WordNet records no synonym and no antonym for cynosure. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
something that provides guidance (as Polaris guides mariners)
“let faith be your cynosure to walk by”
something that strongly attracts attention and admiration
“if he was the cynosure of all eyes he didn't notice”
Synonyms, antonyms and the broader and narrower relations for cynosure come from Princeton University’s WordNet. Full sources and licences.