study and comparison of h-r diagrams of open clusters
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Study and Comparison of H-R Diagrams of Open Clusters
What is a star cluster/star clouds?
• A group of stars• Can be distinguished into two types: -Globular Cluster Open Cluster
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
Comparison of Star ClustersParameter 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
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
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
Formation
Collapse of part of giant molecular
cloud
Undergo hierarchical fragmentation into
infrared dark clouds
New star formed inside the collapsing
infrared cloud
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
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
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
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
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
Significance
• Key object for studying stellar evolution -same ages -same chemical composition -same distance from Earth -only difference between members is mass
Hertuzsprun-Russel Diagram
Background
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
H-R Diagram of M15
Interpretation• Main
Sequence: region where most stars occupy (90%)
• Giant stars: upper right corner
• White dwarfs: left of the main sequence
Study Process
Results
NGC4755
IC4651
NGC6791
NGC6633
NGC6067
M103
M21
M11
M6
M7
Analysis
Combined H-R Diagram
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
-16
-14
-12
-10
-8
-6
-4
-2
0
H-R diagram of 10 open clusters
NGC4755 IC4651 NGC6791 NGC6633 NGC6067 M103 M21 M11 M6 M7
Comparison
• Similarity– The majority of stars
appears on the main sequence and follow its route
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
-16
-14
-12
-10
-8
-6
-4
-2
0
H-R diagram of 10 open clusters
NGC4755 IC4651 NGC6791 NGC6633 NGC6067M103 M21 M11 M6 M7
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
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
-16
-14
-12
-10
-8
-6
-4
-2
0
H-R diagram of 10 open cluster
NGC4755 IC4651 NGC6791 NGC6633 NGC6067M103 M21 M11 M6 M7
What leads to those similarities and differences?
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
NGC4755
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 oldFeatures: white dwarfs
IC4651
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
NGC6791
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 giantsWhite dwarf
NGC6633
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
NGC6067
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
M103
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
M21
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
M11
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
M6
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
M7
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
-16
-14
-12
-10
-8
-6
-4
-2
0
H-R diagram of 10 open cluster
NGC4755 IC4651 NGC6791 NGC6633 NGC6067 M103 M21 M11 M6 M7
14Myr1.2Byr 8Byr 660Myr 102Myr 25Myr
4.6Myr
220Myr 100Myr
220Myr
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
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