Sometimes after getting a star in Super Mario Galaxy 2 for the Nintendo Wii, often after playing as Luigi, I will get a message similar to:
A
ghost
has appeared in thesome galaxy name
galaxy!
What does this mean?
It happens whenever you play through a level as Luigi.
It means you've unlocked the developer time trial ghost -- a translucent Luigi near the start of the level that races to the end when you get near it.
You don't get anything for beating the ghost, however.
It is a Nintendo staff time for you to race against. It appears if you beat the level as Luigi. It will appear on all levels if you get 9999 coins.