This is the lite version of Galaxy Collider. The full version adds more galaxies and stars, features such as stereo rendering and accelerometer interaction.

Brace yourself for the collision of the Milky Way with the Andromeda Galaxy. Galaxy Collider simulates interacting galaxies (tap image below to read a Wikipedia article) by computing the gravitational forces between the stars and the heavy galaxy centers. The initial galaxy positions and velocities are computed randomly by Galaxy Collider. All other parameters for the simulation can be modified in the Settings tab.

Controls:
Double tap restarts the simulation. Tilting the iPad or one finger dragging moves the camera around the galaxies. A two finger pinch zooms the camera and a three finger drag translates the camera. A four finger tap pauses and unpauses the simulation.
After enabling "Shake Mode" in the Settings you can mix up the galaxies by shaking your iPad.

Settings:
For quick access to the settings, tap the upper left corner of the render screen.
You can change the number of stars per galaxy that are rendered. Note that this does not change the galaxy mass in the simulation, since the galaxy mass is defined by the mass of the galaxy center. Note also, that a higher number of stars and galaxies decreases the frame rate.
The render primitives for rendering the stars can be chosen between PointSprites (best quality), Points (medium quality) and Lines (low quality).
The simulation can be slowed down by selecting a the simulation speed.
You can change the mass of each galaxy center by changing the "Mass of Galaxy" sliders.