ISO
go to http://www.dxomark.com/index.php/Cameras/Camera-Sensor-Ratings/%28type%29/usecase_sports
Find your camera and see what iso settings you can push your camera to and still retain descent image quality, this is especially important when shooting auroras, a high iso sensitivity means you can push the shutterspeed under the magical 8 seconds to retain some of the movement of the aurora in the image.
(I personally recommend 6 or less)
Preventing STAR TRAILS
Use the rule of 600 to keep stars in the picture from trailing/streaking.
Divide 600 by the true focal lenght of your lens, true is measured by 35mm full frame, so a 30 mm full