Andromeda didn't "become" a constellation in the way that constellations are created by humans. The constellation of Andromeda has always existed, as a pattern of stars in the sky. It's just that humans eventually noticed it and gave it a name.

Here's how that happened:

* Ancient Astronomers: Ancient civilizations, like the Greeks, observed the night sky and began noticing patterns of stars. They used these patterns to tell stories, track time, and navigate.

* Mythology: The constellation of Andromeda got its name from Greek mythology. It was named after the princess Andromeda, who was chained to a rock as a sacrifice to a sea monster. The story of Perseus rescuing her is told in the stars of the surrounding constellations.

* Formalization: Over time, various cultures developed their own constellations and star names. Later, these were catalogued and standardized by astronomers, making the constellations we recognize today.

So, Andromeda wasn't created, but rather identified and named based on the visible arrangement of stars in the sky and linked to a story from Greek mythology.