And they name it as the shape the septuple Primary Stars.

北斗、牛、女、虚、危、室、壁，其形如龟，称玄武。

In fact, they divided Heaven into four parts and each part is composed by septuple Primary Stars.

西奎、娄、胃、昂、毕、角、参形状如虎，称西宫为白虎。南、鬼、柳、、张、翼、轸联为鸟形，称朱雀。