Let's fix your sentence!
Original: people imagine pictures by joining stars in the sky and call them constellations.
Corrected (easy): People imagine pictures by joining the stars in the sky, and they call those pictures constellations.
Another short version: People join the stars in the sky to make pictures called constellations.
Why this is better (in simple words):
- Start with a capital letter: "People" not "people."
- Add a comma before "and" here to make the sentence flow better.
- Say "the stars" (not just "stars") to sound natural: we mean the stars up in the sky.
- Use "they call those pictures constellations" so it's clear what "them" means.
Try it yourself:
Write a sentence like: "I see a dog when I join the stars." Then change it to: "When I join the stars, I see a dog."