Nova Fragments are a Hardmode, post-Golem crafting material. It is one of the Lunar Fragments, dropped from the Nova Pillar during the Lunar Events. The Nova Pillar drops 12-60 fragments on normal mode, and 18-90 on Expert Mode. These fragments can be used to craft a lategame armor set that benefits alchemic weapons, and can also be used to create its respectively themed Tools, Weapons, wings, etc.
Crafting all of the Nova gear, weaponry, and tools requires 121 Nova Fragments and 68 Luminite Bars (272 Luminite).