globular clusters. a globular cluster is an almost spherical conglomeration of 100,000 to 1,000,000...
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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
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f M
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f ste
llar
lum
inosit
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llar
lum
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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)