It is a rule in a photography that helps you to calculate an ideal exposure to capture stars at night and avoid star trails. It can be calculated with the following formula:
Exposure time = 500/(crop-factor * focal length)
This method, however has one drawback and that is the actual exposure time. Most of the time, that calculation will provide you with an exposure time, that is unnecessarily too long and will result with star trails. For that reason, it is better to use NPF Rule.