The Live Wallpaper version of the Standalone N-Body Simulator.
Have a real gravitational particle simulation in the background of your phone where particles interact with each other by means of gravity.

Features:
-Real n-body simulation, pure gravitational interactions between particles.
-Allow or not phisically realistic collisions and mergers between particles.
-Set particle and background colors.
-Set gravity strength and particle mass.
-Accelerometer support.
-Shoot particles or create repulsive forces using touchscreen.
-Enable the action to expand beyond your screen limits without losing particles.
-Enable or disable the central black hole which exerts a neat attractive force towards the center.
-Enable or disable particle trails (disable to improve performance).

Take into account we're talking about a real n-body simulation. I recommend keeping the number of particles reasonably low for the live wallpaper version (about 10-20), even though it only executes when the screen is on. Does not drain your battery keeping loops or listeners when sleeping.

Give it a try!