From the Super Mario Wiki
Starbags are enemies that appear in Super Mario Galaxy and Super Mario Galaxy 2. They are blue or green sack-like creatures with a pair of eyes resembling to those of a Blooper and legs with big gray feet. They commonly appear in places with sand, dirt or beaches, such as Deep Dark Galaxy or Starshine Beach Galaxy. They appear invisible, revealing only their footprints that leave over the soil.
If Mario can successfully Spin at the spot where the Starbag is, it comes out and remains dizzy for a few seconds. Mario can also toss a shell or a fireball at it, but Starbags can duck them. Once Mario defeats them, they give away twenty Star Bits. If the player keeps moving, the Starbag won't appear, so they need to not move for being able to defeat it.
In Super Mario Galaxy 2 Starbags are rare. Only 2 of them appear in Starshine Beach Galaxy.
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