What helped Perseus to defeat the Gorgon Medusa?

In Greek mythology, Perseus is the legendary founder of the Perseid dynasty. He beheaded the Gorgon Medusa for Polydectes and saved Andromeda from the sea monster Cetus. He was the son of Zeus and the mortal Danaë, as well as the half-brother and great-grandfather of Heracles.

When Perseus was grown, Polydectes came to fall in love with the beautiful Danaë. Perseus believed Polydectes was less than honorable, and protected his mother from him; then Polydectes plotted to send Perseus away in disgrace. He held a large banquet where each guest was expected to bring a gift. Polydectes requested that the guests bring horses, under the pretense that he was collecting contributions for the hand of Hippodamia, daughter of Oinomaos. Perseus had no horse to give, so he asked Polydectes to name the gift; he would not refuse it. Polydectes held Perseus to his rash promise and demanded the head of the only mortal Gorgon, Medusa, whose gaze turned people to stone.

Perseus found his way to the lair of the gorgons. Following Athena's advice he looked only in his shield and saw the gorgons asleep. Swiftly he chopped off Medusa's head and thrust it into the bag. To his surprise the winged horse Pegasus and the warrior Chrysaor sprang from Medusa's dead body and woke her sisters. Perseus donned the helmet, became invisible, and flew away.