Black+Hole+Hunters

Stellar Orbit Data comes from @http://astro.uchicago.edu/cosmus/projects/UCLA_GCG/ Animations of these orbits are available at: @http://www.astro.ucla.edu/~ghezgroup/gc/ These versions of this lesson plan use the specialized astronomy software IDL, which is not easily accessible in schools. Need to update to make more teacher friendly. PDF of an alternate, more up-to-date, version of the lesson below: ** Black Hole Hunter Project **  ** Authors: Daniel DuBrow & Jason Hwang, 2011 **


 * Essential Question: **** How do researchers determine the mass and location of the supermassive black hole at the center of our Galaxy? **


 * Goal: **** Using authentic observational data, determine to the best of your ability the mass and location of the central black hole. **


 * Materials: **
 * IDL code, [[file:gk12northwestern/BHHunter.zip|BHHunter.zip]] **
 * If we're interested in extending this lesson into a full 4-6 week unit, ideas for doing so are listed [[file:gk12northwestern/extendingBHhunter.docx|here]]. **


 * Procedure: **
 * 1) Log onto one of the computers in the classroom. If you are on a computer along the back wall, use “wildkit/student” as your username and password.


 * 1) Go to the Start Menu, All Programs, then find “IDL 8.0” and click on “IDL”. This opens up a data analysis program. You may see a message stating that there is an error due to no license. You don’t have to worry about this, but note that you will have to reopen the program after 7 minutes.


 * 1) At the bottom of the screen you will see a small area labeled “current directory”. Click on the folder icon next to this, and navigate to **My Computer -> Local Disk (C:) -> BHH**. Then click “ok”. You have selected the Black Hole Hunter folder.


 * 1) Now actually find and open the folder, “BH Hunter”, by clicking on the Start button, then “My Computer, Local Disk (C:), BHH, BH Hunter . Notice that there are several files in this folder. Open “BHParam.txt” by right clicking on it and selecting “Open with Wordpad”. There are notes in “BHParam.txt” that you should read. Each note starts with a ‘;’. You’ll also be making changes to this file as you try to find the black hole location and mass. Also note the existence of “odata.txt” which contains the star positions.


 * 1) Now go back to the IDL window. Click on the lower left part of the screen, so your cursor is next to the “IDL>” prompt. Then type: (and hit enter)


 * .run BHHunter.pro **

You will see a movie of the orbits of 13 stars near the center of the galaxy. There are two types of paths that can be shown - a solid orbit and a dashed orbit. The dashed orbit represents the actual **observed** orbit of the star and the solid orbit represents a simulated orbit, based on your “guessed” location of the black hole. The first time you run the program, you will ONLY see the actual **observed** orbit. Let the program run until it is finished. This will take about 2 min. You can also stop the program when you feel you have enough for a good guess, by hitting “stop” in the toolbar in IDL. Write down your initial guess for the location of the black hole.

X: ___ Y:___


 * 1) Now go back to “BHParam.txt”. Change “view” to “2” which represents the Y-Z plane. Now re-run the program and repeat step 5, but this time guess the Y and Z coordinates of the black hole.

Y: Z: _


 * 1) Repeat Step 6 but use view 3 this time, which represents the X-Z plane. Run the program and make a guess for X and Z.

X: Z: _


 * 1) Which stars do you think would be the best for getting a 'ballpark' guess? Which stars would be best for refining your estimate and why?


 * 1) Now it’s time to get down to business! Take your guesses for X, Y, and Z, and input them into the BHParam file. For now you can leave the mass unchanged, because the value we gave you is kinda close (but not quite). Also, pick one of the 13 stars. We recommend Star 1 to start with. Type this number in place of the ‘0’ that is in “ostar” and the ‘-1’ in “sstar”. These refer to the Observed star # and the Simulated star #. The 0 means that we were looking at all 13 actual observed orbits to begin with and the -1 means that we weren’t simulating any orbits at first. RUN the program again and look at the difference between the simulated and observed orbits. Write down your observation of this difference – include which orbit has the smaller radius.

If the **observed** orbit (dashed) is closer to the black hole, how should you change the mass of the black hole once you’ve figured out its location?

If the **simulated** orbit (solid) is closer to the black hole, how should you change the mass of the black hole once you’ve figured out its location?

X: Y: __Z:__ __ (by the way, 1.00 unit of length = 1.2342721 × 1015 meters = about 10000 times the Earth-Sun distance)
 * 1) This is the part where you really have to work. Start tweaking your location and mass guesses by looking at stars other than #1. Remember that you can change the views to X-Y (1), Y-Z (2) or Z-X (3). When you think you have arrived at your final guess, write it down here: (after all groups are done we will compare between all groups and come to a class consensus) M:

Discussion/Analysis Questions: (Answer in lab books at length)

1) a. How would you choose the timestep the program uses? Larger timesteps give you faster results but less accuracy and vice versa. Keep in mind you were using a timestep of 10, and you have limited computational resources (slow computers!).
 * 1) Compare the timestep you should use at first with the one you should use when you’re close to your final guess.

2) Why can we are ignore the gravity between the stars? Hint: what is the ratio of the force between stars to the force between star and black hole. Think of a scenario where you would NOT want to ignore it?

3) What are the different types of orbits that you see? Why are some of the orbits ellipses, some circles, and others parabolas?

4) If the black hole were replaced by a massive star of the same size, what would be the difference in the orbits? (hint, remember, black holes are NOT cosmic vacuum cleaners).

5) What would happen if you set the mass to be negative? Try it and write down your observations. What does this remind you of? (hint: think about the HW you just completed for today)

6) What would happen if there is a very small error (but not zero) per timestep over the course of 10000 timesteps? Why is this important?