Nice sentence! Here is a clearer, correct way to write it:
Corrected: People imagine pictures by joining the stars in the sky. Those pictures are called constellations.
Why this is better (in simple words):
- Start with a capital letter: use 'People' not 'people'.
- Put a period at the end of each sentence so the reader knows the sentence is finished.
- Say 'the stars' to show you mean the stars in the sky.
- Use 'Those pictures' instead of 'They' so it is clear what you mean.
Great job talking about the stars! You can also say one short sentence: 'Constellations are pictures people make by joining stars.'