EDUCATIONAL ACTIVITIES - EPOCH 2

Determining the Age of Open Clusters and Globular Clusters (PDF file format—requires Adobe Acrobat Reader)

Star clusters are groups of stars that form from the same nebula at the same time. Since star clusters contain stars of different mass, the evolution of the cluster is not uniform; the more massive stars evolve more rapidly and reach the main sequence first. As time passes, stars of lower and lower mass reach the main sequence. By the time the low-mass stars reach the main sequence, the upper main sequence stars will have exhausted their core hydrogen and started evolving towards the supergiant region of the H-R diagram. As the cluster ages further, more and more main sequence stars (starting with the most massive stars) will evolve to the supergiant region. If all the stars within a cluster are plotted on an H-R diagram, the star that is about to evolve off the main sequence is at what is called the "turnoff point." The lower the "turnoff point" of a star cluster, the older the age of the cluster. So the ages of star clusters may be ascertained by an analysis of their turnoff points. The following activity provides information for two open clusters and a globular cluster. Plotting the data for the stars in the three star clusters on the H-R diagram provided will allow you to determine the three "turnoff points" and therefore the age of the clusters.

ME C 2000 HR Diagram (PDF file format—requires Adobe Acrobat Reader)

For use with the Determining the Age of Open Clusters and Globular Clusters activity.

 


 

Questions for discussion or short essays:

Students will find these exercises akin to a pre-test, largely evaluating one's understanding either based on prior knowledge or on the brief presentation in the Prologue. They are therefore meant to provoke critical thinking. Much more information regarding each of these questions will be developed in greater detail in the principal epochs of this Web site.

 

1. What are the two major ways of classifying galaxies?

 

 

 

2. Outline the several different models of galaxy formation presented here, including their major strengths and weaknesses. Which one seems to fit best with the observations?

 

 

 

3. In what ways are hurricanes similar to spiral galaxies? How might, or might not, these similarities be useful to astronomers?

 

 

 

4. Define thermal energy, and then gravitational potential energy. Describe their frequent tug-of-war.

 

 

 

5. Describe briefly how galaxies might have formed from swirling eddies in a primordial gas clouds.

 

 

 

6. Define linear and angular momentum. Explain why a contracting object would tend to increase its rate of spin—and how, ultimately, a disk results.

 

 

 

7. Can the four major morphological types of normal galaxies be fit into an evolutionary sequence? Why or why not? Are there any clear evolutionary progressions yet observed among the galaxies?

 

 

 

8. What intrinsic processes likely affect galactic evolution? What environmental effects do galaxies experience?

 

 

 

9. How can we be sure that galaxies did not form via the chance accumulation of atoms?

 


 

True or False:

1. True or False: Galaxies are still being formed today. _____ Explain your answer.

 

 

 

2. True or False: The contraction of a gas cloud occurs when the gravitational potential energy exceeds the thermal energy of the cloud. _____ Explain your answer.

 

 

 

3. True or False: Although thermal energy competes with gravitational potential energy when a galaxy begins its initial contraction, it is the rotation of the galaxy which ultimately balances gravity when the galaxy is fully formed. _____ Explain your answer.

 

 

 

4. True or False: Galaxies evolve because of environmental effects, in addition to intrinsic processes. _____ Explain your answer.

 

 


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