The constellation Centaurus is one of the largest in the southern sky, and its mythology is inseparable from the story of Chiron. Chiron was born of divine blood on both sides, though his birth was strange and shameful. His father was Cronus, the Titan who had castrated his own father Uranus and seized the throne of the cosmos, and his mother was Philyra, an Oceanid nymph. When Rhea, Cronus's wife, surprised the two together, Cronus transformed himself into a horse to escape notice. The result was Chiron, a creature with the torso and head of a man and the body and legs of a horse. Philyra was so horrified that she begged the gods to transform her into something else, and she became a linden tree. The infant Chiron was abandoned, but Apollo, the Olympian god of music, prophecy, and medicine who drove the sun across the sky in his golden chariot, and Artemis, his twin sister, the virgin goddess of the hunt whose silver bow never missed, found him and raised him on Mount Pelion in Thessaly.

Under Apollo's tutelage, Chiron learned to play the lyre, to read the signs of the stars, and to heal wounds that would kill ordinary men. From Artemis he learned the art of the hunt. He grew into a creature unlike any other centaur. The other centaurs were the wild offspring of Ixion, the king of the Lapiths, who had tried to assault Hera, the queen of Olympus, and was tricked into lying with a cloud made in her shape. Those centaurs were drunken, violent, and lustful. But Chiron was their opposite. He lived in a cave on Mount Pelion with his wife, the nymph Chariclo, and he welcomed mortal heroes into his home to teach them the arts of war, music, and medicine. The list of his pupils reads like a catalogue of the greatest names in Greek myth: Achilles, the son of Peleus and the sea-goddess Thetis; Jason, the captain of the Argonauts; Asclepius, the mortal son of Apollo who would become god of medicine; and Heracles himself.

The tragedy that made Chiron a constellation began with a dinner party. Heracles, during his fourth labour, the capture of the Erymanthian Boar, visited the cave of Pholus, another civilised centaur. Heracles asked for wine, and Pholus produced a sacred jar that Dionysus, the god of wine and ecstasy, had entrusted to the centaurs. The aroma of the sacred wine drifted outside and intoxicated the wild centaurs who were gathered nearby. They attacked the cave with stones and tree trunks, and Heracles repelled them with his bow, shooting arrows poisoned with the blood of the Lernaean Hydra, the many-headed serpent whose venom was fatal to any mortal and agonising to any immortal. In the chaos, one arrow passed through a fleeing centaur and struck Chiron in the knee, or in some versions in the foot. The wound would not heal. Because Chiron was immortal, he could not die, but the Hydra's poison burned through him with unending pain.

For nine days, Chiron suffered. He tried every herb and remedy he knew, but the poison of the Hydra was beyond even his skill. In the end, he went to Zeus, the king of the gods who ruled from Mount Olympus with his thunderbolt, and begged to be allowed to die. But there was another dimension to his sacrifice. Prometheus, the Titan who had stolen fire for mankind, was chained to a rock in the Caucasus Mountains where an eagle ate his liver each day. Zeus had decreed that Prometheus could only be freed if an immortal volunteered to take his place in Tartarus, the dark pit beneath the earth. Chiron offered himself. He gave up his immortality, died, and in his place Prometheus was released from his torment. The gods, mourning the loss of the wisest of the centaurs, placed his image in the night sky as the constellation Centaurus, the figure of a man-horse holding a wolf above an altar.

The constellation itself is a remarkable sight. In the sky, the centaur is visualised as about to sacrifice a wild animal, the adjoining constellation Lupus, on the altar of Ara. Eratosthenes, the Greek astronomer who composed the Catasterismi, said that this was a sign of Chiron's virtue. The constellation contains two of the brightest stars in the sky: Alpha Centauri and Beta Centauri. Alpha Centauri is a triple star system that includes Proxima Centauri, the closest star to our own sun. Some ancient writers identified the constellation with Sagittarius instead, placing Chiron among the stars as the archer rather than the centaur. Others said the constellation represented not Chiron but Pholus, the centaur who had hosted Heracles, or even Centaurus, the son of Ixion and Nephele, who had mated with the Magnesian mares to produce the race of centaurs. But the dominant tradition, followed by Ptolemy in his Almagest and by virtually every later writer, holds to Chiron.