Fig3_8 Absorption and Emission Lines

Fig3_8 Absorption and Emission Lines

Fig. 3.8 . The spectrum of a star or other cosmic object displays the intensity of its radiation as a function of wavelength, denoted by the Greek symbol ?. Any hot gas radiates at all wavelengths, producing a continuum spectrum with an intensity that varies with the wavelength and depends on the temperature. When this thermal radiation passes through an outer, cooler layer of a star, the atoms in this layer can produce absorption at a specific wavelength. This feature is called an absorption line because it looks like a line in the spectrum. When atoms are excited at high temperatures, they can radiate an emission line. The line wavelength tells us which atom is responsible for the absorption or emission. The intensity of a stellar line is related to both the number of atoms and the physical conditions in the starís atmosphere. The lines have a width and a shape that can be observed under close scrutiny in wavelength, and they provide information about the local physical conditions. The motion of the absorbing or emitting atoms, for example, broadens the lines (see Fig. 3.13).

Copyright 2010, Professor Kenneth R. Lang, Tufts University