Answer:
Spectroscopy
Explanation:
Spectroscopy is a tool that scientists typically use to determine the mass, structur and the temperature of stars or other objects within the observable universe.
In general, scientists take a spectrum of the object they observe using a spectrograph. It is based on the theory that the lines of absorption at a given wavelength are special for each element.
Hence, the correct answer is spectroscopy or spectrograph.