1. Use the star (filled with a soul) like any other soul gem. Unlike normal soul gem, the star
will not vanish, it will change to an empty star instead. In the other words, the star is a perpetual soul gem that can be used for enchanting, recharging, and soul trapping again and again. A very useful soul gem
2. The star can hold any soul, including Vivec. Note: Vivec will not fit into a grand soul gem, you must have the star.
3. The star is the highest level soul gem. In the other words, if you have some normal soul gems in your inventory then the trapped souls will go there preferably. If you want a soul to be trapped by the star, you
must not have a "suitable" normal soul gem in your inventory. Drop the corresponding soul gems on the ground if you see a creature that you want in the star instead in the normal soul gem.
4. Unlike Daggerfall, there isn't an option in Morrowind that would allow you to empty the star after trapping an unwanted soul by the star. You can use recharging instead: recharge a magical item by the unwanted soul and the star becomes empty again.