Work out the longest exposure you can use before the stars start to trail — for your exact lens and camera.
These are sharpness limits, not a full exposure recipe. Seeing conditions, focus accuracy and lens quality all affect the final result — treat this as a starting point you refine in the field.
Your lens
Use the focal length you're actually shooting at — if it's a zoom, the widest end is usually what you'd use.
Your camera
Megapixels matter more than you'd expect — a higher-resolution sensor records star movement that a lower-resolution one physically can't, so it needs a shorter exposure for the same lens.
Where you're pointing
Stars near the pole trace tiny circles and barely move; stars on the celestial equator move fastest. If you're not sure, leave this on the worst case. Northern or southern hemisphere makes no difference — stars turn at the same rate everywhere, only the part of the sky you can see changes.
Sky brightness (only affects the suggested ISO)