Nikon D3100 Long Exposure

I'm trying to take pictures of star trails with my Nikon D3100. I have a shutter release and I set it to take 150 45s exposure photos. For some reason it won't take the 45s photos back to back, it seems as though the camera has to take some time to save the photo. I was wondering if there was any ways to get around that. When I get every other photo my star trails look like dots rather than lines. Help would be greatly appreciated, thanks.

Added (1). I can't do longer than a 45s exposure because otherwise my sensor will overheat and I get 'hot pixels'. I use an image stacking program to get the star trails.

How do you change the noise reduction?

See page 134 in your manual regarding the noise reduction setting. This needs to be off. You also need to watch your ISO setting. At some higher ISO setting, noise reduction will kick in again, even though you have noise reduction set to off. However, it's supposed to be a less aggressive noise reduction which may be fast to process. You'll need to read up on when this happens. Nikon likes to omit and obfuscate important details about the operation of their cameras, so you'll most likely depend on reading blogs about this issue.

You need to figure out how much time it takes to process the image. If it takes 3 seconds, then you need to set your intervelometer to 49 seconds (add an extra second to take care of round-off issues in the arithmetic). Hopefully, this 4 second gap will not be noticeable in your final image.

Do practice runs in your house to see what the cycle timing is. Better to figure this out ahead of time than wasting a couple hours shooting.

Are you using any noise reduction in the camera? This may be causing a delay, if the delay is 45sec.

I presume you're using an intervalometer for this.

If you want trails that are lines you're going to need a lot more than 45s for an exposure. Try 10 minutes or more.

Edit

where does the 45 sec come from? Nikon or experience? Certainly, if 45sec is enough to get the right exposure and you can join all the photos together then do that.

You are much less likely to get sensor overheating with a long exposure than you will running the sensor continuously like you do in video. There you're reading the sensor 24 times a second and there's a lot of electron activity. With the sensor just there receiving light there's no reading of the sensor by the camera. Any hot pixels you may see are usually because there's not enough light to trigger the sensor element properly and it's being effected by the low background current activity in the sensor. The sensor may get warm if the camera is warm, or the weather is warm. Long exposure in itself doesn't directly cause sensor heating.