Astra is the sister wand to Mornstar, both possessed by the Wizard King. Both Astra and Mornstar are associated with light magic. Of the two wands, Astra is the more powerful wand, alongside its accompanying light magic spell of the same name.
Astra is acquired after defeating Gallus in the Ivory Tower. In this setting, 'Gallus' is actually the ghost of the Wizard King, whose soul is preserved in the wand.
The description of the item confirms this: A wand that contains a wizard's soul, entrusted to Oliver so that he might save Cassiopeia from her fate.
Astra has +75 magic attack, making it the best wand in the game, even above the Sky Tree Wand, which only has +62 magic attack.