Globular ClustersGlobular Clusters

Globular ClustersGlobular Clusters A globular cluster is an almost spherical conglomeration of A globular cluster is an almost spherical conglomeration of

100,000 to 1,000,000 stars of different masses that have 100,000 to 1,000,000 stars of different masses that have practically the same age and chemical composition. practically the same age and chemical composition.

The stars orbit around the center of the cluster, and the The stars orbit around the center of the cluster, and the cluster orbits the center of the parent galaxy.cluster orbits the center of the parent galaxy.

So far, about 160 globular clusters are known to exist in a So far, about 160 globular clusters are known to exist in a roughly spherical halo around the center of our Galaxy.roughly spherical halo around the center of our Galaxy.

Globular clusters are very old. There is a straightforward Globular clusters are very old. There is a straightforward method of determining their age, and this provides a very method of determining their age, and this provides a very interesting lower limit on the age of our universe of about 14 interesting lower limit on the age of our universe of about 14 billion years. billion years.

Also, because one can also determine their chemical Also, because one can also determine their chemical composition, one can derive information (together with the composition, one can derive information (together with the age information) about the chemical evolution of a galaxyage information) about the chemical evolution of a galaxy

OLD STAR CLUSTERS

                          

       

Hodge 11

                           

              

                     

            

47 Tucanae (NGC 104)

                           

              

                           

              

NGC 6522 in Sagittarius

Density about1,000,000times greater than in the Solar Neighbourhood

NGC 3603 : A globular-like cluster just born? NGC 3603 : A globular-like cluster just born? Massive star formation site in Milky Way Massive star formation site in Milky Way

The HR diagramThe HR diagram

The HR diagram shows the distribution of the photometric properties of the cluster stars on a plane that is equivalent to a luminosity-effective temperature plane. 

Various characteristic regions of the HR diagram which correspond to different evolutionary stages are identified: Main Sequence (MS); Turn off (TO); Red Giant Branch (RGB); Helium flash occurs here at tip of RGB (Tip); Horizontal Branch (HB); Schwarzschild gap in the HB (Gap); Asymptotic Giant Branch (AGB);

the final stellar remnants, White Dwarfs (WD), will lie off the bottom of the diagram. These regions show the main phases of stellar evolution. By comparing such diagrams with stellar evolution theories, one can derive information on the age, chemical composition, and distance of a globular cluster.

The observed Hertzsprung-Russell Diagram of The observed Hertzsprung-Russell Diagram of an old coeval stellar populationan old coeval stellar population

Measu

re o

f M

easu

re o

f ste

llar

lum

inosit

yste

llar

lum

inosit

y

Measure of surface effective Measure of surface effective temperaturetemperature

MS turnoffMS turnoff AGEAGE

Red Giant Branch Red Giant Branch Metallicity +ageMetallicity +age

RR-Lyrae variablesRR-Lyrae variables Distance (+Z)Distance (+Z)

Stellar EvolutionStellar Evolution

Schematic HR diagram of a coeval Schematic HR diagram of a coeval population of different agepopulation of different age

Effective temperature

Lu

min

osit

y

Medium-size black holes in globular Medium-size black holes in globular clustersclusters

                                                                                             

       

BH of 4000 solar masses

BH of 20,000 solar masses

Globular cluster dynamics

What determines the structure of a cluster Two types of relaxation:

• Initial mixing• Stellar encounters

Galactic tidal forces:• Limiting radius of a cluster

Encounters with giant molecular clouds At later stages, formation of medium size central black holes in dense clusters change the stellar orbits and the density profiles

Empirical formula by King (1962) involves three parameters: fo (central surface density) rc (core radius)

rt (tidal radius)

Number density profile of NGC 7789 Number density profile of NGC 7789

Log f(Surface number density)

Log r (arcmin)