Green Stars, or Cosmic Jewls are items you collect in Super Mario Galaxy and Super Mario Galaxy 2. They are just like regular stars, as collecting them does count as beating a level. In Super Mario Galaxy, three green stars appears, and collecting them unlocks the Trial Galaxies. In Super Mario Galaxy 2, after collecting 120 Power Stars and defeating Bowser, they will be hidden somewhere in all galaxies, and you can collect them to get 240 power stars and to unlock the Grandmaster Galaxy. This time around they make a unique sound, different from that of yellow stars.