Our story ends where it began, with physicists scrutinizing the night skies for clues about the universe we inhabit. In the first two chapters, we saw Galileo confirming with his telescope the Copernican message that Earth and other planets orbit the Sun, and Newton building his universal gravitation theory to calculate the planetary orbits and the motion of all the other heavenly bodies. In this part of the book, we see some of the great strides taken in the twentieth century by physicists in their efforts to map our universe, define its dynamics, and write its history. The main characters in the modern story are an astronomer, Edwin Hubble; an astrophysicist, Subrahmanyan Chandrasekhar; and a cosmologist, Stephen Hawking.
Edwin Hubble was the first to identify galaxies beyond our own. He used the greatest telescopes of his time to estimate distances in this extragalactic realm. Then he made a careful study of the colors of distant galaxies and found that the greater the distance to a galaxy, the more its color shifted toward the red. He proposed a simple linear relationship between distance and this “redshift.” Because the redshift of a galaxy can be interpreted to mean that the galaxy is moving away from us, Hubble's data suggested that the universe as a whole is expanding. Hubble was at first cautious about adopting this interpretation, but others were more easily convinced, and the first steps were taken in the development of the now dominant “big bang” theory of the origin and history of the universe.
Subrahmanyan Chandrasekhar (or “Chandra,” as he was known) was a man who excelled in probing the complexities of stellar physics. In his long career as an astrophysicist, he studied stellar structure, dynamics, and evolution. One of his last efforts was an investigation of the mathematical theory of “black holes,” massive