This simulation illustrates the formation of the partial spiral structures in a galaxy that result from the influence of a massive external body. The galaxy's core is represented by a single large object, the rest of the galaxy is represented by a disc of 1,000 stars, and a second galaxy is represented by a single massive object. It was created by making a new simulation with the single object edited to have a radius of 5,000 light years, a luminosity of 1.0e11 Solar luminosities, and a mass of 1.0e11 Solar masses. The Edit / Add Family... command was then used with the '2D Protoplanetary Disk' quick setup, edited to a family size of 1,000, a radius of 0.001 light year, and a semi-major axis from 5,000 light years to 50,000 light years. The second galaxy was added with the Edit / Add Object... command, edited to a luminosity of 1.0e10 Solar luminosities, a mass of 1.0e10 Solar masses, a semi-major axis of 120,000 light years, an eccentricity of 0.25, a true longitude of 180 degrees, and other orbital elements equal to 0.
If you set the simulation running, you can watch how it evolves, and see how the gravitational field of the second galaxy distorts the star disk, and spiral structures form and dissolve. The speed of the simulation is quite fast (because the stars have all been given zero mass) and it should take no more than a few minutes to see what happens. Since different computers run at different speeds, you may need to edit the evolution time step to get the simulation to run at an acceptable rate.
You can also experiment with editing the second galaxy in various ways - for example, by changing its mass, semi-major axis, eccentricity, and inclination. Changing the eccentricity to 0.5 so that the second galaxy actually passes through the star disk results in quite impressive spiral arms, although many stars are also ejected with hyperbolic orbits in the process. You can even try making the second galaxy more massive than the main galaxy - for example, if you change its mass to 1.0e12 Solar masses, its semi-major axis to 300,000 light years, and its other orbital elements to 0, you will find that impressive spiral arms form, followed by the second galaxy starting to cannibalize the stars of the main galaxy. Adding a third galaxy is another interesting area for experimentation.
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