Normally you want to take a star photo at the lowest ISO possible with the largest aperture possible. Generally with a stopped down aperture (smaller opening) you are letting less light thus less stars even with a longer exposure. So generally as wide open as possible to get as many stars as possible. Unless you want a certain look.
But in regards to ISO you can do a test.
Leave the lens cap on
Take a 5 minute exposure (or about) at your lowest ISO
It theorectically should be perfectly black But it might show streaks , pixels, etc
Then try it at ISO 800
or even higher
This will give you an idea of what your max ISO should be for taking astrophotography stuff.
for instance I used ISO 25,600 @ f/5.6 @ 0.6 secs to get a noisy shot of Uranus using a regular NIkon 70-300 VRII cropped to the max. A longer exposure and the planet was elongated (movement), lower ISO and you can't see it. and f/5.6 becz that is what it is at 300mm.
versus the Orion Nebula which was pretty clean (except slightly out of focus) at ISO 6400, 10 seconds.
It also depends upon your environment you are in. How much pollution, clouds, lights from light poles, etc etc.
I've seen tutorials for shooting with pollution and you overexpose, and various other techniques/tips out there for various issues. I shoot mostly in my backyard and there's only a couple spots I can shoot otherwise light coming in from an angle interferes. Or across the street and the police come and look at what I'm doing.
Even though I love astronomy .. I'm kinda a newbie with the camera on it. just got back into it after failing a decade ago with film. So I'm still learning the digital stuff.