DETERMINATION OF THE DISTANCE
AND THE AGE OF A STAR CLUSTER

Rainer C. Gaitzsch
"EAAE Summerschools" Working Group

Teacher's Academy of Bavaria (Germany)

Abstract

Examples will be shown how to get spectra of bright stars, easily made with ordinary material available at school. In order to obtain a Hertzsprung-Russell diagram (HRD) of certain stars, spectra have to be analysed.

By means of data-banks of several star clusters on given papers, the participants for themselves will work out an HRD of a certain star cluster. In this diagram the apparent magnitude is plotted against the surface-temperature (resp. the color-index B-V). The group will then discuss their results. By comparing their own HRDs with the standard main-sequence of stars given in absolute magnitudes, the participants will derive the distance of their own cluster and learn about how to estimate its age.

Special computer simulations with the participants being involved will illustrate some questions to phenomena concerning the evolution of stars, i.e.: What happens in the star's core? Where do the cluster-stars shift their location in the HRD during the process of aging? Why do all open star clusters disperse at last?

 

Exercise:

Determine the distance and estimate the age of the open cluster NGC 6025.

Additional exercise:

The globular cluster M13 (Fig. 1b) has an apparent radius of 5.0 ' and the apparent magnitude m = 5.7. M13 containes variable stars of RR Lyr-type (abs. mag. M = 0.0), given with their apparent mag. m = 14.9.

Table 1

table 2

table 3

Fig. 7 : HRD-Simulation program of the evolution of a star cluster;
time : 300 Mio. years.

Fig.8 : HRD, indicating the turnoff point of several star clusters. At those points the most massive stars leave the main sequence for the red-gainst region.