It was an odd, whimsical sort of question, and Rosario, who tended to disavow any speck of whimsy in her, would ordinarily have shrugged it loose, but her mouth was already saying, "A Classical Cepheid. Eta Aquilae or Polaris. They're standard candles, they help us establish distance scales and galactic structures, measure the Hubble constant. They help us see the universe for what it is, you know?"