AstroGrav Help /Tutorial /Exercise 7 - Adding New Families of ObjectsVersion 5.1

Exercise 7 - Adding New Families of Objects

This exercise teaches you how to add new families of objects, and should take about 30 minutes to complete.

Creating a New Simulation

In previous exercises, you have been using the Tutorial simulation, but in this exercise, you are going to create a simulation from scratch. Go through the following steps to create a new simulation to work on.

If you don't already have AstroGrav running, double-click on the AstroGrav application icon () to start it running.
Cancel the Open Simulation file chooser dialog, because you are going to create a new simulation rather than open an existing one.
If AstroGrav doesn't automatically created a new simulation with a table window titled Untitled: Table, create it manually by choosing the File / New menu item (or the tool).
If you have any AstroGrav windows open other than Untitled: Table, close them to get them out of the way.

You will now have AstroGrav running with just a single window titled Untitled: Table, and with just a single object named 'Star' present in the table.

Adding a New Family with Quick Setup

Go through the following steps to learn how to easily create an entire family of objects using the Quick Setup option.

Choose the Edit / Add Family... menu item, which will result in the New Family dialog being displayed. At the top-left corner of this dialog are fields for entering the parent and the number of objects that you want in the new family, and below this are two large boxes labelled Physical Elements and Orbital Elements. Each of the data elements occupies a single row, and has two fields for specifying the minimum and maximum values, together with a bias slider for specifying the distribution of values, and a checkbox for specifying whether the values are to be distributed evenly or randomly. Figuring out how to fill in this dialog in order to achieve a desired effect is far from simple, so a selection of Quick Setup options are provided in the top-center of this dialog to make things easier.
In the Quick Setup popup menu, select '2D Protoplanetary Disk', and notice how the fields, sliders, and checkboxes are changed automatically to reflect your choice.
Make the radii of the new objects ten times as big by adding one to the exponents of the two 'Radius' fields. This is necessary to ensure that collisions are reasonably common when the simulation is evolved.
Accept the dialog, and notice how the 400 new objects are added to the table.
Choose the Window / New View menu item (or the tool), and notice how a new Untitled: View window appears.
Experiment with the 3D model, particularly by rotating it, and by moving your viewpoint towards and away from the system. If you show the orbits, you will probably notice that the 3D model is less responsive than before, because of the time it takes AstroGrav to display the orbits.

Evolving the New Simulation

Go through the following steps to see what happens when you start the new simulation running.

Choose the Evolve / Settings... menu item (or the tool), which will result in the Evolve Settings input dialog being displayed.
In the 'Collisions' popup menu, select 'Combine', and then accept the dialog. This is so that when the simulation is run forward, colliding protoplanets will be combined to form larger protoplanets, and eventually whole planets.
Now, choose the Evolve / Run Forward menu item (or the tool), which will start the simulation running forward in time.
Watch how the simulation evolves while experimenting with the 3D model, particularly by rotating it, and by moving your viewpoint towards and away from the system. You should find that collisions between the objects in your new family slowly result in the formation of a smaller number of larger objects.
Now, choose the Evolve / Settings... menu item (or the tool), which will result in the Evolve Settings input dialog being displayed.
Make the time step ten times as big, accept the dialog, and notice how the simulation now runs much more quickly.
Experiment with other settings for the time step. The optimum value will depend on both your computer and your personal preferences, as well as on the number of objects that are left in the simulation. You will notice that the simulation runs faster and faster as the number of objects gets smaller and smaller, because AstroGrav has fewer and fewer calculations to perform for each time step. You can also speed up the simulation by closing the Untitled: Table window, so that AstroGrav only has the one window to keep updated.
Finally, when you have tired of watching the simulation, stop it running by using the Evolve / Stop menu item (or the tool).

Adding a New Family Manually

The protoplanetary disk that you have just experimented with was created with objects whose masses were less that the mass threshold in the evolver settings, which means that when evolving, the mutual gravitational attraction of the protoplanets was not taken into account. Go through the following steps to learn how to create a more realistic family of objects by filling in the New Family for 'Star' dialog manually.

Choose the File / New menu item (or the tool), which will result in a new simulation being created with a table window showing a single object named 'Star'. If the single row is not selected, click on any one of its cells to select it.
Choose the Edit / Add Family... menu item, which will result in the New Family dialog being displayed.
In the Quick Setup popup menu, select '2D Protoplanetary Disk', and notice how the fields, sliders, and checkboxes are changed automatically to reflect your choice.
Click on the 'Copy' button between the two 'Color' fields, and notice how the right-hand field changes to the same color as the left-hand field. Having the two fields the same ensures that all objects in the new family will have the same color.
Edit the left-hand 'Mass' field to 8.0e24.
Click on the 'Copy' button between the two 'Mass' fields, and notice how the right-hand field changes to the same value as the left-hand field. Having the two fields the same ensures that all objects in the new family will have the same mass.
Edit the left-hand 'Radius' field to 1.0e8.
Click on the 'Copy' button between the two 'Radius' fields, and notice how the right-hand field changes to the same value as the left-hand field. Having the two fields the same ensures that all objects in the new family will have the same radius.
Accept the dialog, and notice how the 400 new objects are added to the table, all with the same color, and all with a mass of 8.0e24kg.
Choose the Window / New View menu item (or the tool), and notice how a new view window appears.
Experiment with the 3D model, particularly by rotating it, and by moving your viewpoint towards and away from the system.
Now, choose the Evolve / Settings... menu item (or the tool), which will result in the Evolve Settings input dialog being displayed. As with the previous simulation, select 'Combine' in the 'Collisions' popup menu, and then accept the dialog.
Finally, start the simulation running and experiment with it in the same way that you experimented with the previous simulation. You can repeatedly increase the time step as the number of objects decreases, in order to speed up the evolution. If you run the simulation forward far enough, you should find that it evolves to have just a few objects (planets) orbiting the central 'Star', rather like the Solar System.

Whenever you choose the Edit / Add Family... menu item, the dialog will show the same settings that you last accepted. This makes it easy to experiment with creating new families by repeating the following sequence until you achieve the desired affect.

Choose the Edit / Add Family... menu item.
Change one or more of the settings shown in the dialog.
Accept the changes and study the resulting family.
Choose the Edit / Undo menu item, ready to try again.

Next: What To Do Next

You've now completed Exercise 7, and are ready to move on to Exercise 8, which teaches you what to do next.

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