Why is the white snake a purple star?

I remember when Xu Xian went to the underworld, the white snake was caught by the hateful queen mother! Xiaoqing begged Guanyin to save the white snake. Guanyin said, "Xu Zi is suffering, White Lady is anxious, but it is difficult for him to prosper! If I am not convenient for them, who is convenient for them? "

Purple MSI should be Xu Xian.

But astrologically, it's different. Purple MSI symbolizes the star of the king, but it definitely doesn't refer to the son of White Snake, because Wenquxing and Purple MSI are two different stars.