There is more than one way to approach really long exposures. Here's a 15 minute exposure taken near Republic Michigan. Sometimes is desireable to capture how the earth moves under the night sky. Really long exposures shows stars as curved streaks of light in the sky. The milky way becomes a soft erratic veil of light among the stars.
Understanding and using long exposures its important to remember about doubles and halves. Cut the sensitivity in half you must double the exposure time. Longer exposure allows for lower ISO. In another lesson series I described the relationship between ISO and shutter speed.Long exposures can use more desireable ISO settings. The lower the ISO the better as there is less grain. By grain I mean the photo begins to look as though its printed on gritty sandpaper.
The example photo above is 902.0 seconds (15 minutes) ISO 800, F4.5, 18mm focal length. 18mm widest angle on my entry level equipment. Thus far I've recommended ISO 1600. But when the goal is to deliberately cause the stars to blur I drop to ISO 800. Doing so allows for a longer exposure time.
Expose too long at too high of an ISO and the trees in the foreground will look like daytime. Moonlight will light up everything. I've read but never tested it: a full moon is about 1/6 as bright as full daylight. So in theory a full moon's light is exposed long enough the photo will look similar to daylight. Imagine moonlight shining on nature the outdoors you can take a photo using moonlight, results will look very similar to daylight. I very rarely edit the color or exposure in my nature photography. The above picture has subtle details worth examination.
A fine detail, look closesly at these trees. They are not exactly black, the pine in the center has a dark green cast. The maple on the left has a slightly yellow cast. It was fall when the photo was taken with colors in abundance. Nighttime obscures colors, this shot was sometime around 2am. Location was rural remote upper Michigan. To the naked eye I could barely make out the details in the trees, the trees had no visible colors. I recall there was some moonlight but not much. So the color cast in the trees is literally the starlight and moonlight refected off the tree's colors.
Pay close attention to natural colors. Even within night photography there is an amazing wide pallet of dark tones. Use the pallet to your advantage. Close examination and not retouching your photos will build dicipline. Dicipline infuses creativity and versitlity into your photography. All too often I find people immediately going to software for color correction. No need when the colors are already correct in nature. Practice and you will learn how to capture colors using your camera. Please consider reading an older essay, I have strong opinons about the ethics of editing color in nature photos.
I can't just leave you here with just theory. Not fair without also showing what I've done wrong and what I've learned from my mistakes. I imagine three or four lessons about troubleshooting are in the near future.