Story of SMG (Spoilers):
Shortly after Mario is invited to the centennial Star Festival by Princess Peach to celebrate the comet that passes overhead, Bowser invades the Mushroom Kingdom with a surprise attack in a fleet of airships.[12] Summoning a giant flying saucer, he rips Peach's entire castle from its foundations and lifts it into outer space.[12] After an unsuccessful rescue attempt, Mario is catapulted across the cosmos and awakens on a small planet. On the planet he meets an enchantress named Rosalina and her companion stars, the Lumas.[13] Rosalina is a watcher of the stars, who uses the Comet Observatory to travel across the universe. However, the Power Stars that act as the Observatory's power source have all been stolen by Bowser, rendering it immobile. Bestowed with the power to travel through space, Mario sets off on an intergalactic adventure across the universe to reclaim the Power Stars and restore power to Rosalina's observatory.
Upon collecting enough Power Stars, the Comet Observatory regains the power to transform into a comet and flies to the center of the universe, where Peach is being held captive. Confronting Bowser, Mario learns that Bowser's plan is to rule the entire universe with Peach at his side, using a newly constructed sun of his own via the power of the Grand Stars. Mario manages to defeat Bowser and free Peach; however, in doing so, Bowser's sun collapses into itself, becoming a supermassive black hole that begins consuming everything nearby. All of Rosalina's Lumas jump into the black hole to destroy it, but sacrifice themselves in the process. The black hole collapses into a singularity and explodes in a supernova. Rosalina appears to Mario as a giantess, stating that dying stars are later reborn as new stars. Mario awakens in the restored Mushroom Kingdom alongside Peach and Bowser, celebrating the new galaxy that has emerged in the skies.
Story of SMG2 (Spoilers):
The plot of Super Mario Galaxy 2 is a follow-up of the original. Princess Peach invites Mario to share some cake at the Star Festival, a time when Star Bits rain down from the skies over the Mushroom Kingdom. On his way, Mario finds a Luma, who immediately befriends him and grants him the ability to spin. Shortly thereafter, Mario's archnemesis Bowser, who has grown to an immense size (after swallowing Grand Stars, unbeknownst to Mario at this point), appears and attacks Princess Peach's Castle. Kidnapping the princess, Bowser escapes into outer space to recreate his empire at the center of the universe. After launching into outer space, Mario is given control of Starship Mario, a mobile planetoid in the shape of his head, made by a crafty Luma mechanic called Lubba, powered by Power Stars, and piloted by other Lumas; his mission is to fly across the universe in pursuit of Bowser and the Princess and to help Lubba find the lost Lumas that were part of Bowser's plot. Along the way Mario meets new Lumas and joins up with his companion Yoshi, the Toad Brigade from the original Super Mario Galaxy, and his brother Luigi.[23]
Upon collecting enough Power Stars, Starship Mario reaches Bowser's main fortification, draining energy from what appears to be a comet. Mario infiltrates the castle and defeats Bowser, retrieving the last Grand Star. Rosalina and her Comet Observatory from the first Super Mario Galaxy appear before Mario and Peach. Rosalina thanks Mario for watching over the Luma that he had found, and the Luma returns to the Comet Observatory, taking Mario's hat with him. Mario and his friends return to the Mushroom Kingdom and celebrate with a cake as Starship Mario sits above Princess Peach's Castle and the Comet Observatory streaks across the sky.
Both from Wikipedia
