Star X is a star that formed billions of years ago. As revealed in OiramOiram12345 and the Dark Star X, a billion years after the Big Bang, a comet hit Star X and it divided into two seperate immortal creatures, The Dark Star X and The Light Star X. It reformed at the end of the movie, when the DSX was stabilized and all Dopples were destroyed.
It was later revealed in late 2013 that OiramOiram12345 stole the Dark Star X from Bowser's body, cloned it to create the Light Star X, and time traveled to create the Star X in the area it would be shattered at. He also TIme Locked the event to keep it from changing.