The Throwback Galaxy is a remake of Whomp's Fortress, the second world from Super Mario 64. The music is also a remix of the track heard on Whomp's Fortress. Six Power Stars can be found here; two normal stars, one Prankster Comet star, and three green stars.
The first star, Return of the Whomp King, has Mario fighting an even larger version of the Whomp King boss from Mario 64. The other normal star is a Silver Star hunt.