Collisions Lesson

Overview

Galaxies are observed to occur in a variety of shapes. They can generally be classified as either spiral or elliptical, with different patterns seen in spiral galaxies. The purpose of this lesson is to explore the role that collisions and near collisions of galaxies play in determining the shape of galaxies. Students will model the collision of disk shaped and spherically shaped galaxies.

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.

GalaxSee is designed to generate a single galaxy, or to use the Galaxy Make tool under "New Galaxy" to make more than one.

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 performed the Rotation and Flattening activity, consider doing that activity before this one.
Review with students the different shapes of galaxies. There are many examples of the tuning fork classification diagram on the web. You can also show them images of different galaxies from the SEDS database.
Lead a discussion that could touch on the following points about galaxy interactions:
1. The process is extremely slow--interactions take place over hundreds of millions of years.
2. Because of this, we cannot watch the event happen.
3. In order to try to understand galaxy shape, 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. Galaxies are still mostly empty space. They do not crash together when they collide, they just pass through each other, and feel each others gravity.
5. If we are to accomplish anything in science, it is extremely important that we are careful observers.
There are a number of sample initial conditions you can try to explore, or you can make your own in Galaxy Maker under the New Galaxy menu item. (right click to download)
Have the students write down what happens to the shape of the galaxies as they interact.
Have the students compare this to models with spheres, and models in which the galaxies are not rotating. Do they get the same effect with two non-rotating spherical galaxies?

Note: As the students try to create galaxies which contain the same amount of mass in a smaller physical space, they will have to use a smaller time step to reduce numerical error. Make sure the students are keeping track of the energy in the info window.
For more information about detecting and controlling error, see the section about the info window in the GalaxSee tutorial.

Discussion of the Simulation

Galaxies are extremely large objects. They are so large that in a close interaction, the near side of the galaxy will be pulled towards the other object more strongly than the far side, stretching the object. Ask the students what would happen if you pulled an object while it was spinning.

Can your students give any examples of objects that are pulled and spun at the same time (A taffy machine)? Can your students give any examples of where an object in space stretches something by pulling more on a close side than the far side (Ocean tides, the break-up of the comet Shoemaker-Levy 9).

Discussion of Observation

Ask the students to describe how galaxy interactions might produce the shapes they observe in images of galaxies. While spiral structure is thought to be due to the collision of two disk galaxies, how might irregular galaxies be formed?

Assign them to write a clear and accurate report of what they observed and modeled. 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 another 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

If disk galaxies are formed by rotation, and spirals by collisions of two disks, why is it that many of the oldest galaxies observed tend to be ellipticals? Have the students collide more than 2 galaxies. What is the result of mergers of multiple galaxies?
Thinking Harder

By now students should know the theory that rotation causes flattening, and that collisions of two spinning disk galaxies cause spiral formation. Ask the students how galaxies start spinning in the first place. Compare two possibilities. One, galaxies collide, causing rotation. Two, galaxies were formed as mass spread out over a very large area collapsed under its own gravity, and in the process, any small amount of rotation would increase as the object got smaller, like a ice skater pulling his/her arms in. How would the student try to set up observations and models to compare these possiblities?