When you think upon a star : FROZEN STARS: OF PULSARS, BLACK HOLES AND THE FATE OF STARS by George Greenstein (Freundlich: $16.95: 274 pp., illustrated)

My mother says, “The more you know, the less you know,” one of the many aphorisms with which she sent me into the world. Though she knows little of science, her insight turns out to be especially true in that realm, where each answered question raises a dozen new questions that scientists didn’t know enough to ask before. And of all the sciences, it is especially true in astronomy, where the explosion of knowledge has been outpaced by the explosion of ignorance it engenders.

Greenstein’s book, written for the interested layman by an accomplished astrophysicist, deals with both the knowledge and the ignorance that has been amassed in this century and particularly in the last 20 years, since radio telescopes turned the universe on its head. It is a beautifully written book, conveying both the facts and, more important, the process and the wonder of science.

Here are scientists at work: Arthur Eddington escaping the draft during World War I to test Einstein’s theory of relativity; Jocelyn Bell, a graduate student, discovering pulsars and wondering what they are; G. Richard Huguenin building a huge radiotelescope on the cheap in the Massachusetts wilderness; Harvey Tananbaum launching a rocket from a platform in the Indian Ocean; Subrahmanyan Chandrasekhar, alone in his room at Cambridge, working out the theory of white dwarfs, and Stephen Hawking, confined to a wheelchair, barely able to move, yet spinning out brilliant and beautiful mathematics.

To Greenstein, the process is almost more rewarding than the results. “Good science does not always involve being right,” he says, “at least not in every detail. It is far more important to paint the broad outlines of a subject, to discover entirely new phenomena and to point the way to new and fruitful investigations.

“Intuition and taste have an important role to play in science,” he tells us. “Such terms as ‘belief,’ ‘faith’ and the like may sound strange. But they should not. The impression that science is solely guided by logic rests on a lack of appreciation of the layers upon layers of ambiguity that actually surround the profession.”

One of his many themes is the interaction of science and technology. Which drives which? On the one hand, progress in astronomy is inextricably tied to the invention and development of telescopes. On the other hand, “If every computer on Earth were magically to vanish, I have no doubt that science would continue to flourish,” Greenstein writes. “The computer is only a tool,” and discoveries are made “by the workings of a creative mind.”

The book illustrates the interplay between theory and observation. Neutron stars were predicted long before they were found. Ditto black holes, only one of which has ever been found, and that by accident. Einstein’s theories of physics have been found to have remarkable explanatory power in the far reaches of the universe, where they are just as paradoxical yet inescapable as they are close by.

In the midst of all this, Greenstein’s book is distinguished popular science, conveying the facts and the theories as they are currently known and believed. Discoveries in astronomy are made with such rapidity these days that it is sometimes difficult to keep all the elements straight and remember what relates to what, or what refutes what. Greenstein draws it all together into a comprehensive whole, and his most characteristic and honest statement is “We don’t know.” Where there are competing explanations for phenomena, as there often are, he gives voice to them all, explaining the strengths and weaknesses of each.

From time to time Greenstein wanders off into literary digressions to illustrate things, which are less successful than his more straightforward prose. At one point he muses about the mind-body problem, which so enthralled Descartes, as if no one ever heard of it before.

But these minor quibbles do not detract from the power of this book and its paean to thought. “I find thinking enjoyable and I do it all the time,” Greenstein says. “Some people do crossword puzzles, others dive down to the model railroad in the basement. Me--I think.”