The main way astronomers determine the physical and chemical properties of an object in the distant universe is through

• A. Photometry
• B. Astrometry
• C. Spectroscopy
• D. Radio astronomy

Answer: (C)

Spectroscopy reveals the chemical fingerprints and physical state of distant objects. Each element produces a distinctive set of spectral lines at specific wavelengths, so when gas emits or absorbs light, those lines appear and tell us which elements are present. The strengths and ratios of these lines reveal abundances, temperature, density, and ionization conditions of the gas. The same lines also shift due to the object’s motion, giving redshift that informs distance and velocity. While photometry can indicate brightness and approximate temperature, it doesn’t provide the detailed chemical information that spectroscopy simultaneously delivers. Astrometry focuses on position and motion, not composition, and while radio spectroscopy exists, the broad and direct method for extracting physical and chemical properties across distant sources is spectroscopic analysis of their light.