This is a topic that I don't intend to spend too much time on as there are a thousand excellent tutorials on YouTube, covering these topics in great detail. However, there are a few fundamental factors to consider when planning a star trail shot:
- Is it only a star trail image wanted from the sequence of images about to be captured?
- Is it possible a time lapse movie will be wanted to show the (apparent) movement of the stars?
- Will certain still images be required from the sequence that have the stars rendered as points of light, and not "dashes"?
These are important questions to answer before the capture sequence is initiated. If it is only a star trail image, and nothing else that is desired, then the process is relatively straight-forward. In this case, there is no issue with the exposure rendering the stars as "dashes" as they will be joined together anyway in post processing. If a time-lapse movie is the sole objective of the exercise, again, some movement in the stars MAY be acceptable, depending on taste, and how movement is to be portrayed in the time-lapse itself. If still images are likely to be required from the sequence, then the exposure details are critically important, as each image taken must be able to stand up to scrutiny as a picture in its own right.
My personal recommendation is to start your time-lapse / star trail journey by exposing for individual images, and then use the software available to create time lapses and/or star trail images if and when desired. This is the approach which gives the greatest flexibility.