Cosmic journey in space-time

FROM WHITE DWARFS TO BLACK HOLES - The Legacy of S. Chandrasekhar: Edited by G. Srinivasan; Orient Longman Ltd., 160, Anna Salai, Chennai-600002. Rs. 350.

IN A sense this book, a collection of 11 essays by some of the leading astrophysicists of today, can be considered a tribute to S. Chandrasekhar (1910-95), the India born U.S. astrophysicist, who is known for his theory of white dwarfs. He studied in Presidency College, Madras and, later, in Cambridge before moving to the U.S. in 1936.

He was barely out of his teens when he wrote his first scientific paper titled ``Thermodynamics of Compton Scattering with reference to the Interior of Stars''. Inspired by Fowler's work based on Fermi-Dirac statistics on the stability of white dwarfs, Chandra, as he was popularly known, turned his attention to investigate the final stages of stellar evolution.

What happens to a star when its nuclear fuel is exhausted? Chandra showed that when the star runs out of its nuclear fuel, an inward gravitational collapse occurs. Normally this collapse is halted by the outward pressure exerted by the star's highly compressed and ionised gases.

Once this stage is reached, the star becomes an extremely dense white dwarf with a peculiar property that the greater its mass, the smaller its radius. This is a situation that excludes massive stars from becoming white dwarfs.

In other words, there is a limiting factor controlling the mass of the star and this limiting stellar mass is called the Chandrasekhar limit and it is about 1.4 times the solar mass. All known white dwarfs in the universe have been found to conform to this limit. Like his illustrious uncle, C. V. Raman, Chandra also won a Nobel Prize, in the year 1983.

Roger Penrose, the British theoretical physicist, who is known for his contribution to the theories on black holes, is among the 11 luminaries who have contributed to this volume. His research revealed many of the properties of black holes. A large star becomes a black hole when it collapses and reaches a density that is so high that not even light can escape the star.

The region within which light cannot escape is known as the ``event horizon''. According to Hawking and Penrose there is a space-time singularity at the centre of the black hole and Penrose further showed that because of the existence of event horizons these singularities cannot be observed from the outside.

In 1970 the Hawking-Penrose refrain was that the universe must have begun as a singularity, the harbinger of the Big Bang if one may call it so.

In this book Penrose's essay ``Chandrasekhar, Black Holes and Singularities'' begins with a historical background tracing Chandra's work on white dwarfs and the controversy it gave rise to in Eddington's mind. Chandra, as Penrose says, chose not to mount a direct attack on the problems of gravitational collapse.

This was in the early 1930s, an era when Einstein's general theory of relativity had not yet caught on, and there were many other issues that would occupy Chandra's attention for the next quarter of a century. In conclusion, Penrose refers to how Chandra's lifetime work comes back a full circle - from massive white dwarfs to collapse of space-time singularities where, in spite of Chandra's ingenuity and industry, some questions yet remain unanswered.

The editor of this collection has himself contributed an essay titled ``Stars: Their Structure and Evolution''. Being the very first essay in the book, it begins with an introduction to the study of stars, the very subject that gave a kick start to the study of astronomy itself.

Like some of the other essays in this compilation, his essay handles the subject with a liberal sprinkling of equations and mass-radius relations while discussing the theory of white dwarfs, the way stars are, the stellar evolution and, finally, what fate awaits the massive stars.

Through these essays the book takes the reader on a cosmic journey that puts Chandra's work in its perspective, giving an insight into the highly cherished work done in half-a-century by one of the most distinguished astrophysicists of our time.

However, this book is not meant for the general reader as it calls for some grounding in physics, with an emphasis on the basics of astronomy.

C. V. SUBRAMANIAM

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