Globular Cluster Ages
Topic/Concept: Stellar Evolution and Globular Clusters
Type of Activity: small group + discussion at the end
Prerequisite knowledge required: star formation and evolution, Hertzsprung-Russell (HR) diagrams, determining stellar lifetimes from masses
Resources required: Copies of cluster HR diagrams, textbooks and notes (for reference)
Learning Objectives: determine the age of a globular cluster based on the turn-off point in an HR diagram
Common misconceptions:
Detailed description of activity:
I used this activity to complement a short lecture-style review of clusters and/or stellar evolution. Going into this, my students knew that all cluster stars are the roughly same age and could draw an HR-diagram without necessarily understanding its nuances. The purpose of this activity is to allow students to process all that knowledge and make the connection between the turn-off point, stars dying, and age determination.
Split the students into 5 groups of 3 or 4. Give each group a printed HR diagram and tell them to determine the age of the cluster. I gave them ~15 minutes (depending on the students) to work on it, during which I visited each group and answered questions as they came up. In my sections students were confused at first because “age” isn’t an axis on the HR diagram :). I often needed to give hints to help them—“why does the turn-off point occur” was the best reminder; even though we’d covered it that point seemed to evade most students in the lecture. The next set of hints were aimed making the connection between dying stars and age; the amount of help varies from group to group! It was also important to show them how to look up information about stars, like mass, and remind them that they know how to get lifetime given a mass. The big task for the TA during this activity is guiding from step to step as they figure it out.
Note on materials— HR diagrams are easy to find—I recommend those with Stellar Classification labeled but if you use one with only temperature on x-axis be sure to help students make connection between temp and stellar classification—they can look it up in text books. Have text books available!
Once it seems like most groups have figured it out, I asked them to write up a short (2-3 sentence) description of how they got the age and turn it in. Alternatively, instead of a write up each group could give a short oral summary.
Discussion at the end covered such things as why massive stars die fastest, why we have a turn-off point in HR diagrams of clusters but not the Milky Way, why knowing the age of a cluster matters, why there might be variation in cluster ages, and any other questions raised by the activity.
Typical time breakdown for this activity—15 min lecture, 20 minute activity, 15 minute discussion.
Assessment: Group interaction with TA, written summary or oral group report
Associated files and images:
Comments: I used this activity in 5 sections of Astro 103 students in discussion sections and it worked well. The success is dependent on the interaction between TA and groups—I didn’t lay out the steps between HR-diagram to age to force them to figure it out themselves, but without guidance and answering questions along the way, students will get frustrated and learning will cease. A short write-up at the end is a good way for the students to review the steps yet again and clear up any remaining questions, plus it let me know if they were “getting it.”