Quick Start
|
Exploring Possibilities
|
Model Details
|
Changing
Your Perspective
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Advanced Features | |
Using the Star List
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More Depth
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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.
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:
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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.
Central Mass: allows you to set a distinct mass at the center of the galaxy with a different mass than the rest of the stars.
Dark Matter: Gives you options for setting a non interacting dark matter component to your galaxy
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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.Changes to the model settings take effect immediately upon confirmation (clicking the OK button). It is not necessary to generate a new galaxy for these changes to take effect.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 gravitationally--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.
The Windows version of GalaxSee sets the shield radius automatically based on the time step and the masses whose interaction is being calculated. You can set the coefficient of the equation which determines the shield radius in each interaction.
Force Calculation Method: The problem with N-body problems is that the straight forward solution requires considering the interaction of each body with every other body. Advanced force calculation methods use approximations to limit the number of interactions which need to be calculated.
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.
One can also simply "grab" the galaxy by clicking on it and holding the mouse button, and "spin" it by dragging the mouse around. If the mouse is still moving when you let up on the button, the galaxy will continue to spin in the direction you were moving it. If you hold the mouse still and then let up on the button, the galaxy will remain in the selected position. This functionality is easily mastered with a little practice.
More information about the
View
and Action menus can be found in Menu
Item Descriptions.
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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:
Editing the Star Properties
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):
The units for these values (when the scale is set to Galactic), as indicated in the dialog box, are as follows:
- 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.
- 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.
- 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.
Go to Help Index.
The
Shodor Education Foundation, Inc.
© Copyright 1996