Physics of the Interstellar and Intergalactic Medium
Bruce T. Draine, Princeton, 2011, 540 pages

any people think of astronomers as staring bleary-eyed all night through their telescopes, watching a screen full of little white dots, and hoping it doesn't get cloudy. In fact, most of them prefer to be called astrophysicists, and most of them are too scruffy and covered with chalk dust to be allowed anywhere near a telescope. Well, at least, that's the stereotype. In reality, they probably spend most of their time, like everyone else, writing grants.

Still, astrophysics is one of the few branches of science where people still talk about semiclassical constructs like the Bohr atom and the Balmer and Lyman series. That's because hydrogen is overwhelmingly the most abundant element in the interstellar medium. There's so much of it that the interstellar space is dominated by hydrogen Lyman-alpha at 121 nanometers in the vacuum ultraviolet. Draine reviews the molecular physics of these gas clouds and, to some extent, the chemistry that occurs in them. The coverage is somewhat abstract, with lots of equations, spectra, and graphs, making it perhaps a tough slog for a layman, but hardly dry considering the magnificence and galactic scale of the subject matter. There are also 13 color plates, stuck in the middle of the book, of objects familiar to all amateur astronomers, such as the Orion Nebula, the M82 galaxy, and the Helix Nebula.

The equations, which require a background in physical chemistry and spectroscopy to fully appreciate, are mostly pulled out of a hat, so to speak, which is to say that the reader must turn to the original paper to find the derivation (and sometimes also for the meaning of the parameters). The author also has a tendency to end paragraphs with equations, without explaining their implications. For example, on page 311, he calculates the precession period of a dust particle to be 10 years. Then he makes refinements to the equation, presents the new equation, and stops, forcing readers to calculate the new answer on their own. This will undoubtedly frustrate readers seeking an intuitive understanding as well as the mathematical formula. Useful appendices include a list of energy diagrams and a big table of collisional rate constants. Readers should also download the errata list.

The Physics and Chemistry of the Interstellar Medium
A.G.G.M. Tielens, Cambridge, 2005, 495 pages

surprising amount of chemistry goes on in the vacuum of space, and much of it is familiar to those of us on Earth who use mass spectrometers and other tools to study organic molecules in a vacuum. Although this book also covers much of the basic atomic spectroscopy of hydrogen and helium as Draine, it's much more engagingly written and somewhat less abstract: in addition to the raw equations, Tielens explains the implications of the equation in terms of what one might observe. However, there are only a few images, and these are of poor quality. The rest are graphs and, as you might expect from the title, emission spectra and chemical reaction pathways.

The interstellar medium is not just gas; there are also vast quantities of dirt, or "dust" as astrophysicists prefer to call it. Chemically, this dust is very much like the dirt you would find in your back yard, albeit with much finer particle sizes: mostly amorphous silicates, oxides of iron and magnesium, and carbon. Unlike the carbon-based dirt your back yard, however, most of the carbon in space is in the form of nanodiamonds.

Because of the intense ultraviolet light in space, one would expect carbon chemistry in space to be dominated by free radical reactions, and such is indeed the case. In addition to well-known carbon molecules such as carbon monoxide, a great deal has been discovered about polycyclic aromatic hydrocarbons (PAHs), coal-like molecules formerly believed to be produced only by the remains of living organisms, but now known to be abundant in interstellar space. Space PAHs, which Tielens specializes in, were first discovered in the Red Rectangle, a protoplanetary nebula in Monoceros. Tielens is unique in being highly conversant with chemistry as well as physics. This makes much of the book exciting territory not just for physicists, but also for chemists, for whom space is just another really big boiling cauldron full of nasty chemicals.

Other highly regarded, but older, books on the subject are Physical Processes in the Interstellar Medium (1978) by the late Lyman Spitzer (after whom the Spitzer Space Telescope was named), Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (2nd ed., 2005) by the late Donald E. Osterbrock and Gary Ferland, and Radiative Processes in Astrophysics (1979) by George Rybicki and Alan P. Lightman.