The Milky Way: An Insider’s Guide, by William H. Waller

In this engaging tourist guide to our home galaxy, the Milky Way, we are taken on a fascinating journey that introduces the structure, origin, evolution and varied landscapes of our local neighbourhood: a large spiral galaxy that emerged from the chaos of the Hot Big Bang about 13 billion years ago. Our galaxy extends to a diameterof about 100,000 light years, and contains some 25 billion solar masses of ordinary matter (stars, gas and dust clouds, black holes) and up to 10 times that amount of massive dark matter.

Historically our insiders’ view was myopic because of the sheer size of the Milky Way (we live ,000 light years from the central supermassive black hole) and the absorption of starlight by the interstellar medium. Galileo’s spyglass resolved the misty veils of the Milky Way into “a congeries of innumerable stars”, and such was his ocular certainty that he felt he had freed philosophers from vexatious wordy debates about stellar realms. He was wrong about that.

William Herschel was the first observational cosmologist to fathom the vast depths beyond our solar system. Less than a century ago, astronomers still thought that the Sun was close to the galactic centre. The distances to the nearest galaxies were unknown. In 1920, at the Smithsonian Museum, an informative debate on the nature of the spiral nebulae took place: Harlow Shapley of Princeton University argued that they were local to the Milky Way, whereas Heber Curtis of the Lick Observatory asserted that they were sidereal systems far beyond our own “island universe”.

A century ago, astronomers still thought that the Sun was close to the galactic centre. The distances to the nearest galaxies were unknown

William Waller’s narrative begins with an excellent historical survey of the discovery of the Milky Way. He stresses the importance of the development of invisible astronomies, such as radio, infrared and X-ray, for probing its contents and structure. Radio astronomy gave us the distribution of hydrogen, and allowed the mapping of spiral structure. With infrared detectors we’ve penetrated the murky clouds of dust, and X-ray sky brims with high-energy phenomena.

Five chapters describe the birth, life and death of stars in the Milky Way. Concise coverage of a bewildering variety of stars is vividly conveyed by visiting the families of stars in the famous Hertzsprung-Russell diagram, which is also used to great effect in descriptions of the evolution of different types of star.

In the concluding chapters the author considers why the Milky Way exists: what processes in the early universe produced this gigantic structure? Did it emerge top-down from primordial irregularities, or bottom-up when cold dark matter began to lump together and spawn dwarf galaxies? This is left as an open question. In a philosophical coda, Waller gives a personal view on the meaning of sentient life, ourselves, in the Galaxy: where do we fit into the grand scheme of the universe?

Many clear diagrams support this thrilling story of our home galaxy, quite the best I’ve seen in popular astronomy books, as well as extensive captions. There is a comprehensive glossary, plus a bibliography that is a useful guide to the literature for those wanting more detail. The publisher has positioned it as popular science, which is correct, but it is also excellent background reading for undergraduates starting astrophysics. That’s because it is written like a guidebook with the aim of showing how the wide variety of objects in the galactic garden (the author’s expression) fit together.

Waller’s writing style reminds me of Isaac Asimov’s, in which analogy and metaphor are overworked: stars are spiced with heavy elements “like so many drops of Tabasco in a roiling pot of gumbo”. I failed to picture that.