Each constellation in the night sky in D&D is truly a constellate, a sentient intelligence with a body made up of stars set into a shape, placed into a celestial position and confined there for eternity. A constellate has a shape that is more the vague suggestion of a shape, a ghostly image of a creature or object that they depict (a constalletion of a swan, for instance, would resemble a ghostly swan surrounding the stars of the constellation), but it can vary in size to be nearly whatever size it wants to be. When in the sky, they can be millions of square miles, but when summoned or when they descend to interact with mortals, they take a more proportional size, and when beset by attempts to imprison or capture them, they may become even smaller or much larger, making securing them nearly impossible.
At human scale, with diffuse stars moving about randomly in the creature's ghostly form, the constellate's statistics and abilities resemble that of the creature or object they represent. A dragon constellate might make claw attacks and breathe fire, a constellate of a famous hero might have that hero's abilities (including spellcasting), a constellate of a sword might wave itself around, etc.They have a THAC0 of 2, make one attack per round, and have an AC of 5 in this form.
At their natural scale, a constellate can make two types of attacks. The first is to crush any creature within its size with its massive bulk (causing 2d20 damage per 1,000 square miles in the creature's size), which is an attack it can use every other round. For more distant creatures, a constellate can blast a cone-shaped beam of stellar energy (called a "sunbolt") 1,000 miles in diameter, for a distance of up to 125,000 miles, dealing 2d12 points of damage per 1,000 square miles in the creatures size to everything within the beam. The contellate can fire the beam farther than this with a penalty (-2 up to 250,000 miles, and -5 up to 500,000 miles). The constellate can use this attack every round, even when it also crushes.
The sunbolt has been known to damage and crush entire planets, creating asteroid belts from their remains, and including at least one case of a constellate group banned from its crystal sphere for destroying a world, whose location is now unknown
A constellate typically tries to avoid combat, fleeing from those spelljammers that persue it, but if it chooses to engage in combat, its assaults are brief - no constellate has been known to attack for more than 5 rounds before resuming its place in the sky. When a constellate isn't interacting with pesky mortals, it is often simply stationary in its place in the sky.
Most of a constellate's thoughts and ideas are unknown to mortal beings, as they don't voluntarily communicate and aren't typically subject to confinement or intimidation for interrogation. A constellate may be created, changed, destroyed, or defined by a deity, but some suggest they pre-date even the gods, being as old as the stars that form their bodies.
A though it doesn't communicate, a constellate exhibits generally low Intelligence (5-7), combined with a fearlessness (19-20 Morale) that makes them easy to fool but difficult to drive off.
Aperusa & Constellates
The group of human spelljammers known as the aperusa have an interesting and mysterious relationship with the constellates. Once per year, by permenantly sacrificing 1 hp, the Beloved Grandparent of an aperusa clan can summon a constellate to their aid. When summoned, the creature is called at human scale, and the constellate fights for the aperusa clan's goals until complete. Why the aperusa have this ability -- and what relationship they have with the constellates -- remains unknown.