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

Reflection

Q&A