astromancy lore questions

[Cleretic]

my question is, do they use aether from some of the seven heavens? or from the stars/space? if so, how exactly do they draw upon this aether? it's not exactly right there for them to use- the aether would, in theory, be quite far and inaccessible. and also, isn't space lacking in aether anyways? besides our star, the rest of the universe and other planets seem to be much less rich in aether and instead lean more towards dynamis.

The answer for where they get their aether is 'from the stars', and the answer for how they do it is... uhm, from the stars, this is confirmed by the Encyclopedia Eorzea. If we use the logic of actual real-world energy, then it is, in fact, 'right there': the fact we can see them means the energy from the stars got here, and so the same logic is probably true for the Astrologians. You probably think other planets are 'less rich in aether' because the main experience of them you have was Ultima Thule, which isn't exactly a representative example.

They attune themselves to the stars in the same way any other style attunes themselves to anything else, that's not especially unique or weird here. I could say that any aetheric style could become 'unsable' by their source getting corrupted or the connection getting weird; if anything, I think Astrologians have better odds of avoiding that, because their power source is a lot of individual sources in aggregate. One star getting corrupted somehow (which already seems to be a pretty hard thing to do) isn't gonna do them a lot of trouble.