Celestian is a non-binary gender from the galactian alignment system. It is the demigender version of stellarian meaning it only has a partial connection to stellarian, with the other part being something else. The "other part" can be another galactian alignment or it can be another unrelated gender. The ‘other part’ can not be singularian, as singularian can only exist by itself.
The term celestian was first coined by Tumblr user Kaispiral in a reply to the post on the blog ask-pride-color-schemes that first coined the terms dawnian and duskian. Celestian was coined at some point on or after January 19th, 2017.
The same user later made the original celestian flag on DeviantArt under the username Mikokit, on or before May 29th, 2017. It has no confirmed meaning.