Rosalina's Story

Rosalina's Story, while never explicitly stated, is the story of how Rosalina made it into outer space and how she became the "mother" of the Lumas prior to the events of Super Mario Galaxy. In the Comet Observatory's Library, Rosalina sometimes reads from the storybook to the Lumas traveling with her. Mario/Luigi can come in to listen to the story. In all, the storybook contains nine chapters.

The storybook becomes available to read once the player clears Bowser's Star Reactor and obtains one more Power Star afterwards. More chapters are unlocked as the player obtains more Power Stars.

Yoshiaki Koizumi, director of Super Mario Galaxy, was the writer of the storybook. He wrote the script late at night when no one was around so no one would know what was in it. The next day, he told Shigeru Miyamoto about the story and presented it to him. Miyamoto was surprised that Yoshiaki wrote the script late at night with nobody around. Yoshiaki Koizumi, in a Nintendo Power interview, mentions that he would enjoy seeing Rosalina's storybook get published as a children's book.

Story
After Mario enters the library, Rosalina begins to read the following story.

Chapter 1: The Celestial Duo
This chapter is available after obtaining 17 Power Stars.

Chapter 2: Star Bits
This chapter is available after obtaining 24 Power Stars.

Chapter 3: The Comet
This chapter is available after obtaining 28 Power Stars.

Chapter 4: The Dream
This chapter is available after obtaining 32 Power Stars.

Chapter 5: Home
This chapter is available after obtaining 40 Power Stars.

Chapter 6: Friends
This chapter is available after obtaining 49 Power Stars.

Chapter 7: The Telescope
This chapter is available after obtaining 58 Power Stars.

Chapter 8: The Wish
This chapter is available after clearing Bowser's Galaxy Reactor.

Final Chapter: Family
This chapter is available after clearing Bowser's Galaxy Reactor.

Other appearances
The storybook appears in the secret ending of Super Mario Galaxy 2, where Rosalina is seen closing the book, having just finished reading it to the Lumas.

The book itself makes a minor appearance in Mario Party 10, where it can be seen in the south-western part of the Rosalina Board. It is sitting on a round table, surrounded by two Lumas.

Super Mario Galaxy Trading Card description
''In the Library, Rosalina reads a story to the Lumas about how a young girl and a Luma built a spaceship to go in search of the Luma's mama. Eventually the little girl takes on a motherly role for this Luma and count-less others, much like Rosalina herself.''

Trivia

 * The starship that is mentioned many times throughout the story appears to be the Starshroom that the Lumas built with the Toads in Super Mario Galaxy. It differs, though, since it has no glass in the portholes/windows. It can be seen, moss-covered, in the extra credits sequence, after 120 Power Stars have been obtained.
 * Baby Rosalina's design may contradict the events of the storybook; she has a crown with jewels, similar the one she wears as an adult, while the crown of the young Rosalina in the story has no jewels. However, this could merely be a limitation of the artstyle of the book's illustrations.
 * In Super Mario Galaxy 2, beating the game after collecting 120 Power Stars results in a cutscene showing Rosalina reading a similar storybook, only this time, the book recaps the game's events. After finishing said book, Rosalina starts reading another one, this time about the Green Stars.
 * In Mario + Rabbids Sparks of Hope, after Rosalina temporarily breaks free from Cursa in Barrendale Mesa, Mario and his friends escape from Cursa. JEANIE then tells the story of how Cursa became, along with pictures depicting how it took control of Rosalina. These pictures have a similar art style to Rosalina's storybook.