In Greek mythology, Andromeda (Ancient Greek Ἀνδρομέδα) is the daughter of Cepheus and Cassiopeia, rulers of Aethiopia. When her mother boasted herself more beautiful than the Nereids, Poseidon sent the sea monster Cetus against the land; Andromeda was chained to a rock as sacrifice and rescued by Perseus, whom she married. She is the mythic princess and later constellation figure — not the Andromeda Galaxy (M31), which was named after the constellation that commemorates her.
Adapted from Wikipedia, "Andromeda (mythology)" · CC BY-SA 4.0
I.Family
Andromeda was the daughter of Cepheus, king of Aethiopia, and Cassiopeia. Perseus became her husband after slaying Cetus; their children, in Apollodorus, include Perses, Alcaeus, Heleus, Mestor, Sthenelus, Electryon, and the daughter Gorgophone. Through Electryon she is ancestress of Alcmene and thus of Heracles; their line rules Mycenae down to Eurystheus.
II.Mythology
Cassiopeia's boast against the Nereids brought Poseidon's flood and the monster Cetus. An oracle of Ammon required Andromeda's sacrifice; she was stripped and chained to a coastal rock (often localized at Jaffa). Perseus, returning from the Gorgons with Medusa's head, fell in love with her, secured Cepheus's promise of marriage, and killed the monster — by sword or by the Gorgon's gaze, depending on the telling.
At the wedding her former fiancé Phineus (her uncle in many accounts) attacked; Perseus showed Medusa's head and turned the attackers to stone. Andromeda followed Perseus to Seriphos and then to Argos/Tiryns, where she reigned as his queen. The Catasterismi tradition places her in the sky as the constellation Andromeda, limbs outstretched as when she was bound to the rock.
Classical pottery, Roman frescoes, and later European painting repeatedly stage the rescue; favoured scenes shift over time from Medusa's death to Andromeda on the rock.
III.Namesakes
The atlas id andromeda is the Aethiopian princess rescued by Perseus. The constellation Andromeda is her catasterism; the Andromeda Galaxy (Messier 31) and other modern astronomical or biological namesakes are later labels that borrow the constellation's or the princess's name — they are not the mythological figure herself.