What exact settings do you have on your DSLR when shooting long exposure shots of stars?

So, I just recently got a Nikon D7100 and I'm trying to learn how to get better shots of the stars. Last night it was a clear night with a full moon and I went out in an open field to take pictures. The first couple of shots I set my camera to 1600 ISO with an aperture of 3.5. I set the shutter speed the highest it could (30 seconds), but the picture didn't turn out all that great. It was pretty black but you could see the stars and a tree in the foreground a little bit.
BUT then, I decided to try out the Bulb mode. I had the same settings except I only pressed it down for 10 seconds and the picture was significantly brighter than when I had it set to 30 seconds. It was so bright it almost looked unreal. What happened there? Is there a difference between bulb mode and doing it the normal way? Or did I accidentally change something in my camera menu between them? I'm just really confused and very new at this haha. Sorry this was so long. Would love if you had an answer!

You may be fighting light pollution from the lights coming from your city or town

If you can move out away from the city (at least five miles from the last of street lights), you should be able to shoot the night sky using an ISO of 400, a thirty second exposure and at f/8.

I'm sure that you know you will have to manually focus the lens as well as manually set the exposure.

I have linked you to an image of stacked night sky photos. Those were shot at 30 seconds, f/8 with an ISO of 800.

Make sure that your noise reduction is turned on in the camera.