Population of Stellar Ninjas (High School, COSMOS)
Topic/Concept: Isochrones, Stellar Populations, Stellar Evolution
Type of Activity: large-ish group activity
Prerequisite knowledge required: HR diagram basics: location of main sequence, giants, white dwarfs; Basics of what is a star and stellar evolution
Resources required: A “game piece” for each student—we used plastic ninjas; A large piece of paper with the HR diagram axes drawn on it (Alternatively, you could do the activity on the white board, I suppose); A card for each student telling them what mass their star is, and when to “evolve”
Learning Objectives: What an isochrone is! That it signifies the location of stars of different masses at a given cluster age. A higher-tier goal was that stellar populations change very quickly and dramatically when they are young, but change very slowly and subtlely when they are old. This is why it’s really hard to accuarately date globular clusters, and much easier to find the age of open clusters!
Common misconceptions: Stars “move” up the main sequence. Isochrones are the path a star takes as it evolves. Isochrones & HR diagrams show the locations of stars in real-space; when the star “moves” on these diagrams, it moves in space
Detailed description of activity: Explain to the students that they are a cluster of stars, and they are all about to be born at the same time. The only difference between them is their masses. Hand out a card to each of them that says something like:
“You are a 5 solar mass star. At 10:07 you evolve into a red giant. At 10:07 and 40 seconds, you begin burning helium. At 10:08 and 20 seconds, you become a planetary nebula.”
Now, these directions assume that the cluster started evolving at 10:00, and 1 second real time = 1 Myr ninja time.
We had 8 (high school “gifted”) students. These were the times we used:
50M_sun (at 10:00 & 1 sec you go supernova)
25M_sun (at 10:00 & 4 sec you go supernova)
10M_sun (at 10:01 & 40 sec you leave the MS. At 10:01 & 41 sec you go supernova)
The rest evolve (Main Sequence—> Red Giant—> Helium Burning—> Planetary Nebula) at the below times:
Mass______go RG______go HB______go PN
5M_sun__10:07:00____10:07:40____10:08:20
3M_sun__10:08:30____10:09:20____10:10:10
1.5M_sun__10:50______10:55_______10:57
1M_sun___12:46________1:03________1:11
0.8M_sun___2:10_______2:35________2:43
We had the good fortune of having the students for the whole day, so we were able to have time for the low-mass stars to evolve. Make sure every student feels responsible for their star ninja, and will watch the clock and evolve on time.
Start all the ninjas on the main sequence before 10:00. Make sure everybody understands what the main sequence is, and why their ninjas are arrayed in a line.
Set the cluster evolving. Every time a star evolves, draw a line connecting the new positions of the ninja, and label this line with the time. At the begninning, it will probably be pretty hectic, but once more high-mass stars evolve, there will be time to start discussing what is happening to the stars as they evolove, what an isochrone is, and how a stellar population changes with age.
It’s fun to make the student say “Bang!” when they go supernova, and “Poof!” when they go planetary nebula.
Later on in the day, move on to a new lesson, and let the very low-mass star ninjas evolve at the appropriate time. This shouldn’t interrupt your class too much.
Assessment: this exercise was done in preparation for students fitting isochrones to their color-magnitude diagrams, so we did not assess it directly.
Associated files and images: None.
Comments: This was for 8 high school students at COSMOS.
Isochrones are really hard! I think this activity was effective, but I think the students could have used more time to synthesize what they learned, because when they had to explain isochrones in their presentations, they really struggled.
Maybe I would have each student write me a defintion of an isochrone for homework. Or present in front of the class their defintion (presenting would only work for small groups).
Additionally, because Emily and I had forced ourselves to start at 10:00 by writing this on the cards, the beginning of the activity was very hurried, and we did not preface it with thorough explanation. This probably confused the students. Leave plenty of time to explain what’s happening before you set the cluster evolving.