Answer:
spectroscopy.
Step-by-step explanation:
The use of light to determine composition of a star is known as spectroscopy. When astronomers look at an object's spectrum, they can find out its composition based on the wavelengths of light. Spectroscopy is the most common method astronomers use for knowing the composition of stars, planets, and other objects so we can conclude that spectroscopy is the method which is used to determine the composition of stars.