Hovik showed me this website where you can play around with a star's effect on gravity (the planets have gravity as well). You can choose from presets or mess around by moving the stars and planets. It's very fun and it's a great way to learn about possible orbits and gravity.
Here is the link: http://phet.colorado.edu/sims/my-solar-s...em_en.html
Here is the link: http://phet.colorado.edu/sims/my-solar-s...em_en.html