University of Michigan - Department of Astronomy

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Galactic Star Clusters Worksheets

Table 1: Cluster Turn off point and Ages
Cluster Name (B-V)o
at Turn-Off		 Age
M 67    
M 45    
M 44    
M 25    
NGC 752    
NGC 6791    
NGC 7044    
Mel 20    

Table 2: Cluster Distances
Cluster Name V Mv Modulus
(V-Mv)		 Distance (pc)
M 67        
M 45        
M 44        
M 25        
NGC 752        
NGC 6791        
NGC 7044        
Mel 20        

Table 3: Reddening/Color Excess
Cluster Name (B-V)o (B-V) E(B-V)
M 67      
M 45      
M 44      
M 25      
NGC 752      
NGC 6791      
NGC 7044      
Mel 20      

Table 4: Distance Errors
Cluster Name True Distance
d (pc) 		Measured Distance
d' (pc) 		d'/d
M 67 800    
M 45 120    
M 44 160    
M 25 600    
NGC 752 360    
NGC 6791 3500    
NGC 7044 3000    
Mel 20 170    

Graph 1: The correlation between distance errors & color excess

Distance Error versus Color Excess Graph

Questions

  1. What do you think are the principal sources of error in your distance determinations? You should be able to identify two human/procedural sources of error as well as one physical/natural source.












  2. Draw a sketch, with a dust cloud and several rays of light (blue light and red light). Include the effects the dust has on the light. Explain what else, besides reddening, could happen to the appearance of a cluster because of dust.














  3. Sketch a plot of the ratio d'/d versus E(B-V) in Graph 1. Describe the trend you see in your data. Carefully and logically explain the physical reasons for this trend.












  4. Suppose you have two clusters that are both 1000pc away, but are in different constellations. One of the clusters has E(B-V)=0.6, while the other has a color excess of 0. What does this tell you about the distribution of dust in the Galaxy?












  5. Draw a sketch of a color-magnitude diagram, and indicate which regions in the chart represent red/blue, bright/dim, and cool/hot stars. What kind of stars live in each region?

















  6. Cluster Mel 20 contains objects in the lower right of the CMD that are not on the main sequence. These are not evolved red giant stars -- how can you tell? What stage of stellar evolution do they represent?












  7. While there was quite a bit of variation in the clusters we looked at, none of them was as old as a globular cluster (these were galactic clusters). If you were to look at a color-magnitude diagram of a very, very old cluster, what would you expect to see? Draw a sketch as well as explain any new features.

















  8. Could you use the technique we have used here to find the age of a single star (not in a cluster)? Why/why not? In general, can you tell anything about the age of a single star from an H-R diagram?

Last Update: 8/30/07 by SAM

original by LA&GB