What instrument does an astronomer use to determine the spectrum of a star? Why is using this instrument better than using only a telescope to view the spectrum?
Telescope and spectroscope have different functions.
To collect more light from faint stars we need a telescope with large aperture. Spectroscope then splits the light into different spectral lines.
Picture shows a combined telescope and spectroscope used in the JPL dwan probe.
picrture JPL nasa/
Every star has its own spectroscopy or light signatures like fingerprints for humans or nose "prints" for dogs.
Compared to optical telescopes, spectroscopes are able to obtain that fingerprint.
From these spectroscopy, we can find the primary composition of the stars. Some have very little helium, some, lots of carbon. Those with lots of heavy elements are nearing their stellar life. The ones with lots of irons will go supernova soon (usually).
Stars with lots of iron will show iron spectral lines and so on.
Hence analyzing the spectrum of stars can show more information than simply looking at stars.