One of my students (Antonio) sent me a link to this superb universe simulator, Universe sandbox. I've only just downloaded it but it looks great. The full version costs $9.95 but so far I've just got the free version and even that enables you to play quite a lot. It's always difficult to get an idea of the scale on things in the universe but sending a light signal from a selected object gives some idea, its a good job there's a fast forward button.