Astronomy and Astrophysics - An Introductory Survey, A lecture series by Prof. G. Srinivasan - 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. ABOUT THE LECTURE 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. ABOUT THE LECTURE SERIES This lecture series is organized by the Astronomical Society of India as a part of its Golden Jubilee celebrations. Aimed at undergraduate and graduate students, this series of talks by renowned astrophysicist and teacher, Prof G. Srinivasan, starts from the first principles of Physics and builds up the subject rigorously to the present state of our understanding. Series website: https://astron-soc.in/srini-ana The website hosts a question bank as well as a list of suggested reading for each lecture (prepared by Dr. Sushan Konar) ABOUT THE SPEAKER Professor G. Srinivasan In a research career spanning more than five decades, his interests have spanned many aspects of physics and astrophysics. He is a passionate teacher and has authored two outstanding Astronomy texts titled ‘What Are The Stars?’ and ‘Can Stars Find Peace?’ He is a Past President of the Astronomical Society of India, and a Past President of the Division on Space and High Energy Astrophysics of the International Astronomical Union. In his distinguished career he has worked at the University of Chicago, USA; the IBM Research Laboratory in Zurich, Switzerland; Chalmers University of Technology, Goteborg, Sweden; the Cavendish Laboratory, University of Cambridge, England and the Raman Research Institute, Bengaluru Follow us on social media at https://twitter.com/asipoec https://www.facebook.com/asi.poec/ This series is also supported by the IAU OAE Centre in India https://twitter.com/astro4edu_India https://www.facebook.com/OAE-Center-India-105698322030674