ABSTRACT
Spectroscopy is a most powerful way to study the heavenly bodies, from planets to comets, asteroids, stars, nebulae and distant extragalactic objects. In addition to observational facts, simple theoretical arguments will be expounded to help with the interpretation and classification of spectra of normal and peculiar stars, assuming basic knowledge of atomic and molecular spectroscopy. As it occurred for laboratory spectroscopy, many classification schemes were devised well in advance of the physical understanding of the stellar atmospheres and interiors. Cometary dust analyzed in the laboratory and through mass spectroscopy in dedicated in situ space mission was proved to be closer to solar abundances being less depleted in light elements with respect to CI chondrites. The interpretation of cometary spectra would deserve a specific section for their peculiar chemical and physical conditions, rapidly varying with the distance from the Sun. Therefore, the energy levels are not populated by collisions as in equilibrium conditions, but by a rapidly varying radiation field.
