Elsewhere in the forum is a link to an Introduction to Astrophotography Workshop including a useful table on exposure times to avoid star trails.
Click here: http://flamsteed.info/forums/topic/astrophotography-exposure-times/#post-7063
I wish to add this useful exposure time calculator called Sceneplanner which calculates the maximum exposure time when given the make of your camera and lens and declination of imaged object.
http://www.sceneplanner.com/tool3.php