Globular Clusters

A globular cluster is an almost spherical conglomeration of 100,000 to 1,000,000 stars of different masses that have practically the same age and chemical composition. A globular cluster is an almost spherical conglomeration of 100,000 to 1,000,000 stars of different masses that have practically the same age and chemical composition. The stars orbit around the center of the cluster, and the cluster orbits the center of the parent galaxy. The stars orbit around the center of the cluster, and the cluster orbits the center of the parent galaxy. So far, about 160 globular clusters are known to exist in a roughly spherical halo around the center of our Galaxy. So far, about 160 globular clusters are known to exist in a roughly spherical halo around the center of our Galaxy. Globular clusters are very old. There is a straightforward method of determining their age, and this provides a very interesting lower limit on the age of our universe of about 14 billion years. Globular clusters are very old. There is a straightforward method of determining their age, and this provides a very interesting lower limit on the age of our universe of about 14 billion years. Also, because one can also determine their chemical composition, one can derive information (together with the age information) about the chemical evolution of a galaxy Also, because one can also determine their chemical composition, one can derive information (together with the age information) about the chemical evolution of a galaxy

YOUNG OPEN CLUSTERS AAO ref AAT 10 The Trapezium stars and inner Orion NGC 2244 and the Rosette nebula NGC 3293 in Carina

OLD STAR CLUSTERS Hodge Tucanae (NGC 104) NGC 6522 in Sagittarius Density about 1,000,000 times greater than in the Solar Neighbourhood

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

The 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 an old coeval stellar population Measure of stellar luminosity Measure of surface effective temperature MS turnoff AGE AGE Red Giant Branch Metallicity +age Metallicity +age RR-Lyrae variables Distance (+Z) Distance (+Z)

Stellar Evolution Schematic HR diagram of a coeval population of different age Effective temperature Luminosity

Medium-size black holes in globular clusters 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: f o (central surface density) r c (core radius) r t (tidal radius)

Number density profile of NGC 7789 Log f (Surface number density) Log r (arcmin)