I found a cool way to very easily demonstrate the different colours of stars - no fancy equipment required. Just my D70 and tripod.
Take long exposures (20sec or so) with a defocused lens. Then the star image is spread out and the colour really clearly demonstrated.
For example here is alpha/beta-centauri and crux.
This was done on purpose after finding out how this would happen accidentally last year - took a bunch of constellation photos only to find the focus was miles (light years in fact) off.......
