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The discovery of quasi-stellar objects (QSOs or quasars) in 1963 represents a landmark in observational astronomy. Thanks to a coordination between optical and radio astronomers, it was possible to discover a new and important class of astronomical objects. Because this text book is all about quasars and related phenomena, it will not be out of place to begin at the beginning of the subject and to review briefly how these remarkable objects were first discovered.
The science of radio astronomy really began after the end of World War II, when some of the scientists and engineers engaged in wartime radar projects used their know-how to follow up the pioneering works of Karl Jansky in the 1930s and Grote Reber in the early 1940s. Thus radio dishes and interferometers appeared in England and Australia, at Jodrell Bank, Cambridge, Sydney and Parkes.
The early observations revealed the existence of cosmic radio sources and by the mid-1950s it became an accepted fact that radio galaxies exist. The nature of their radiation was non-thermal, and its polarization properties indicated that its origin lay in the synchrotron process. As we will discuss in Chapter 3, in this process radiation comes from electrons accelerated by a magnetic field. Thus a typical radio source has as its energy reservoir the dynamical energy of relativistic particles and magnetic field energy.
In 1958 Geoffrey Burbidge drew attention to the enormous size of this energy reservoir. Since we will go through his argument in Chapter 3, we simply state the result here. He found that the minimum energy available is of the order of 1060 erg! This estimate would go up further if one included the energy of the protons as well as that of the electrons. Indeed, so large was this estimate that it was the last nail in the coffin of the collision hypothesis, which sought to explain the radio source phenomenon as the outcome of colliding galaxies. The typical gravitational potential energy of a pair of colliding galaxies of masses M1 and M2 separated by a distance R is -GM1M2 / R. For typical galactic mass ~ 1011 M, where M is the mass of the Sun, and R ~ 10 kpc, we get an answer ~ 1059 erg. This calculation also assumes an optimistic situation where one energy reservoir can be converted to another with perfect efficiency, whereas astrophysical processes seldom exhibit high efficiency. The mass-energy conversion in the thermonuclear fusion of hydrogen to helium operates with an efficiency of 0.007. Thus even a 10 per cent efficiency would require the primary source of energy in a radio source to be of the order of 1062 erg. What can such a source be like?
Burbidge himself had suggested that the source could lie in a chain reaction that triggers off one supernova after another in the nuclear region of a galaxy. Since the nuclear region is expected to be much denser than average, and supernovae do pour out a lot of energy in an explosive fashion, this idea seemed plausible. However, it was not pursued in detail.
Instead, in 1962, Fred Hoyle and William A. Fowler proposed that the nuclear region of a galaxy may permit the formation of a supermassive star having, say, a million or more solar masses which would evolve to become a ``super-supernova'' thus generating the requisite energy from a thermonuclear source. However, by this time, Hoyle and Fowler had come to realize that a thermonuclear source of energy will not be as efficient as a gravitational one for masses of this order. This is clear from a qualitative argument to start with: the nuclear energy increases in proportion with the mass whereas the gravitational potential energy increases as the square of the mass. Thus while at the solar-mass level the former wins over the latter (vide the historical argument that dethroned the Kelvin-Helmholtz contraction hypothesis in favour of thermonuclear energy), the situation is the exact opposite for supermassive stars. But how can one tap the gravitational energy?
In a pioneering paper in Nature in early 1963, Hoyle and Fowler proposed that the energy for such sources was of gravitational origin, being derived from the collapse of very massive objects under their own strong gravitational fields. They pointed out that for very massive objects the internal pressures are inadequate to withstand this force of gravity. The objects therefore start contracting and as they contract the force of gravity grows, thus widening the gap between the inward and outward pressures. The situation becomes unstable, leading to the phenomenon of gravitational collapse. Thus the stage was set on theoretical grounds for highly collapsed supermassive objects as sources of high energy.