Activity Created by Jessica R. Lu


Audience: Introductory astronomy class for majors, 1 hr discussion group

Content Goals: Kepler's 3rd Law

Topic: Stars Orbiting the Supermassive Black Hole at the Center of the Galaxy

Materials: Handout (see attachements)

Teacher Notes:


I conducted this activity as close to inquiry-style as I could get given the 1 hour time limit. First, I handed out ONLY the plots page and I didn't give them ANY information (they had already seen a lecture on Kepler's 3rd law). I asked them what they thought the plot contained. Several said planets orbiting a star. I prompted with a response about whether planets have such elongated and diverse orbits. Then I told them they were stars. They wondered what the stars were going around and some mentioned binaries. One mentioned the idea of a black hole. I encouraged that and suggested they might figure out how big the black hole was. They struggled with period, distance, etc. since the units are in arcsec. I let them work through it until they asked me for distance and period information (I thought this was a key discussion point for them to work through). Then once they had the distance it was pretty much plug and chug. For the speedier ones, I prompted them with questions about geometry (inclination, eccentricity)... they couldn't really do anything about it other than use an ensemble average; but it really made them think about the problem a second time. We did all of this in a 1 hour discussion section... it was VERY fast and I had to manage very actively to keep people moving. The timing I used is described below.


  • 7 minutes for initial inquiry
  • 7 minutes for discussion after intro of concept of stars + black hole — and objective to get mass of black hole. Ended when they started asking for distance/period information — I gave them a second hand out with a table of stellar orbit properties 
  • 30 minutes working on conversion from arcsec to physical units and then deriving black hole mass
  • 10-15 minutes wrap up having presentations of results, and brief discussion of problems encountered. Also reviewed here the current research. They REALLY enjoyed the idea that they just worked on real data and did what a REAL astronomer had just recently done as well.




Page last modified on Wednesday 14 of March, 2012 16:53:28 EDT

Attached files

ID Name Comment Uploaded Size Downloads
28 gcExcercise-1.doc jlu Wed 14 of Mar, 2012 16:55 EDT 135.50 Kb 1689