The part of Harry's head that was hurt as he hit this concrete was his occipital lobe.
This is the part of the brain that is responsible for visual processing. This part of the brain was what got hurt that was why he saw stars.
The action that led to his fall must have affected his vision. The effects that he got is what is explained above.
Read more on the occipital lobe here:
https://brainly.com/question/18732797