Final answer:
Astronomers determine a star's chemical composition by using a spectrograph to analyze its unique spectral fingerprint.
Step-by-step explanation:
Astronomers can determine a star's chemical composition by using a spectrograph. A spectrograph is an instrument that separates light into its component wavelengths, allowing scientists to study the patterns and intensities of different colors. Each chemical element absorbs and emits light at specific wavelengths, creating a unique spectral fingerprint. By comparing the spectral pattern of a star to known spectral patterns of elements, astronomers can identify the elements present in the star and determine its chemical composition.
Learn more about Determining a star's chemical composition