Image stacking

Hi all,

I'm *extremely* new to astrophotography, and appologies for what may be
an off-topic question.

For my studies I am required to take a series of photographs of flames
and get an average. A bit of hunting around on the net lead me to
discover astrophotography. Very different subject matter, but similar
process. The photos I saw blew me away, so I started taking some shots.

I tried to use "Registax" to stack my images (both of flames and the
sky), but am having troubles with the "align" process. If I'm taking
shots of flames, or the sky at night (eg just the clouds) without a
telescope then there aren't any particular features I want to align to.
I want to be able to stack the images, but without aligning them to a
particular feature. Sure, if I had a really high powered scope, or I
wanted to track something moving then I understand why it would be
necessary, but otherwise, could someone please explain to me why I have
to align the images, and why can't I just overlap the images directly on
top of one-another?

Is there other software out there that would do what I want? I've tried
downloading Astrostack, but kept getting Java errors, although this
doesn't seem to do what I want anyway.

Cheers,
Steve