ASTROPHYSICISTS, unlike most experimental physicists, cannot influence the subject of their 'study. Instead they are confined to analysing the electromagnetic radiation emitted by objects in the sky. The primary analytical tool is spectro-< sc.opy, and most of what we know about the velocity, composition, density and temperature of any object beyond our solar system is .determined from its emission or absorption spectrum. Spectroscopy was for a long time" only possible in the optical, near-ultraviolet and near-infrared regions of the electromagnetic spectrum, where the atmosphere is transparent. Although the recent development of spectroscopic techniques at radio and X-ray wavelengths has changed the situation to some extent, optical spectroscopy remains our main analytical tool.
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