Even with the more modern cameras and advanced sensors, noise is still a big issue when it comes to dark long exposures.
Most cameras have Dark-frame subtraction – a mechanism which should handle noisy photographs. The way DFS works is by capturing a second photograph right after the exposure with similar conditions (ISO, shutter speed) so it can detect noisy pixels and adjust them.
But, as Jim Goldstein says, sometimes the camera cannot do that extra exposure. Either because it is a time lapse / star trail shoot where you don’t want to create gaps, or because the exposure is too long and the battery cannot handle another exposure.
“…taking advantage of Dark-Frame Subtraction (DFS) isn’t always possible. If you’re shooting a sequence of images either for a night time-lapse or startrail photo DFS will introduce unwanted gaps or breaks. If you’re shooting extremely long exposures then your camera battery might not last long enough to complete your exposure, the dark-frame exposure and record the exposure to your memory card“
Jim shares a technique where he uses a noise reduction technique in post. By overlaying two layers: the first is the original and the second is an almost identical layer but with Dust & Scratches filter applied. He then sets the blending mode to darken. The results are pretty impressive.
You can read the full process over at JMG-Galleries.