Presentation Speech by Professor Sven Johansson of the Royal Academy of Sciences

Translation from the Swedish text

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

Astrophysics is one of the areas in physics which has developed most rapidly during recent years. Through satellite technology it has become possible to study the different physical processes which are taking place in stars and other astronomical objects. Space has become a new and exciting laboratory for the physicist. It is true that experiments, in the proper sense of the word, cannot be carried out, but one may observe phenomena which can never be observed in terrestrial laboratories. In space we find matter in the most extreme forms; stars at immensely high temperatures and with enormously high densities, and particles and radiation with an energy which we cannot reach, even with our largest accelerators.

The common theme for this year’s prize in physics is the evolution of the stars. From the moment of their birth out of interstellar matter until their extinction, the stars exhibit many physical processes of great interest. In order to put this year’s prize in perspective, it is perhaps appropriate to give a short description of the evolution of the stars.

Stars are formed from the gas and dust clouds which are present in galaxies. Under the influence of gravity, this matter condenses and contracts to form a star. During these processes energy is released which leads to a rise in the temperature of the newly formed star. Eventually, the temperature becomes so high that nuclear reactions are initiated inside the star. Hydrogen, which is the primary constituent, burns to form helium. During this process pressure builds up which prevents further contraction, the star stabilizes, and may continue to exist for millions or thousands of millions of years. When the supply of hydrogen has been used up, other nuclear reactions come into play, especially in more massive stars, and heavier elements are thus formed. A particularly effective type of nuclear reaction is the successive addition of neutrons. Finally, the star is, to a large extent, composed of heavier elements, mainly iron and neighbouring elements, and the supply of nuclear fuel is exhausted. When the star has evolved this far it can no longer withstand the pressure of its own gravitational force and collapses, the product of collapse depending on the mass of the star.

For lighter stars with a mass roughly equal to that of the Sun, the collapse results in a so-called white dwarf. The star is so named because of its reduction in size, leading to an increase in its density to about 10 tons per cubic centimetre. The mechanism for the collapse is that the electron shell structure is crushed, so that the star consists of atomic nuclei in an electron gas.

For somewhat heavier stars, the collapse can lead to an explosion, the visible result being a supernova. This is accompanied by a short-lived but intense neutron flux which leads to the formation of the heaviest elements. In these heavy stars the collapse can go even further, the atomic nuclei and the electrons combining to form neutrons. This results in a so-called neutron star which has the enormously high density of 100 million tons per cubic centimetre. A star with a mass of 1 to 2 times that of the Sun may be compressed so that the radius is only about 10 km. A neutron star is essentially a sphere of neutrons in a fluid form surrounded by a solid crust which is very much harder than steel.

The collapse of still heavier stars can lead to an even more exotic object, a black hole. Here the gravitational force is so strong that all matter which is sucked into the hole loses its identity, and is compressed into an infinitely small volume, i.e. a mathematical point. Not even light, emitted from within the black hole, may escape into the outside world, hence the name, black hole. The existence of a black hole may be revealed through the radiation which is emitted by matter which, when being sucked into it, undergoes a considerable increase in temperature before finally disappearing. Certain strange objects called quasars may possibly be a black hole in the centre of a galaxy.

It should now be clear that during their evolution stars exhibit many different physical processes of fundamental importance. Many scientists have studied the problems involved with these processes, but especially important contributions have been made by Subrahmanyan Chandrasekhar and William Fowler.

Chandrasekhar’s work is particularly many-sided and covers many aspects of the evolution of stars. An important part of his work is a study concerning the problems of stability in different phases of their evolution. In recent years he has studied relativistic effects, which become important because of the extreme conditions which arise during the later stages of the star’s development. One of Chandrasekhar’s most well known contributions is his study of the structure of white dwarfs. Even if some of these studies are from his earlier years, they have become topical again through advances in the fields of astronomy and space research.

Fowler’s work deals with the nuclear reactions which take place during the evolution of stars. Apart from generating the energy which is emitted, they are important because they lead to the production of the chemical elements from the starting material, which mainly consists of the lightest element, hydrogen. Not only has Fowler carried out a great deal of experimental work on nuclear reactions of interest in the astrophysical context, but has also worked on this problem from a theoretical point of view. In the 1950’s, together with a number of colleagues, he developed a complete theory for the formation of the chemical elements in the Universe. This theory is still the basis of our knowledge in this area, and the latest advances in nuclear physics and space research have further shown this theory to be correct.

Professor Chandrasekhar and Professor Fowler,

Your pioneering work has laid the foundation for important developments in astrophysics and you have both been the source of inspiration for other scientists working in this field. The remarkable achievements of astronomy and space research in recent years have vindicated your ideas and demonstrated their importance.

It is my privilege and pleasure to convey to you the warmest congratulations of the Royal Swedish Academy of Sciences. May I now ask you to come forward and receive your prize from the hands of His Majesty the King.