astronomical spectroscopy; astronomical photometry
Technical term covering both spectroscopy and photometry in astronomy. In practice, 天体分光法 is sometimes used for spectroscopy specifically, but this term encompasses both.
Astronomical spectroscopy is used to investigate the chemical composition of stars.
彼は天体分光学の研究で博士号を取った。
He earned his doctorate with research in astronomical spectroscopy/photometry.