I hit my head and saw stars. is this a concussion?

Probably: A concussion is defined as a brief alteration in brain function that may occur after a mild traumatic brain injury. There does not have to be any loss of consciousness. This can be manifest as the stars that you see. Other associated findings include dizziness, trouble concentrating, headache, nausea. These symptoms should improve over time.

Possible: Some consider seeing stars to be an indication of a focal concussion. This is usually very transient. If not associated with any other symptoms of concussion, I would not be worried.