Rotation and Flattening Lesson

Overview

As we observe the thousands of galaxies in the visible universe, we find that many of them are flattened disks. Because the process of galaxy formation is too slow to watch, scientists study it with computational models. This lesson/activity demonstrates how one might explain the appearance of so many flat galaxies with a computational model.

Preparation and Materials

The teacher should be familiar with the GalaxSee application (for those unfamiliar with this software, there is an online tutorial), have it loaded on a computer, and have some means of displaying the monitor to the class.

Objectives

Students will

use a computational model to discover possible answers to a question about a natural phenomenon
practice accurately observing and recording data from a scientific experiment
communicate and defend a scientific argument while collaborating with other students.

Standards

This lesson fulfills portions of the following standards and curriculum guidelines:

NSES content standard (9-12 A) on science as inquiry, including:
- Use technology and mathematics to improve investigations and communications.
- Communicate and defend a scientific argument.
NSES content standard (9-12 D) on origin and evolution of the universe.
NSES content standard (9-12 E) on understandings about science and technology.
DoDDs Science 9 objectives, including:
- Use of computational models.
- Use careful systematic observation and data collection to obtain valid information.
- Relate force, motion, energy and power.

Activities

If the students have not observed through previous activities the fact that many galaxies (including our own) are flat disks, show them images of several galaxies (available at numerous sites on the World Wide Web) and allow them to discover this fact.
Make the following points about galaxy formation:
1. The process is extremely slow--it takes billions of years for a galaxy to form.
2. Because of this, we cannot watch the formation happen.
3. In order to try to understand galaxy formation, scientists model galaxies on computers and, by watching the model evolve, they hope to learn why real galaxies have the features that we observe.
4. If we are to accomplish anything in science, it is extremely important that we are careful observers.
With the monitor displayed so that the students can see it, make sure that the Galaxy Setup is for a spherical galaxy of, say, 200-500 stars that are 100-200 solar masses each, with the rotation factor set to zero. Generate a new galaxy you can use the pre-generated galaxy "no_rotation.gal" at this web site. It may be easier to see what is going on if you turn on the Toggle Box. This is done under the view menu. The Toggle Box is a 3-D box surrounding the stars that can give you a helpful frame of reference. Run the simulation, and have the students watch what happens. Rotate the galaxy for them so they can see it from different angles. Let the simulation run until the stars clump together in the center. Stop the simulation, and have the students write (briefly) what they observed.
Note: As the stars clump to the center, their speeds will increase rapidly, and error will start to accumulate in the model. If this is allowed to continue, the model will become unrealistic, and could, for example, throw all of the stars out beyond the original boundaries. This is an inaccurate result, and if you allow the simulation to run beyond the initial clumping, you should be sure to discuss the implications of error in the model.
For more information about detecting and controlling error, see the section about the info window in the GalaxSee tutorial.
Now change the setup described in #3 so that the rotation factor is 1. Generate a new galaxy (or use the one named "rotation.gal" that came with this distribution), run it, and have the students observe the results. When a significant amount of flattening is observable (note that you will have to rotate your view of the galaxy to observe the flattening), stop the simulation, and again have them write what they observed. Use the Info Window to "measure" the time it takes for the disk to form from the spherical initial shape.

Discussion of the Simulation

Ask the students to discuss the differences between the two simulations. Ask them if they can think of other situations in which rotation flattens things. (Like hand-tossed pizza crust, spinning a Koosh® ball or key ring, for example).

Discussion of Observation

Ask the students to look at what they wrote about the two simulations. Tell them to suppose that they were the first people in the world to discover that rotating galaxies flatten over time. Is what they wrote adequate to explain this new discovery to the public or to other scientists? What would they need to include in or change about their writing in order to make it useful and clear to another person? Are they sure that they understand the relationship? Should they do more or different tests? Have them critique their own writing from this standpoint, and discuss what they learned from this.

Assign them to write a clear and accurate report of what they observed. Emphasize that it is important that they know what software was used, and what parameters were set. Be sure to go through the setup procedure again so that they can record this information.

Collaboration

After they have polished their reports, have the other group of students attempt to repeat the experiment as described in the report, verify the findings of the first group, and provide feedback about their methods and conclusions. Encourage both groups to ask questions of each other's procedure and observations. If another group of students is not available, you could split one class into two large groups and require them to communicate only through writing.

Extensions

Further Experimentation
Have the students use GalaxSee to try different values for the rotation, number of stars, dark matter, distributions, etc, observing and recording the effects.
For example, have each student create a different galaxy, iterate it, and record the experiment (including initial conditions). Then have the students exchange reports with one another, and attempt to repeat each others' experiments, comparing and reporting on the similarities or differences of the results.
Thinking Harder
Ask the students how they would verify the conjecture that flattening in galaxies may be a result of spinning. One way to do this is to see if the flat galaxies we observe in nature are spinning (they are, and this is observable with photometric tools attached to telescopes). However, not all of the galaxies that we observe are flat. What would we expect to find if we observed them to see if they were spinning? How might what we find from these observations cause us to modify our conjecture?