Photometric study of  globular clusters in our galaxy using the 1.3m telescope



NGC 6426


- Production of homogeneous data base for all clusters in B, V, R, I. 

- Derivation of ages, metal abundances, distances, and interstellar absorption towards globular clusters

- Determination of lower limit for the age of the Universe 

- Deeper understanding of early galaxy formation 

- Detailed study of various stages of stellar evolution, their duration and dependence on metal abundance

HR diagram for NGC 6426
hr_diagram_ngc6426.gif (38457 bytes)
Results for NGC 6426
Age: one of the oldest star clusters known (about 0.7Gyr older than M92)Metal abundance:[Fe/H]=-2.33 dex:the most metal poor star cluster known in the Galaxy




General Information about 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. 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. 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 system (i.e. the way that the interstellar medium is enriched in heavy elements)

Shown on the left is an example of an HR diagram for a globular cluster.
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 and are explained below. By comparing such diagrams with stellar evolution theories, one can derive information on the age, chemical composition, and distance of a globular cluster.