The deeper the aperture, the more stars that will be visible. My standard procedure is 2.5 to 3 hours, f5.6 to f8, using Provia 100 in 35mm with a 24mm lens, unfiltered. Reciprocity failure will cast the photos you get, chiefly pinkish with Provia, green with Velvia, unearthly reddish hue with Velvia 100F and cool to cold blue with Tungsten (T64) film. Spoilt for choices, though Tungsten film is hard to get in 35mm. Star trails are experimental and a lot of fun: the fun comes out of not knowing exactly what you have until the film comes back! My first star trail (Yandoit, below) was surprisingly good despite having had a little too much wine with friends that night...
It was found during a trip into the outback here in Australia that the stars were absolutely beautiful to look at, but problems with the lens misting over in the very cold air and kangaroos or some other animal(s) bumping into the tripod caused some problems. I imagine The Arches National Park would be a stupendously beautiful place to set up star-trails, but do get some groundwork done in scouting around for important foreground subject e.g. one of the many arches, perhaps the star trails recorded with the arch "framing" the scene. Lots of possibilities obviously (my XP screensaver has 25 images of various US national Parks, Arches NP among them).
• Yandoit, central Victoria, February 2009: Southern Cross/South Celestial Pole
Provia 100, 2.5 hour exposure, Canon EOS1N f8, TS-E 24mm prime.
• Walker Creek, Cascade #7 (campsite in trees to left), Litchfield National Park, Northern Territory, Australia
While I was sound asleep...Unattended intervalometer image (2.30am), one of three. Velvia 100F
2.45 hour exposure under waning gibbous moon (hence the brightness); a few gusts of wind caught in trees.
Canon EOS1N with 17-40 f4L zoom (17mm), f6.7, unfiltered.
Camping here was adventurous: some quite big snakes active during the hot days (this pic shows my personal plunge-pool!) and nights,
the nights being a little troublesome from the bats, small animals and tree-climbing snakes.