I have alot of questions about Azura's star.

I know that it has unlimited uses, but how large is its capacity? (lesser, greater, grand)

Also, you can choose to bring the brozen star to either Aranea or Nelacar. I know that if you bring it to Aranea, you will get Azura's star.
What happens if you bring it to Nelacar? I heard that you will get some sort of a black soul gem and Aranea will no longer be avaliable as a companion?

Well, I know the star had Grand capacity in Oblivion so it most likely will in Skyrim. As for the other question, I don't know.

"Azura's Star is one possible quest reward for The Black Star. Bringing the Broken Azura's Star to Aranea Ienith instead of Nelacar grants the Dragonborn with this Soul Gem-like Daedric artifact. Similar to Soul Gems, Azura's Star has the ability to trap the souls of non-humanoid creatures, known as "white souls". These souls can be used for enchanting and for recharging weapons. Unlike ordinary Soul Gems, Azura's Star is not destroyed after use: it is simply emptied, and must be filled with another soul to be used again."

Source: http://elderscrolls.wikia.com/wiki/Azura%27s_Star

So basically, the only downside to getting Azura's star is that you will only be able to absorb animal souls?

I didn't know it had unlimited uses, thats awesome.

I think if you bring it to Nelacar, it can only absorb human souls (all grand quality) and nothing else.

You're basically choosing between good and evil. Trapping human souls is evil and associated with necromancers that commits murders and torture.

Black Soul Gems are not evil. There is no confirmed universal guaranteed afterlife in TES. Their souls would just go to waste anyway, possibly get eaten by Sithis or clog up Sovngarde. I see recycling them into powering my Destruction improving rings as the Eco-friendly choice

Lol agreed

I didn't get that quest. That wizard guy was like, " Those Daedra are evil and you can't trust them, here, let me make it so your artifact can only run on human souls."