WordNet records no synonym and no antonym for sunburst. 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.
a sudden emergence of the sun from behind clouds
a design consisting of a central disk resembling the sun and rays emanating from it
a jeweled brooch with a pattern resembling the sun
Synonyms, antonyms and the broader and narrower relations for sunburst come from Princeton University’s WordNet. Full sources and licences.