Synesthesia also known as shifting consciousness, is to describe objective things , with the language of image and the transfer of feeling, the human hearing, smell, taste, touch and other different feelings of each other to move the conversion,  which will originally represent the A feeling of the words used to represent the feeling of B, so that the image is more lively, novel a rhetorical style
"Your eyes are like stars in the sky" does not use the synonym according to the above definition.