From Ancient Greek καταστερισμός (katasterismós, “star legend”),

from καταστερίζω (katasterízō, “to place among the stars”),

from κατά (katá, “down”) + ἀστήρ (astḗr, “star”).

Catasterism (plural catasterisms) (Greek mythology) 

The transformation of a hero or mythological creature,  

into a star, constellation, comet or other celestial object.