Those little chaps called quasars

It is a sobering thought that if forces balancing the gravity of the Sun were to disappear, the Sun would collapse to a point within 29 minutes!

The phrase “gravitational collapse” might suggest some kind of disease. Indeed, my article on this topic in a popular science journal elicited several reprint requests from medical practitioners. This phrase was established in astronomy in 1963, when astronomers began to discover a family of strange, new kind of objects that were subsequently named “quasars”.

For me, quasars and gravitational collapse bring up memories of a ringside seat from where I could watch the unfolding of phenomena hitherto unimagined in the annals of astronomy. My story begins in 1961, with the visit paid by English astronomer Fred Hoyle and me to Hanbury Brown, an experienced radio astronomer at the Jodrell Bank Observatory, UK. The celebrated 250-feet diameter radio dish greeted us as we drove Fred’s sporty car into the observatory’s porch. Hanbury led us to his spacious office littered with sheets and sheets of data.
Hanbury lost no time in coming to the point. Showing us a graph paper with several points marked on it, he stated the puzzle: “These points over here are the standard radio sources. We know them as large systems. (A typical source may extend as far as 500 thousand light years!) But here is another new population of compact objects. They are small but powerful emitters of radiowaves. They may be as powerful as the big ones over here, in spite of being very small in size. So what are these little chaps?”
This was indeed a challenge to theorists like Hoyle and an incentive to a new entrant to the field like me. However, there was one problem. What did these compact sources look like when seen through an optical telescope? So far, their studies had been carried out by radio astronomy. And in the early 1960s, astronomers were still guided by the old adage “seeing is believing”. That is, they would like to see these sources through an optical telescope. An optical telescope sees the universe through the same type of lightwaves that we humans do.
To find the optical object corresponding to the radio one, the astronomer needs a very precise location of the latter. To see why, imagine a classroom in which the teacher hears a whistle. To find the perpetrator of this unruly act she directs her attention to the bench from where she heard the noise emanating. There are three boys sitting on that bench. Which one of them sounded the whistle? If the teacher has good hearing, she will be able to identify the boy; otherwise she cannot be sure. Likewise, if the position of the radio source is known accurately, the astronomer can more easily locate and identify the optical object with the radio one.
As it happened, in the following year, one of Hanbury’s compact sources was going to be occulted (the passage of a celestial body across a line between an observer and another celestial object) by the moon. That is, the lunar disc was going to come between the Earth and the source, just as it does at the time of a total solar eclipse, blocking out the Sun. Because the astronomer knows the moon’s path accurately, such an occasion provides us with a unique opportunity of measuring the position of the source very accurately.
The source in question was catalogued as 3C273, telling us that this was the 273rd source in Third Cambridge Catalogue of Radio Sources (3C). Realising that the information about the source would increase considerably if they could see it through an optical telescope, astronomers were keen to perform the lunar occultation experiment. The experiment was carried out in Australia. Fred Hoyle has described how the event was managed.
For hours before the occultation, all local radio stations broadcast repeated appeals: that no one should switch on a radio transmitter during the critical period of observation. All roads leading anywhere near the telescope were patrolled to make sure that no cars were in motion in the vicinity. A final, somewhat macabre touch: After the observations the observers carried duplicate records back to Sydney, in separate planes.
Did the final outcome justify all these precautions? It did, more than expected. With the position of the radio source known accurately, optical astronomers were able to identify a compact star-like object with it. More detailed optical studies showed that the object was not a star but a highly compact and much more luminous source, the like of which had not been seen before. Because of its stellar appearance it was called a quasi-stellar object (QSO or a quasar) and its luminosity was estimated as even more than that of our Milky Way galaxy.
Thanks to several new methods of observing, the number of QSOs recorded in various catalogues has long crossed the 10,000 mark. But the breakthrough in understanding them was made by Fred Hoyle and William Fowler in early 1963. Hoyle and Fowler had wondered whether very massive stars may exist in the universe and may explain the QSOs. Can one, for example, think of a QSO as a star as massive as a million or even a billion Suns? In the case of the Sun, the internal forces are strong enough to withstand the tendency to shrink under gravity. It is a sobering thought that if forces balancing gravity of the Sun were to disappear, the Sun would collapse to a point within 29 minutes!
Hoyle and Fowler found that in the QSOs, the force of gravitation of such a massive star far exceeds any other force present with the result that the massive object starts shrinking. Worse, the dominance of gravity grows in the process and the slow shrinkage gives rise to rapid collapse. This is the origin of the phrase “gravitational collapse”. What is the end state of this collapse? The theory of relativity tells us that such an object would end up as a black hole. Because of its strong gravity, a black hole prevents even light from escaping out from its surface.
Black holes cannot be seen but their existence is inferred from the dynamical activity around them. For they have strong attractive force pulling matter towards them. Thus, the “little chaps” of Hanbury Brown opened strange new vistas of the universe.

The writer, a renowned astrophysicist, is professor emeritus at Inter-University Centre for Astronomy and Astrophysics, Pune University Campus

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