I cannot capture grand souls with Azura's star. I can get common, petty, and lesser, but not greater or grand. I'm told via message on the screen that I don't have a soul gem large enough to hold the soul.

First off, it's not the black star - just Azura's star. I have no other gems in my inventory. I can grab deer, saber cats, bears, mud crabs, etc. But mammoths or Giants - no go.

I should also mention that I'm not using soul trap. I'm using the soul tear shout. (I do not have a Mage or any spells.)

From what I understand, the difference between the two stars is the type of souls one can capture (white/black), not size. I also understand that I cannot capture black souls (humanoids) with the star that I have.

What am I not understanding?

Is there already a soul in it? If there is then you have to use it first. Can only have 1 at a time.

Hmm, not sure why it seems to be acting up for you. I too have the 'white' Azura's Star, but it works fine all the way up to include greater and grand critter souls. One difference is that I am using a soul trap enchantment on my character's bow and have never used the soul tear shout. My character doesn't kill giants or mammoths so most of the greaters and grands she gets are from high level draugr.

Thanks.

That's what it was. At first I didn't realize that if you didn't do the killing move to the creature with the actual shout (Soul Tear) or spell (Soul Trap) then you don't get the soul. So, I was shouting at giants and mammoths and nothing was happening because it didn't do wnough damage. So then I moved to smaller things and it worked, because the shout itself instantly killed them. Through all the confusion of figuring that out, I forgot that the star was already loaded with a lesser soul. So when I'd get a giant down to almost nothing and do the shout, it would tell me that I didn't have a gem big enough to hold the soul. I tried it again with my newfound knowledge and it worked. Thanks.

I usually don't kill the peaceful creatures either but I was trying to test the star thing out. I had never used it before, and the first thing that came to my mind with large souls were mammoths and giants. They are usually also almost always found together so it made things easier. (Giants provide Greater souls, mammoths provide Grand.)

You should try the soul tear shout sometime. To my understanding, it does the same exact thing. I would imagine it's a different experience hearing your player shout at someone and stealing a soul.