Capturing star trails is easy and, for me, very exciting. The best, and safest way for you to get these images is to stack many 30 second exposures into a single frame, rather than using the bulb mode of your camera.
Shooting in blub mode can cause sensor damage, especially in the duration required to capture these photos. By taking many exposures, you are allowing the sensor a little "break" between the shots, and keeping things from getting too abused...
Things you will need...
1. Digital Camera (SLR)
Must be capable of taking timed exposures. My camera is a Nikon D3000, with the 18-55 zoom lens that came with it.
Needed to hold the camera still. You'll want one with a fully articulating head, so that you can point it at the stars. You'll also want good functioning legs, so that you can level the shot(s).
Point your browser to the following link:
It's 100% free and totally awesome, because that's how German photographer/programmers roll.
4. Rubber Band
5. A Post-It Note
Step 1: Make a Button Holder
You'll set your camera release mode for continuous shooting, so holding the button down will keep taking pictures. So, we will need a way to hold the button down for quite a while...
Enter the rubber band and the post-it note...
Fold your post it in thirds like so.
Then half again, like so.
Then half again, but this time, like this kind of so.
Now, get your roll on, and roll that little guy up.
And check for fit.
Now, rubber band that little fake finger in place.
Check and test your finger substitute with a faster shutter speed, so you don't have to wait 30 seconds before you know whether or not it's actually working. You should hear it clicking off photo after satisfying photo.
Step 2: Gather the Photos
Get your gear, and find a sweet spot for some shots.
You'll want to be away from pretty much all sources of light, including but not limited to street lights, highways, businesses, full moon, etc. You will have your camera aperature wide open, and be shooting at an elevated ISO, so limit the excess light. If the moon is indeed full, you can still do this, but try to set up in the "shade" of a tree that will block the moonlight. I also recommend a hill top, if you can find one. It's just better.
Set your camera and tripod up, and select a section of sky to shoot. The startrail photos you commonly see (like the title image) have large concentric arcs. Those were shot using Polaris (North Star) as the center of the photo, which won't show a trail.
You will need to make the following adjustments to you camera settings:
ISO: 400-800 (depends on how well your camera shoots at higher ISO's. I use 400)
Aperature: Full open
Exposure: 30 Seconds
Shutter Release: Continuous
EDIT: Went out tonight, and discovered another setting you might need to adjust. Turn off Noise Reduction if your camera has it. This feature processes each image before writing it to the card. This slows down the camera. Instead of shooting and shooting and shooting, it shoots, processes, shoots, processes, etc.,which is very frustrating. It took me half an hour in the dark before I got this sorted out. I forgot I enabled it. =/
And turn off the photo preview. It'll just drain the batteries faster.
Once you've got everything set and ready, get your finger imposter out, and rubber band him in place. You'll have to throw away your first few photos, as they'll probably be all shaky and whatnot.
So, you'll be taking 30 second exposures, in a sequence. Lets do some math.
30 seconds/exposure X 60 Mins per hour =120 Photos in one hour.
Make sure your card can hold around an hour or two worth of photos. I think I've got an 8 GB card, which will hold about 1,700 photos for me.
I recommend at least an hour of shooting to give a good trail.
EDIT: Also ran into another issue which has never appeared before: Dew. Tonight was very humid. Once the camera cooled down, dew began to settle on it, blurring about half my shots. If anyone has any ideas on how to work around this, let me know.
Step 3: Get Your Shots Ready
Pull all the shots off your camera, and get them ready for processing. Get that free awesome program up and ready, and load the photos. You'll also want to shoot a blackframe to stack in there with them. You get one by taking a picture with the lens cap on.
Step 4: Save Your Photo
You just got a sweet photo mosaic that captures the motion of the earth under the stars!
If you have any questions, just ask in the comments.