Since this is a rather unusual character, please let me know if you have any objections to her.
An unearthly-looking mare who appears to be an earth pony, although she claims to be something called a Starmare. She has a dark blue coat with star-like sparkles strewn across it, a long, pale lavender mane and tail, gold eyes, and a comet cutie mark.
Stella can fly to some extent, although she is rather clumsy at it, or create tiny, colourful sparks of light.
Allegedly, she has phenomenal power over outer space, but there aren't any ponies around who have actually seen it.
Stella, as mentioned above, claims to be a Starmare, a species of pony from beyond the atmosphere. How much truth there is in this claim is unknown. While she is rather strange for an earth pony, nopony has seen her in space, Starmares probably don't exist, and even if they did, what would a Starmare be doing here, in a school, teaching a class, instead of roaming the void?
Whatever the case may be, though, she is the astronomy teacher at the school, and knows a fair bit about the subject.
Stella is a rather enigmatic pony. While she has her moments of wisdom, she is also very spacey, and rarely can keep her thoughts connected to reality. She could just be light-headed, or she could be acting it to try to keep her students interested.
Mostly professional so far.