Green Stars are Stars that can only be found during the Christmas period of 2014.
How to trigger them
During the Christmas period of 2014, when a player delivers a Holiday Spirit to the Santa NPC, the surrounding players will get a temporary aura with stars. While the aura is in effect, every normal(yellow) star will generate a Green Star, effectively doubling the amount of stars the player gets.
Forms of Green Stars
Green Stars only show up in the regular form of stars. In other words, hollow green stars or (mega) Green Star wheels cannot be found.
Every time a player delivers a Holiday Spirit, the aura triggered only last for approximately an hour of game time. When the aura is gone, the player no longer gets Green Stars, and only get the regular yellow stars. Therefore, it is suggested that the player maximizes the aura effect to get as much stars as possible, as it will take twice the time to get the same amount of stars when the player does not have the aura.