In just over 200 pages, Neil deGrasse Tyson takes his readers, who are presumably in a bit of a rush, on a grand tour of the cosmos, with a refreshing emphasis on what scientists don’t know. He’s bumptious, conversational, unafraid of including personal opinions about people in the field and commendably clear even when describing mind-expanding notions. He’s also a bit cheeky, titling his first three chapters “The Greatest Story Ever Told,” “On Earth as in the Heavens,” and “Let There Be Light.” Apparently he’s always been that way. The first essay he wrote about the wider universe was about diminutive galaxies that are companions to the Milky Way. He titled it “The Galaxy and the Seven Dwarfs.”
He mentions that essay in a chapter on intergalactic space, which concludes that it “is, and forever will be, where the action is.” (p. 74) Not only is there far more intergalactic space than the other kind, there’s a lot more in it than one might think, “dwarf galaxies, runaway stars, runaway stars that explode, million-degree X-ray-emitting gas, dark matter, faint blue galaxies, ubiquitous gas clouds, super-duper high-energy charged particles [cosmic rays], and the mysterious quantum vacuum energy.” (p. 64) How much mass does it all add up to? “Nobody knows for sure. The measurement is difficult because the stars are too dim to detect individually. We must rely on detecting a faint glow produced by the light of all stars combined. In fact, observations of [galactic] clusters detect just such a glow between the galaxies, suggesting that there may be as many vagabond, homeless stars as there are stars within the galaxies themselves.” (p. 67) Astronomers have also seen more than a dozen supernovas far away from presumed galaxies. Tyson notes that in ordinary galaxies, for every supernova there are one hundred thousand to one million stars that do not explode in that fashion. The isolated supernovas may point to “entire populations of undetected stars.” (p. 67) They may be even more numerous, because to date systematic supernova searches have monitored known galaxies, rather than intergalactic space.