There’s too much gold in the universe. That’s the conclusion of a recent study that compared the abundances of gold measured in our solar system with the known mechanisms for producing gold. The primary way to create it, astronomers believe, is when two neutron stars collide (supernovae don’t help, since any star massive enough to produce gold through fusion will end up as a black hole, trapping the gold within it.) But, the study’s authors noted, neutron star collisions don’t appear to be frequent enough to produce the gold we do see. Either another process creates gold, or neutron star collisions create more gold than astronomers expect.

If it’s the latter, it maybe shouldn’t be surprising. Neutron stars remain among the most enigmatic objects in the universe. While black holes have attracted both professional and public attention—and, as of last week, the Nobel Prize in Physics—neutron stars are far less understood or appreciated. In her new book Neutron Stars, science journalist Katia Moskvitch provides a detailed overview of what we know, and have yet to find out, about neutron stars and their place in the universe.

The concept of the neutron star emerged in the 1930s just a few years after the discovery of the neutron itself, when astronomer Fritz Zwicky suggested a star made of neutrons might be created in a supernova (see “Review: Zwicky: The Outcast Genius Who Unmasked the Universe”, The Space Review, September 30, 2019). However, it took more than three decades after Zwicky’s proposal before a graduate student in England, Jocelyn Bell, discovered a pulsing radio source, or pulsar, that astronomers soon concluded was a rapidly spinning neutron star—a discovery that resulted in a Nobel Prize whose recipients, infamously, excluded Bell.

In the roughly half-century since Bell’s discovery, astronomers have found many more neutron stars, but are still only beginning to understand the stars themselves. “Our problem is that the physics of how matter behaves inside neutron stars is so extreme that our models struggle to truly explain their behavior,” Moskvitch writes in one chapter, where she outlines what we do know about the structure of neutron stars, from an iron crust a kilometer thick to a superfluid mix of neutrons and other particles in the outer core (scientists “have absolutely no clue” about the inner core of neutron stars, she writes.) Other parts of the book recount the discovery of planets orbiting a pulsar, a finding that predates the discovery of exoplanets around more Sun-like stars, as well as how neutron stars have been spoilers of sorts for studies of dark matter.

Moskvitch crisscrosses the globe for the book, visiting observatories from England to Australia and from Chile to British Columbia that are involved in studies of neutron stars. She also recounts the first observation of a neutron star collision made by the LIGO and Virgo gravitational wave detectors. That helps create an engaging book about the science of neutron stars, as well as the scientists involved in that research.

In the book’s epilogue, Moskvitch says that most stars disappear at the end of their lives, either by becoming black holes or simply fading away. “But then there are the cosmic zombies—the neutron stars,” she writes. “Ultra-dense, a smidgen or two away from turning into a black hole, they have a life beyond death: they send out radio waves, gamma ways, x-rays, and maybe the enigmatic fast radio bursts.” After reading Neutron Stars, you can see why they are so intriguing, if difficult to understand.

It is not nice to say: "...My, you are _so_ heavy!". Unless, of course, we are referring to a neutron star, because in this case it would be the plain truth.
Theorized by Walter Baade and Fritz Zwicky in 1934, for thirty years neutron stars were virtually ignored, because it was thought almost impossible to observe them. Reversing the situation was in 1967 Jocelyn Bell, with the discovery of the first pulsar, one of the forms in which a neutron star can manifest itself. From then on, thousands of such stars have been catalogued: spherical "spinning tops" no larger than a city, they rotate very quickly, they are composed of really really really compact matter ("_so_ heavy"), they have a very strong magnetic field, they often have another star (or two) as companion, they may have planets...
All this and much, much more, is presented in the book written by Katia Moskvitch. It is a book that somehow looks like a neutron star itself... not because "heavy", but because _very dense_ with data, names, locations, numbers... But this must not dissuade you from reading it: go into the subject according to your interests and preparation (in my case, although I don’t know much about physics and astronomy, I think I have sufficiently understood the fundamental concepts - If instead you are a fan of astrophysics, you will feel like going to a party!).
It is a wide-spectrum book, a tribute to astronomers, physicists, engineers... to all the people engaged in the "great game" of "cosmic spinning tops".