Originally Posted by ichigo_m.
Twilight Sparkle's Gala Gown from My Little Pony. (:
The bottom of that dress is covered
in stars. So much fun! Twilight's "Halloween Costume" of Starswirl the Bearded
would also be fitting, as would Princess Luna
(if you're going for MLP).
You also might get lucky searching for series related to mythology, given the connection of a lot of those stories to constellations. (Ex, Lady Palutena
from Kid Icarus has a lot of references to the sun, as she is the goddess of light!)
((Also, high-five, fellow astronomy lover! xD))