What does a septrograph do?

Answer 1

Spectrographs (assuming that's what you mean) are mostly used in astronomy.

By analyzing the spectrum of radiation of a star, astronomers can learn how hot it is, and how much of it is composed of hydrogen, helium or other elements.

For example, "red giant" stars are relatively cool enough for molecules to exist. The spectra of molecules have been detected in red giant stars.