How to hold open the shutter on a nikon D3100 from a mac?

I would like to do a star trail shot as demonstrated here (http://www.exposureguide.com/night-sky-photography-tips.htm) but the maximum shutter speed is 30 seconds. Is there any way to remotely hold opn the shutter for a longer period of time from my computer so i don't have to hold the shutter button myself?

To take star trail shots you are better off taking lots of 30 sec exposures and stacking them, rather than taking a single long exposure. Reasons?
- you would not need a remote shutter release (which is what you would need to hold the shutter open in 'bulb' mode)
- the sensor would not heat up so much so you get less noise in the image
- if necessary, you can swap out batteries quickly between exposures (if the battery fails during a single long exposure, then you've lost the whole shot).
This is a shot of mine using 129 exposures, each of 30 secs and stacked using this free software; http://www.startrails.de/html/software.h…

Wow, I didn't realize that this was an option!

Startrails is a great program that will stack you images within a couple of minutes. Otherwise, you can buy an intervalometer. You plug this into your camera and program it to take pictures at regular intervals, or for however long, provided your camera has bulb mode. Extremely long exposures can damage your sensor or leaves spots on your picture because the sensor begins to heat up when the shutter is left open for long periods. That's why its better to stack multiple images taken at 30sec. A remote or intervalometer is useful for that too. They are cheap, around $10 on ebay.