The Rule of 500

Divide 500 by (equivalent) focal length to calculate max exposure time in seconds - without generating star trails.

Max exposure time = 500 / (Focal Length x Crop Factor) in seconds

For 50mm full-frame sensor camera, max exposure time = 500 / (50) =  10 seconds
For 50mm APS-C sensor camera, max exposure time = 500 / (50x1.6) =  6.25 seconds