Why "stack" frames?

This is a single 30 second exposure from the set of 45 that went into the pictures in the last post, brightened to the point that the core of the galaxy is as bright as it is in the final stack. Notice that there is nearly zero difference between the brightness of the galaxy itself, and the sky.

In these conditions, the long exposures used for faint objects just won't help; the sky keeps getting exposed just as much as the target. Stacking, or adding a lot of short exposures, is not quite the same as having a long exposure. Instead, it keeps track of which pixels are the same in each shot, and which are different-the ones that are different are considered "noise" and ignored (or replaced with something-usually an average of all the pictures' color at that particular pixel.) So the final picture may be just as faint as a single short exposure, but when you brighten it a lot, like with this photo, the background noise has been reduced enough so that the sky still looks black.

In a nutshell, longer exposures capture more light from the target, which makes the target brighter. (That's the traditional way when film was used.) Stacking improves the ratio of picture to noise, so you are able to "amplify" the picture, so to speak, without drowning in the noise. The best photos use both; with shorter exposures, it's possible that some actual photons from the target are being tossed out as noise. So I still want to get to where I am shooting a minimum of 60 seconds per sub, and as much as 180. But there are times, like last night, that the ambient light prevents that, and that is when it is nice to rely on stacking alone to help draw those faint things out of the slop.