The size of small comets, astral mermaids “swim” through space via gigantic globs of water. Sterre, as they are known, travel from star system to system, exploring the cosmos and cross-pollinating planets with aquatic magic.
Heat, drawn from stars they visit, is stored in their float, the ocean-sized water globs that the sterre live in. Creatures and algae living in the float provide the sterre with food and atmosphere.
Sterre grow throughout their lifetime. It is rumored that they cannot die of aging, simply growing more powerful until they no longer require a physical form. Sterre regularly travel to the astral plane, using it to communicate with water fey all over the universe, so it is conceivable that some remain there permanently, acting as guardians to their planet-based aquatic counterparts.