[…] the quickest way to determine the longest exposure that is possible for any given focal length lens, without the stars streaking, is to divide that focal length into 600. (This is the formula for 35mm. Larger formats are laxer, smaller formats more unforgiving). [#]
For example, with a 50mm lens on a full frame camera, you can only expose for 12 seconds (600/50=12) before the stars turn into star trails. It’s a good rule of thumb to keep in mind!