Rosalina's back-story is chronicled in a storybook told by her in the Comet Observatory's Library in Super Mario Galaxy. Nine chapters are unlocked throughout the game, and the last is unlocked by completing the game.
Rosalina's story begins when she was a young girl living on Mushroom World (possibly in the Mushroom Kingdom). One day, Rosalina discovered a rusty spaceship (resembling the Starshroom) with a Luma inside. The Luma introduced itself, revealing that it was waiting for its "Mama". Rosalina bonded with the Luma, and promised that she would wait with it. Armed with her father's telescope, Rosalina stargazed for years, searching for the comet that carried the Luma's mother. Fearing it was taking too long, Rosalina and Luma fixed the spaceship then set off into space.
Rosalina collecting Star Bits.Traversing through asteroids, Rosalina grew very hungry. She had brought essentials for the trip: the telescope, food, her stuffed bunny (resembling a Space Bunny) and tea, but forgot water. At the sight of a pouting Rosalina, the Luma then told her that Star Bits would be all she needed, and began to share them with her. Leaning out the spaceship the two collected Star Bits to eat.
One night a shimmering comet made of ice flew past the ship. Hoping Luma's mama would be there, the two set foot onto it. Mama was nowhere to be found, but discovered Star Bits clustered to the south.
One night in a dream, Rosalina remembered her mother. Rosalina's mother greatly resembled how she herself would come to look. In the dream Rosalina's mother was going away. She told her not to worry, because she would always be watching her. Clinging to her mother, Rosalina cried. Upon waking up, Luma noticed tears coming from Rosalina's eyes (and mistook them for Star Bits). However, Rosalina explained what they actually were. She was crying because she would never see her mother again. At this, they both broke down into tears over their lost mothers.
Continuing the search, Rosalina comforted the Luma, and told the little star she would take care of it, and with that a spark in her heart went off. She became determined to mother the Luma and then started to make the home out of the comet. With tools found inside the comet, Rosalina built what would become Domes that would link to various galaxies. For now, however, it was a shelter for Luma. Once completed it seemed very large for her and just one Luma.
One day another comet flew close to Rosalina's new home, and from the comet another Luma appeared. The Luma affectionately called out and said Rosalina was it's "Mama." Soon the other Luma joined in: "My Mama, My Mama!" Rosalina found this so touching she couldn't help but laugh. Out of the comet many more Luma came, and all of them called her Mama. She took them all under her wing, named them, and moved them all into her newly built house on the comet.
One night Rosalina remembered her home planet of Mushroom World, and wondered if everything was all right there. She looked through her telescope until she could make out the planet's surface. She found her favorite spot for stargazing, on top of a hill with a lone tree, and it reminded her of her family. She wanted to go home, but she knew she'd never see her mother again. At this time it was revealed her mother had died, and was buried under the tree in view. Rosalina had been aware of this all along, but was unwilling to truly believe it.
The Luma comforted the crying Rosalina, assuring her that she shouldn't be sad. They told Rosalina that her mother is a part of her and will always be close by. It was at this time the Luma she met inside the ship had an idea. The Luma transformed the comet into the Comet Observatory to help in finding all their Mamas. The Luma explained that when they grow up, they transform into various things, including stars, comets, and planets. Rosalina was so overcome with emotion due to this selfless act she thanked all the Luma that had huddled around to hug her. From that day on Rosalina never cried again.
The observatory grew and grew into a better home, and Rosalina and the Luma were proud to call it their own. At a welcoming party for a new Luma, Rosalina and the others baked a cake topped off with Star Bits. Coming together in such a way made her realize the Luma were her new family, and she was determined to raise them and help them all fulfill their destinies. It was at this time she began to call herself the mother of the Luma.
Rosalina was reminded yet again of her home one night. Even though she had a new goal in her life, she thought it'd be nice to visit her favorite spot from her past. So every one hundred years the Comet Observatory passes over the Mushroom Kingdom so that she can re-visit her homeworld.