There are 15 Secret Stars hidden outside the main 15 courses in Peach's Castle. You can find a comprehensive checklist on the Super Mario 64 Wiki .
In Galaxy , these are often referred to as . They appear as a question mark icon on the level select screen.
: Collect 8 Red Coins in the Tower of the Wing Cap , Cavern of the Metal Cap , and Vanish Cap Under the Moat levels. The Talos Principle Secret Stars
: Because these are notoriously difficult, many players use Steam Community Guides which offer tiered hints before giving away the full solution. Super Mario Galaxy (Hidden Stars)
: In the room for Jolly Roger Bay, backflip into the small hole high in the wall to the right. Collect all 8 Red Coins underwater to get the star. There are 15 Secret Stars hidden outside the
: Catch the golden rabbit in the basement twice (appears at 15 and 50 stars).
: Enter the 1-star door on the second floor of the castle and jump into the right-hand Peach painting. They appear as a question mark icon on
Secret Stars - Hints + Complete Solutions [With spoiler tags]