What interval times for Star time lapse?

I'm going out Star shooting tonight, with my Nikon D5100 and i'm wondering what i should set the intervals to (software driven through the camera)? Because i'm not sure how fast stars will move so i have no clue what the intervals should be. I want to make a time lapse Video, with about 500 or so photos. So if you could tell me the interval that would be the best?

The earth rotates 360 degrees in 24 hours. It takes 4 minutes to rotate one degree. The angular width of the sun or the full moon as seen from earth is about half a degree. So stars will appear to have moved a full-moon-width at the celestial equator in two minutes. However, stars closer to the north celestial pole will appear to be moving slower, because the circle in which they are rotating is smaller.

Just to add to Hersh's answer… Apparent movement of stars is also dependent on lens focal length… Using a shorter focal length the stars will appear to stay still for longer, and in fact at 14mm you can take an exposure of 1 minute without any discernible star trailing… On the other hand at 300mm a star will apparently shift after just 2-3 seconds… So, for time lapse you'd be better using a medium (normal) focal length somewhere around 50 mm…

The equation for time of exposure of the night sky without trailing is 1000/fl x cos D = T
fl = Focal Length of lens
D = Declination of object you are looking at (Sirius is approximately -16° [ignore the negative sign])
T = Time in seconds

For Sirius 1000/50 x cos16 = 19.225 seconds (maximum exposure time… Before motion blur)

This should allow enough time for the stars to register on your sensor without trailing but experiment with a single exposure, you may be able to get away with less exposure depending on ISO settings which should be quite low. Then l would think about the gap you want between the images… 10-20 seconds would probably suffice… But you could experiment on the night…

Nice idea!