if i have 400 iso film and i want to take pictures of the stars at f2, how many seconds should my exposure be?

Shoot at least 4 exposures to ensure the best capture. 1 to 4 minutes.

This is a nice guide I use, http://www.monep.org/newsletters/Dan%20Zarlenga%20...

Bracket your exposures 1 stop apart. Bear in mind that any shot over about 30 secs will show the stars 'trailing' due to motion blur (the Earth moving in relation to the stars).

You will also need to take reciprocity failure into account with film - it means that the relationship between aperture/shutter sped breaks down at longer exposures due to the film emulsion taking longer to react. Reciprocity charts are available for each film type - you'll need to Google yours. It does mean that exposures will be longer that you expect, as there's a 'factor' to add in.