May the Stupid Question of the Year award go to this question.
Do you only have to fill it with a soul once and then get unlimited uses out of it or is it simply just a soul gem that doesn't get destroyed after you deplete the soul within it?
People in various discussion forums talk about how Azura's star will help their enchanting. Well, if it's unlimited uses out of a single soul then yes. If you have to refill it after you use the soul within it, though, then no.
I ask this because, this one time, I filled Azura's star with a grand soul that I sucked off a mammoth. After I recharged an enchanted weapon with it, it was empty.
That was a playthrough ago and now I am starting a new playthrough. I want to get Azura's star right away so I can start enchanting like mad, but it doesn't seem like you really do get unlimited uses out of a single soul.
Best Answer
Azura's Star is a soul gem that can be emptied and reused. Every time you want to use it for enchanting (or recharging) you must soul trap something into it. The soul is consumed, but not the Star.
UESP's page on the Star backs this up:
It also doesn't matter if you take the "black" variant or not for the purposes of your question. Both variants work the same way, just on different souls.
The advantage is that you don't have to find or keep a ton of soul gems on hand. If this isn't an advantage to you, then there's nothing particularly special about the Star. I got it in my playthrough, but I had so many soul gems that I rarely made use of it.
Incidentally, this is the way Azura's Star works in Daggerfall (TES2), Morrowind (TES3), and Oblivion (TES4) as well.