stars increase the velocity of a
member beyond the escape velocity of the
cluster
• Disruption: when a cluster passes through or
close to a molecular cloud, it is disturbed by the
tidal force generated. The stars no longer stay
close together as a cluster

13. Significance
• Key object for studying stellar evolution
-same ages
-same chemical composition
-same distance from Earth
-only difference between members is mass

14. Hertuzsprun-Russel Diagram

15. Background

16. Diagram
• X-axis: Temperature of
stars
• Originally spectral class
but now replaced by
color index (mostly B-V
color) of stars
• Y-axis: Luminosity of
stars

17. H-R Diagram of M15

18. Interpretation
• Main
Sequence:
region where
most stars
occupy (90%)
• Giant stars:
upper right
corner
• White dwarfs:
left of the
main
sequence

19. Study Process

22. Results

23. NGC4755
-16
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

24. IC4651
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

25. NGC6791
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5 3 3.5
V
B-V
B-V plot

26. NGC6633
-14
-12
-10
-8
-6
-4
-2
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
V
B-V
B-V plot

27. NGC6067
-14
-12
-10
-8
-6
-4
-2
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
V
B-V
B-V plot

28. M103
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5 3
V
B-V
B-V plot

29. M21
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

30. M11
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

31. M6
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
V
B-V
B-V plot

32. M7
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

33. Analysis

34. Combined H-R Diagram
-16
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
H-R diagram of 10 open clusters
NGC4755 IC4651 NGC6791 NGC6633 NGC6067 M103 M21 M11 M6 M7

36. Comparison
• Similarity
– The majority of stars
appears on the main
sequence and follow
its route
-16
-14
-12
-10
-8
-6
-4
-2
0
0 1 2 3 4 5
H-R diagram of 10 open clusters
NGC4755 IC4651 NGC6791 NGC6633 NGC6067
M103 M21 M11 M6 M7

37. Comparison
• Differences
• Stars of NGC 6791
mostly distributed on
the right of the main
sequence
• Some stars of
NGC4755, M6 and
M103 are out of
main sequence
-16
-14
-12
-10
-8
-6
-4
-2
0
0 1 2 3 4 5
H-R diagram of 10 open cluster
NGC4755 IC4651 NGC6791 NGC6633 NGC6067
M103 M21 M11 M6 M7

38. What leads to those similarities
and differences?

39. NGC4755 (Jewel Box)
• RA: 12h 53m 42s
• Dec: −60° 22.0
• Distance (from Earth): 6.4 (kly)
• Constellation: Crux
• First Observer (s): Nicolas Louis de Lacaille
Age: 14 million years old (youngest known)
Features:
 Blue super-giant
 Super-giant

40. NGC4755
-16
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

41. IC4651
• RA: 17h 24m 49s
• Dec: −49° 56′ 00″
• Distance (from Earth): 2,900 (ly)
• Apparent Dimension: 10.0'(arc min)
• Linear Extension: 15 (ly)
• Constellation: Norma
• First Observer (s): John Louis Emil Dreyer
Age: 1.2 billion years old
Features:
 white dwarfs

42. IC4651
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

43. NGC6791
• RA: 19h 20m 53s
• Dec: +37° 46.3
• Distance (from Earth): ~13,000 (ly)
• Apparent magnitude: +9.5
• Apparent Dimension: 16 (arc min)
• Linear Extension: 15 (ly)
• Constellation: Lyra
• Age: 8 billion years old
• Features:
 Iron to Hydrogen abundance ratio that is more than twice that
of the Sun
 Groups of white dwarf

44. NGC6791
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5 3 3.5
V
B-V
B-V plot

45. NGC6633
• RA: 18h 27.7m
• Dec: 6° 34′
• Distance (from Earth): 1.04 (kly)
• Apparent magnitude: 4.6
• Apparent Dimension: 27’
• Constellation: Ophiuchus
• First Observer (s): Philippe Loys de Chéseaux
• Age: ~660 (million years old)
• Features:
 Red giants
 White dwarf

46. NGC6633
-14
-12
-10
-8
-6
-4
-2
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
V
B-V
B-V plot

47.  RA: 16h 13.2m
 Dec: -54°13’
 Distance (from Earth): 4,621 (ly)
 Apparent magnitude: 5.6
 Apparent Dimension: 12.0 (arc min)
 Linear Extension: 15 (ly)
 Constellation: Norma
 First Observer (s): Charles Messier and Pierre Méchain
 Age: 102 (million years old)
 Features:
 Red giants
NGC6067

48. NGC6067
-14
-12
-10
-8
-6
-4
-2
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
V
B-V
B-V plot

49.  RA: 0.1h33.2m
 Dec: +60°42’
 Distance (from Earth): 8,500 (ly)
 Apparent magnitude: 7.4
 Apparent Dimension: 6.0 (arc min)
 Linear Extension: 15 (ly)
 Constellation: Cassiopeia
 First Observer (s): Charles Messier and Pierre Méchain
 Age: 25 (million years old)
Features:
 Red giants
M103

50. M103
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5 3
V
B-V
B-V plot

51. M21
• RA: 18h 04.6m
• Dec: -22°30’
• Distance (from Earth): 4,250 (ly)
• Apparent magnitude: 6.5
• Apparent Dimension: 13.0 (arc min)
• Linear Extension: 10 (ly)
• Constellation: Sagittarius
• Age: 4.6 million years(relatively young cluster)
• Features:
 White dwarfs
 Blue dwarfs

52. M21
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

53. M11
• RA: 18h51.1m
• Dec: -06°16’
• Distance (from Earth): 6,200
(ly)
• Apparent magnitude: 6.3
• Apparent Dimension: 14.0 (arc
min)
• Linear Extension: 21 (ly)
• Constellation: Scutum
• Age: 220 (million years old)
• Features:
 Red giants

54. M11
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

55. M6
• RA: 17h40.1m
• Dec: -32°13’
• Distance (from Earth): 1,600 (ly)
• Apparent magnitude: 4.2
• Apparent Dimension: 25 (arc
min)
• Linear Extension: 20 (ly)
• Constellation: Scorpius
• Age: 100 (million years old)
Features:
 Red giants
 Orange Giants

56. M6
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
V
B-V
B-V plot

57. M7
• RA: 17h53m51.2s
• Dec: -34°47’ 34”
• Distance (from Earth): 800 (ly)
• Apparent magnitude: 3.3
• Apparent Dimension: 80 (arc
min)
• Linear Extension: 18 (ly)
• Constellation: Scorpius
• Age: 100 (million years old)
Features:
 Yellow Giant

58. M7
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5
V
B-V
B-V plot

59. -16
-14
-12
-10
-8
-6
-4
-2
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
H-R diagram of 10 open cluster
NGC4755 IC4651 NGC6791 NGC6633 NGC6067 M103 M21 M11 M6 M7
14Myr
1.2Byr
8Byr 660Myr 102Myr 25Myr
4.6Myr
220Myr 100Myr
220Myr

60. Source
• http://adsabs.harvard.edu/abs/2007AJ....133.1490W
• http://www.csupomona.edu/~pbsiegel/phy303/hrpic.jpg
• http://en.wikipedia.org/wiki/H-R_diagram
• http://www.docstoc.com/docs/4194598/Laboratory-The-Hertzsprung-Russell-H-R-Diagram-Ejnar-Hertzsprung
• http://en.wikipedia.org/wiki/B-V_color
• https://www.mtholyoke.edu/courses/mdyar/ast100/HW/hr_diagram_big.jpg
• https://portal.utpa.edu/portal/page/portal/utpa_main/daa_home/cosm_home/physics_home/physics_files/astr1402-
lab4i.pdf
• http://abyss.uoregon.edu/~js/ast122/lectures/lec11.html
• http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_hrintro.html
• http://www.seasky.org/astronomy/astronomy-messier-1to10.html

61. Reflection

62. Q&A