Lecture 5 - Interstellar Medium - Atomic Gas
More than a century has elapsed since the discovery of ISM in 1904 to its appearance in popular culture ('Arora' - an SF novel) today. But the science behind our understanding of this tenuous medium remains as fascinating.
Ultraviolet radiation from newly formed hot stars ionizes the relic gas from which they formed. When the ions 'recombine' with electrons, spectacular gaseous nebulae are produced. The vast space between the stars is not empty. There is widespread diffuse gas, as well as gas clouds - just like in our atmosphere. In this lecture, we shall discuss the distribution of 'atomic gas' in our Galaxy. This gas, mostly Hydrogen, emits, and absorbs, radio radiation at a wavelength of 21 cm. The basic physics of how this radiation arises is discussed in detail, as well as how this radiation is detected. The resultant model of the interstellar medium, the "Raisin Pudding" model is described.