I haven't done one in ages, but if you use fairly short exposures (say 2 minutes) and a lot of them (at least 15) you can stack them with good results. You can search for a LR/Photoshop action that will do the grunt work for you.
Oh yeah, don't forget to turn long exposure noise reduction off or you'll have big gaps in your trails.
When I use the BULB, used a remote control, so mount the nex in the tripod and set up to BULB, and then the apertura and ISO, so I think you can't lock the shutter open on BULB, so when I'm ready to stop the time of exposure, I mean finish or cut, depending on how many minutes you want, I press the shutter button again without touching the camera from the remote control, (S shutter button ) this way do not shake the camera and do wrong.
An important detail that happened: before shooting, put the focus on manual. If not, the AF will not be able to focus to have the filters almost "opaque". if you use a filter.
Star trails ? Well I've done a few but not with the NEX-C3 I have now. The problem with the C3 is that it has no IR remote input and the BULB mode requires you to hold down the shutter release during the exposure ( which is rubbish, Sony what WERE you thinking ? ). For previous star trails, I used my Canon 500D running an intervalometer script. I took a 20 second exposure every 10 seconds for about an hour then merged all the shots in Starstax. I aligned the camera / tripod to the Pole star using a night sky app on the Iphone - not very scientific but it worked ok. Here's a result :-