In Greek mythology, Cassiopeia (Cassiepeia, Cassiope) was queen of Aethiopia, wife of Cepheus, and mother of Andromeda. Her boast that she—or her daughter—surpassed the Nereids in beauty brought Poseidon's flood and the sea-monster Cetus; after Andromeda's rescue she was set among the stars in the W-shaped chair.
Adapted from Wikipedia, "Cassiopeia (mother of Andromeda)" · CC BY-SA 4.0
I.The boast and the rock
Origins vary: Nonnus can call her a nymph; Stephanus links an alias Iope, daughter of Aeolus, to the town of Joppa. Apollodorus has her vie with the Nereids and claim superiority; Poseidon answers with flood and a sea-monster. An oracle (of Zeus/Ammon) decrees that only the sacrifice of Andromeda will save the land—so Cepheus chains the princess at the shore.
Hyginus sometimes puts the boast on Andromeda's beauty rather than the queen's. Perseus slays Cetus, frees Andromeda, and marries her; the mother's vanity has already forced the exposure.
II.Constellation
Poseidon places Cassiopeia in the heavens chained to a throne that recalls Andromeda's ordeal; the constellation near the pole never sets for northern latitudes and later artists give her a mirror of vanity or a palm. She sits in the sky beside Cepheus, Andromeda, Perseus, and Cetus.
The atlas id cassiopeia-ethiopia keeps the Aethiopian queen distinct from the constellation as a bare astronomical label.