This, of course, is a newbie question, as I am nothing more than a hobbyist. But I was quite surprised to see the recently-released "before and after" pictures of the so-called "Pillars of Creation" in the Eagle Nebula (here and here). For something that looks like such a "nebulous" cloud of gas (pardon the pun), I would have expected at least some visible sign of change in the 25 years between the two photos. But at the pixel resolution which I had available, I could not detect even the slightest difference between them. Of course, I am accustomed to terrestrial clouds which are in constant motion, so I (mistakenly?) expected something analogous at the astronomical level. Can someone provide, in layman's terms, how my understanding of this cosmological feature is deficient? In other words, how might I adjust my instinctive intuition that this object should be more dynamic? Is it just the sheer scale that I am not comprehending? (By the way: what is the distanse between the three spires, as compared to, say, our solar system?)
It appears to be static because it's huge beyond your imagination.
The distance to the nebula is 7,000 light years. Its apparent size is 7 arc minutes. Therefore its linear size is about 14 light years.
Think about that. The whole nebula is so big, it takes light 14 years to cross it. Any motion therein must necessarily be much, much slower. No wonder you're not seeing much change.
To add to Florin Andrei's answer, with an image height of 7,000 pixels for 14 light years, that's 17.5 light hours per pixel. That's 20 billion kilometres per pixel. To make a change in a single pixel over that time, something of that size must have either changed composition dramatically (to give a different colour or opacity) or it must have moved by a comparable distance.
Given the timeframe, that's a billion kilometres per year, or 123,000 kilometres per hour. (77,000 miles per hour, if you prefer) Few things that large are moving that fast relative to their neighbours.
If you go to this site, http://heritage.stsci.edu/2015/01/supplemental.html , there is a set of comparison photos. The movement that is detectable is very slight but it is there.