Thank you! I watched the episode where they mentioned why tianhe's mother named her son tianhe. At the same time I saw the translation saying Milky way which I didn't quite get(I didn't know it was the name of our galaxy in English so thank you @Jenniffer and @Mehnaz). I also saw the same translation in the song of tianhe's dream by liu yuning. Then, I found what you have mentioned about the silver river or heavenly river. Now, everything makes sense ^^