This node group is able to bring lens flare for your space scenes. This is a quiet complex one, but you could play with the last mix nodes to vary the effect. The "sun" detection is done by a luminance key node ; then the star glare and the flares follow separate paths before to be mixed with the original image.
You can see here a short animation with the node rig : http://www.youtube.com/watch?v=DiyAYj_gKEc