How can scientists study the composition of distant stars?

They use spectroscopy - the study of light from an object to work out the object is made.

They pass the light from a star through a spectroscope, which splits the light into its component colours.

The chemicals in the star emit radiation at specific colours or wavelengths.

The outer layers of the star absorb the emitted light and create absorption lines - dark marks at specific wavelengths.

Each chemical has its own characteristic set of absorption lines, so the absorption lines tell you the chemicals that are in the star.