Star Bit

Star Bits (called Star Shards in the beta of Super Mario Galaxy) is a item that's collected to progress through the SMG series.

Purpose
They're used for food to the Lumas (Don't ask why). During the series, and tv show, they're used to feed Hungry Lumas to get new galaxies, almost as though you bought that galaxy. Mario can also spend 30 to buy a item from Lumalee.