1. Combine sugar, star anise, and 1 cup water in a small saucepan. Simmer, stirring occasionally, until sugar dissolves, about 3 minutes. Chill syrup, uncovered, until cold, about 1 hour.
2. Puree watermelon in batches in a blender until smooth, transferring to a pitcher. Whisk in juice and 1 cup syrup (or to taste). Pour over ice in glasses. Makes 8 servings.