COLOR-MAGNITUDE DIAGRAM ACTIVITY


ISP 205, Section 1, Spring 1997
Color-Magnitude Diagram Activity
March 1997 

                                      Name: __________________________


                                      Student Number: ________________

The Color-Magnitude Diagram for a Milky Way Star Cluster

Star clusters are groups of stars that were born together in the same place and time from the same cloud of dust and gas. The following diagrams give the best information for stars that are all members of one cluster in the Milky Way. Note that the vertical axis appears to be backward, in that the brighter stars have smaller "V" values. This logarithmic scale follows the convention set by Arab astronomers in the middle ages, because this is how your eye judges brightness. The B-V value or "color" is a measure of stellar temperature. Our whitish Sun has B-V = 0.64. Stars with large numbers are cooler and would look reddish to you (Fig.18-28 in your text), and stars with negative B-V are hotter and would look bluish (Fig.17-16). 
CLUSTER 1
See Figure 18-30 in your textbook.
CLUSTER 2
See Figure 18-26(a) in your textbook.
1. This Color-Magnitude Diagram (CMD) is the modern equivalent of the Hertzsprung-Russell Diagram (HRD). This shows the same patterns of stellar evolution as does an HRD. Sketch a line through the points on your plot to show your cluster's "main sequence." Now sketch lines to show the "red giant branch" stage of stellar evolution. What does it mean if all your cluster stars are only in the main-sequence evolutionary stage? 
A cluster with even its most massive stars still on the main-sequence 
(still fusing hydrogen to helium in the core) must be young.
The CMD for each stellar cluster is unique. Get together with the other pair of students who have a different cluster and compare your CMDs. After discussion with the other people in your group, answer the following questions for yourself using complete sentences.

2. (a) Which cluster is older? 

Cluster #1 is older than Cluster #2.
(b) How can you tell? 
Cluster #2 has only stars on the main-sequence, which must mean 
it's younger.  Also, Cluster #2 has red giants, which are always evolved
stars.

A third way to tell is by comparing the color of the top of the main
sequence, called the "turn-off," for these two clusters.  Cluster #2 has
a much bluer "turn-off," meaning that the stars now leaving the main
sequence are more massive and hence younger.  You may also have noticed
that the "turn-off" color for Cluster #1 was about the same color as
the Sun, meaning that its age must be at least that of the Sun when it
will begin to turn to a red giant about 5 billion years in the future.  If you
add that to the current age of the Sun, 4.5 billion years, this cluster must
be at least 10 billion years old.
3. (a) Which cluster is further away from the Earth? 
Cluster #1 is further away than Cluster #2.
(b) Which information did you use to determine this? 
The left-hand axis of the clusters is given in V magnitudes.  The larger
the value of V, the less bright the star appears.  The entire scale of
Cluster #1 is shifted downward, or dimmer, compared to Cluster #2, meaning
that it's further away.  (B is proportional to Luminosity/Distance2)

Next unit when we study our galaxy, the Milky Way, we'll learn that 
ancient clusters populated with many stars (like Cluster #1) tend to be 
further away from us than are young, blue clusters with fewer stars, like 
Cluster #2.
4. When your group is done, the class as a whole will discuss the answers. Correct your answers if necessary, and then hand in your work for credit. 




The photometry for the Pleiades is from Johnson & Mitchell 1958, ApJ, 128, 31
and for 47 Tuc from Hesser et al. 1987, PASP, 99, 739.  The textbook referred
to is Discovering Astronomy, 1995 3rd Ed., Robbins, Jefferys, & Shawl
(John Wiley & Sons).




Beth Hufnagel's 
home page,
email:  hufnage4@pilot.msu.edu




Bob Stein's  home page ,
email:  steinr@pilot.msu.edu