Ancient mythology had a peculiar sub-genre known as catasterism. Catasterism is the process of transforming a regular, terrestrial person or object into stars, comets, planets, etc. Two notable examples of this are the story of the Norse god Odin, wanting to atone for the slaying of the giant Thjazi, cast the giant’s eyes into the heavens, where they became the stars Castor and Pollux. In Greek mythology, the god Dionysus (or Zeus, depending on the version of the myth) set Erigone, daughter of Icarus, into the heavens as the constellation Virgo, after she committed suicide.
In Greyhawk, while the planets are well-described in the book Greyspace, we have precious little hard information on the stars. There are a few mentions of in-universe works that are supposed to deal with them (Secrets of Ye Stars Revealed, etc.), but the information we do have (outside of fan-created material, of course) is the passage from Ivid the Undying that states:
Anxann is convinced that astrology is the only way to predict when, and how, Ivid can be overthrown. He has become very reliant on a complete charlatan, Zwingell, who makes all manner of predictions pertaining to “The Dancer of Swords in the Heavens,” “The Singing Sisters of Revenge in the constellation of the Druid,” and the like.
There’s also a reference to the constellation of the Swan in Howl from the North. Interestingly, nothing in Greyspace pertains to constellations per se, but there is a description of The Sisters, which is a group of 9 bright objects that star together in a set pattern as they move through Greyspace’s crystal sphere. Of the stars themselves, we’re told they are immense gem-like objects, 10-25 miles across, studded across the interior of the crystal sphere. It’s possible they are made from material of the Positive Material Plane, or the Quasi-Elemental Plane of Radiance.
Now, if we apply the principle of catasterism to Greyhawk, it means that at least some of those star gems were mortals transformed by the gods into these timeless and beautiful objects in the heavens. Or at least they are believed to be. If true, it would give some clue as to the origin of those star gems. Here is but one example of what such a myth might look like…
Once upon a time, the Suel Imperium had been destroyed and great waves of people were roaming the Flanaess in search of peace, and Ralishaz the unlooked-for reveled in the suffering of the people. For he, more than any other of the gods, loved to see chaos and bad fortune, and these were to be had aplenty by all of the people who had been forced to flee their homes.
Among these were the young prince and princess of the Oeridian tribe of Relq. The Relq had reached the banks of the Artonsamay River, and were seeking a way to cross, but every time they thought they had found a likely place, Ralishaz would hurl some obstacle in their path. When a shallow for was found, suddenly a bulette would appear on the far bank. When a deep and calm place was found and the people of the Relq started to build rafts, suddenly a great storm would arise and swell the river. And Ralishaz laughed, and the people wept, and none moreso than the prince and princess, who had no hate in their heart for anything save the sorrow of their own people. For it is good to hate sorrow and suffering.
And then one day the prince and princess devised a plan to fool Ralishaz and give the Relq people a chance to cross the river. The prince would go upriver, making as if to cross at a likely point, crying out the name of Ralishaz all the while, and daring the god to try to stop him, to attract the eye of the god to him. While that was done, the princess would take the people downriver and quietly and quickly cross at the shallow ford.
And this they did. The prince went upriver, shouting the name of Ralishaz and cursing and daring the god to stop him. And the princess went downriver, and the people of Relq crossed the river swiftly and silently. And when they had done so, the princess went upriver, now on the far side, to see what had become of the prince. And there she saw that he had indeed reached the far side of the river, but that some creature of the river had attacked him as he did so, and he breathed his last on the far side of the river, having thwarted Ralishaz but also having paid a price to do so.
And the princess wept and raged in her grief, and wished that she could see her beloved prince once more. And the goddess Myhriss head the lamentations of the princess, and took pity on her, and appeared as a shining rain of glittering silver, saying to the princess that her prince would indeed be raised up, and that not a night would pass that she could not see him. And with that the prince’s body disappeared, and from that night on, that star known as the Eye of the Swan appeared in the sky, and all knew from its light that Ralishaz was not to be tempted, but that he could be overcome, as long as one was willing to pay the price.