Overview

Quick Start

Before You Begin

In order to save you time getting started, we highly recommend that you take five minutes to familiarize yourself with the look and feel of GalaxSee. For this exercise, don't worry about understanding everything that you see and do. Trying out a few of the options now will make them easier to understand later. By following these ten steps, you will be better prepared for the explanations in this tutorial.

1. Follow the Software Simulation link and download the JAVA installation of the GalaxSee software. Double-click the GalaxSee icon on your desktop to start the program.

2. Choose Galaxy Setup... from the Galaxy menu. Set the parameters to something like the following:

    

3. To create a new galaxy, choose New Galaxy from the Galaxy menu, then choose Run Simulation from the Galaxy menu to start the bodies moving.
   

   You should see a window open with a bunch of dots in a distribution corresponding to that selected in the Galaxy Setup... dialog box, something like this:

   

4. Iterate the galaxy by selecting Run Simulation from the Galaxy menu. If rotation factor was set to 1 in the Galaxy Setup box when you created the galaxy, you will see motion immediately. If the rotation factor was zero, it may take a few steps before the motion of the stars is apparent. Select Step Simulation from the Galaxy menu to go through the simulation one iteration at a time or hold it to go through the simulation at a slower speed.

5. Stop the galaxy from running by selecting Pause Simulation from the Galaxy menu. Click on the galaxy and drag the mouse around. You should see the galaxy rotating in response to your movements.

6. Select Galaxy Setup from the Galaxy menu.

7. Change the distribution to 3d grid.

8. Change the number of stars to 64 and click the OK button.

9. Repeat steps 4 and 5.

10. Select Quit from the File menu.

Exploring Possibilities

Galaxy Setup

As you learned from the Quick Start exercise, choosing Galaxy Setup from the Galaxy menu will enable you to change many of the characteristics of the galaxy. These changes will have no effect until the next time New Galaxy is selected from the Galaxy menu. Let's briefly exaimine each of the options in the setup dialog box. You may find it helpful to stop and try out an option you are unsure about to see its effect.

Distribution: Specifies the original, overall shape of the galaxy.

You can choose, for example, whether you want the stars to start out in random positions within a sphere, at fixed grid points in a box, in a flat disk, or in various other configurations.

Stars: Specifies the number of stars in the galaxy.

Star Mass: Specifies the masses of the stars in the galaxy.

The units depicted here are solar masses (1.99e+30 kg, the mass of our sun), so entering 200 in this blank gives each star a mass 200 times that of our sun.

Rotation: Allows you to impart an initial "rotation factor" to the entire galaxy.

A factor of 1 imparts a rotational speed that balances gravity to create a fairly stable configuration. A factor of two imparts a rotational speed twice that, which causes the stars to be thrown outwards, while a factor of .5 allows the stars to be pulled in to the center.

Note: The rotation imparted to the galaxy is dependent on the Dark Matter setting.

What if I want more configuration options?

GalaxSee is intended to allow the user to vary the actual details of the model such as the inclusion of dark matter, size of the time step, etc. An important aspect of scientific modeling is choosing a mathematical algorithm that is fast enough to give results in a reasonable period of time, but accurate enough that the results are useful. This options available in the Model Settings dialog box are something like the dials and switches on traditional laboratory equipment. By varying the settings, we can choose exactly what kind of measurement we want to make.

Choosing Model Settings... from the Galaxy menu brings up this dialog box:

Time Step: This sets the "time step" for the current integration method. The numerical models used in GalaxSee model the universe as if it were evolving in discrete steps rather than a continuous flow. This is sort of like when you put a picture on a computer monitor. The monitor can't draw a round curve--it makes all of its pictures with square pixels. But if the pixels are small enough, they can make something that looks like a round curve. Similarly, with numerical models we attempt to see what the behavior of a continuously evolving system would look like, even though we are taking discrete steps forward in time.

Setting the time step is somewhat like deciding how precisely you want to measure some physical quantity--if you decide that you want to measure, say, the length of a brick to within a millimeter of accuracy, you have to work a lot harder than if you only want to measure it to within, say, a few centimeters of accuracy. With a little practice, you can probably learn to look at a brick (or any similarly sized object) and guess its length to within a few centimeters of accuracy. But getting the measurement accurate to the nearest millimeter is going to require a little more effort.

The situation with numerical models is similar. If you want a really accurate model, you (and/or the computer) must work harder to get that accuracy. For the computer, this often translates into taking more time.

The units depicted here are are megayears, that is, millions of earth years. (For other scale settings, the units may be in days or hours.) A smaller time step gives a more accurate result, but makes the program run more slowly.

Shield Radius: Allows you to change the size of the shield radius.

(In order to more accurately model the motion of real astronomical objects, it is sometimes helpful to temporarily "shield" them from each other gravitaionally--that is, they continue to move past each other, but do not affect each other's motion until they are beyond the shielding distance from each other. This is a crude (but easy to implement!) method, and will be changed in a later version.) This is sometimes called "softening" the force.

This option allows you to specify the radius (in the specified units) of this shielding effect.

Dark Matter: This allows you to include the mysterious "dark matter" in your galaxy. Scientists have good evidence to suggest that in addition to all of the visible dust and stars there is a great deal of other matter in the universe that we cannot see with our telescopes. To add dark matter, simply enter a percentage into this box--for example, putting a 25 in the box would mean that the total mass of all the stars would be multiplied by 25%, and that amount of matter would be added to the model. In this model, the dark matter is added as a single mass at the center of the galaxy (that is, the point (0,0,0)). (For example, if you have 100 stars of 1 solar mass each, a setting of 25% will add 25 solar masses' worth of dark matter to the model).

Note: It is important to realize that if the dark matter is set, for example, to 100%, and a galaxy is generated with rotation factor 1, the rotation imparted to the galaxy will take the gravitational effect of the dark matter into account. If, after the galaxy has been generated, the dark matter factor is changed to zero, the stars are likely to go sailing off, since they were initially given speeds that assumed there was a great deal of mass at the center of the galaxy which was acting on these stars gravitationally. Thus, to get the expected behavior from a particular rotation factor, make sure that the dark matter setting is adjusted before the galaxy is generated.

Integration Method: Just as there are many tools for measuring lengths, such as rulers, vernier or micrometer calipers, laser devices, etc, there are many different ways to model a force such as gravitation. Some methods are more accurate than others, and some are better suited to particular situations than others. This popup menu allows you to choose which method the computer will use. There are currently three methods available: Euler, Improved Euler, and Runge-Kutta 4.

Changing Your Perspective

More information about the View and Action menus can be found in Menu Item Descriptions.

Using the Star List

There are two main functions of GalaxSee's star list -- viewing the information on mass, position, velocity, and color for the stars in the galaxy, and changing this information.

Viewing the Star Properties

To view the properties of the stars in a galaxy, simply select Show List from the Galaxy menu. You should see a window something like this:

The star list provides one way to directly set the properties of individual stars in a galaxy. These modifications can be made "on the fly", that is, while the program is running. It is also possible to use a text editor or spreadsheet program to edit the properties of one or more stars in a galaxy. See Creating a Text Galaxy for details.

To use this feature, simply follow these steps (assuming there is a galaxy currently being displayed-- if not, then refer to Quick Start, above):

1. If the galaxy is running, stop it by selecting Stop from the Galaxy menu.

2. If the Star List window is not showing, select Show List from the Galaxy menu.

3. If the Star List window is not in front, bring it to the front by clicking the mouse anywhere in the window.

4. Double-click in the box corresponding to the value you wish to set the parameters for. This should bring up a text box where you can enter the value you would like. Hit the enter key followed by OK. The box looks somewhat like this:
   

   The units for these values (when the scale is set to Galactic), as indicated in the dialog box, are as follows:

   • mass: solar masses

   • x,y, and z positions: kilo-light-year, or, kly. One kilo-light-year is one thousand times the distance that light can travel in a year (9.46e+18 meters).

   • x, y, and z velocities: billionths of a kly/year. These values are displayed and entered as billionths of a kly/year for ease of input and readability. They are used internally and saved in files in units of kly/year--see "Creating a Text Galaxy" for more on this.

5. Click the OK button to confirm the change. If you made a mistake or changed your mind about one of the changes, simply click Cancel to leave things the way they were. Clicking either of these buttions will close the dialog box, and the galaxy will be redrawn, reflecting any changes.

6. If desired, start the galaxy running again by selecting Run Simulation from the Galaxy menu.

In order to address deeper questions such as the accuracy of the model, there are options to do "bookkeeping" (checking the conservation of energy, for example) as well as changing the time step on the fly in order to adapt the algorithm to the current situation. (This has the added value of giving the teacher the option of showing what can go wrong when a computational model is implemented incorrectly. For example, by changing the time step to a large value and letting the simulation run, you can see radical changes in the energy. This violates the law of conservation of energy, indicating inaccuracy in the model. See Time Step, below, for more on this). Selecting Show Info from the Galaxy menu will open the box so that you can see the energy, and the results will be continuously updated on screen.

About the information window

Galaxy Type

Shows what the initial shape of the current galaxy was when it was created. When a user opens a saved galaxy, this always shows "Sphere".

Stars

The number of stars in the current galaxy.

Box Edge

The length of one edge of the view box (see Show Box under the View menu).

Time Step

The current time step, explained above.

Integration

Shows what method of integration is being used. Explained above.

Shield Radius

The current shield radius, in light-years, explained above.

Elapsed Time

How long the current galaxy has been running.

Energy

The total mechanical energy, that is, the kinetic energy plus the potential energy. This value is included to help the user monitor the accuracy of the model. Since this system should conserve energy perfectly, this value tells us something about how much error is being introduced as the simulation proceeds. If the model were perfect, this number wouldn't change at all. Our numerical model has a finite amount of accuracy, partly because we have made some approximations in the model, and partly because the model is being solved on a digital computer. If the energy starts to change by more than, say, ten percent, this indicates that the errors are accumulating, and you may wish to decrease the time step in order to increase the accuracy. Note that this can be done with the command key ([) indicated in the menu. The units of energy (in galactic scale) are 10e-15 solar masses * (kilo-light-years) / year^2

Momentum and Center of Mass Measurements

These values are also included as a check on the model. Momentum should be conserved, and the center of mass should not move (unless the there was non-zero momentum in the initial conditions--for example, if the entire galaxy was moving in the x-direction in the beginning). Some amount of error is expected, as is the case with most numerical models.

The center of mass is given in kilo-light-years and the momentum is in solar masses * (kilo-light-years) / year (or in units appropriate for the current scale).