Just to answer the initial question.
No - several short exposures does NOT add up to one single longer exposure.
Reason - longer exposures record fainter stars!
By stacking several images noise is reduced - the sky background becomes less noisy - and slightly fainter stars become visible as the noise floor is reduced. But the stack will still ONLY show the stars recorded by the shorter exposure. So not even thousands of very short exposures add up to one single long exposure.
Simply take images with the same setup and stack 30 one second exposures and compare that to 1 single 30 second exposure - the longer exposure will show fainter star regardless of the amount of short exposures stacked - just try and see...
And even 300 or 3000 one second exposures will be inferior to one single 30 second exposure - just try and see...
I wholeheartedly agree with this. It's a sad fact that long exposure astrophotography is complex and often very expensive. Without some sort of tracker, your results will be limited by the fact we are sitting on a moving platform (the Earth). You deal with that movement by:
- Limiting the effects of movement by short exposures, high ISO or short focal lengths
- Counteracting the movement with a tracker
It is interesting to consider the viewpoint of an observer stationary in space looking at an Earth-bound telescope or camera. If the imaging setup is not on a tracker, the pointing direction is constantly changing. On a tracker the telescope or camera is stationary in space while the Earth turns underneath it.
While some may point to results coming from heroic efforts without tracking, it's a losing endeavor due to the reasons stated above by Trollman. It's best to recognize the limits imposed by the lack of a tracker and do what you can within those limits. As your subject gets smaller and fainter, increased focal length (image scale) and exposure (i.e. tracking, high ISO) becomes more necessary.
With short focal lengths, the jump to tracking is less expensive. For wide field tracked images (i.e. Milky Way) a
home-made barn-door tracker or relatively inexpensive commercial tracker can get you well equipped without tremendous expense. For zoomed in shots of a bright object (i.e. Sun or Moon), medium focal lengths with fast optics can have success without tracking, as some of Jack's recent images have shown. But for most galaxies (other than the Andromeda Galaxy) and nebulae some sort of tracker will be required.
As your subject gets faint and small (the vast majority of deep-sky objects), a telescope on an equatorial mount involves a big jump in expense and complexity. Even with expensive equatorial mounts, some sort of guiding (manual or auto-guiding) will be required for long exposures.
These are just the facts faced by all beginning astrophotographers. There's no free ride. Yet even getting a camera well-suited for wide field astrophotography is not inexpensive.
