The origin of Tianhe’s name: “The Milky Way hangs in the sky”
The name Tianhe was given by her mother. Why did Suyu choose a name that was only one character different from her husband's name: Yun Tianqing V Yun Tianhe?
This was decided by Tianhe’s mother, Suyu.
If you read the branch plot of Phoenix Flower Branch, you will understand that Xuan Xiao and Su Yu were very affectionate (or admired each other) back then.
Su Yu said that he thought senior brother Xuan Xiao only knows how to observe the sky at night;
Xuan Xiao said, "The Milky Way hangs in the sky, and the stars are brilliant, which naturally makes people look forward to it and open their hearts." However, he also likes the beautiful scenery on good days, and the two of them often come to enjoy the flowers. The words were deeply imprinted in Suyu's heart. (Xuan Xiao’s dream in Huan Ming Palace can be seen)
A lot of things happened later, and everyone knows that in order to avoid killing, Suyu and Tianqing fled, Xuanxiao went crazy, Suyu married Tianqing, and their children She died soon after going to Tianhe, but Tian Qing probably didn't know why she chose such a name. Before she died, she wanted Lingguang Zaoyu (who was a pair with Xuan Xiao) to be buried with her.
Tianhe met Xuanxiao and told him his name. Xuanxiao once again chanted the name "Tianxuan Galaxy", which shows Suyu's inner feelings for him.
Xuan Xiao probably missed the other person and liked Tianhe better, so he and Tianhe became sworn brothers