The terminology is a bit confused. Astronomers speak of "a spectrum" where they mean "a measurement of a spectrum", and thus "spectra of a star" where they rea

The terminology is a bit confused. Astronomers speak of "a spectrum" where they mean "a measurement of a spectrum", and thus "spectra of a star" where they really mean "measurements (plural) of the spectrum (singular) of a star". So it really comes down to whether you speak Latin or Astronomerese.