EJ Öpik, one of the most outstanding astrophysicists of his generation, came to Armagh Observatory in 1948 as a refugee from Eastern Europe. He had for a while been head of the Astronomy Department at the University of Tashkent, Astronomer at the Tartu Observatory, Estonia and later Rector of the Baltic University, Hamburg.
In the tranquil surroundings of Armagh Observatory, he wrote prolifically on a wide range of astronomical subjects until he retired in 1981. His published papers on Astronomy extend over a period of over seventy years from 1912 to his death in 1985.
His discoveries, which were many and various, include:
- The discovery of degenerate stars, i.e., white dwarfs, in his calculation of the density of o² Eridani, in 1915.
- The first ‘proof’ of the extragalactic nature of M31, the Andromeda galaxy, in his calculation of the distance of M31 as 450,000 parsecs from the Sun. He obtained this result, which is close to the modern value, by dynamical considerations, in 1922.
- The computation of evolutionary models of main-sequence stars into giants, carried out in the 1930’s, over a decade earlier than the computations of Hoyle and Schwarzschild.
- The prediction of the density of craters on the surface of Mars which were confirmed 15 years later by planetary probes.
Öpik’s theory of the Ice Ages. - His theory of the internal structure of the Sun, which he developed to explain the Ice Ages, was recently resuscitated when it was realised that it was capable of explaining the solar neutrino flux.
His theory of the internal structure of the Sun, which he developed to explain the Ice Ages, was recently resuscitated when it was realised that it was capable of explaining the solar neutrino flux.
In recent times there has been considerable media coverage of possible changes in the earth’s climate brought about by man’s activities. This might lead one to suppose that the earth’s climate would normally be stable and unchanging. This, in fact, is not the case, as through unknown natural causes, the earth’s climate has often changed in the past. Most notable amongst these changes are the periodic ice-ages, when vast ice sheets covered much of Europe and North America.
From geological studies we know that the ice-ages typically last several hundred thousand years and are separated by long hot periods lasting for up to 250 million years. The ice-ages themselves are broken up by short, temporary, recessions of the ice, known as ‘inter-glacial periods’. It is just such a period that we are now in. The inter-glacial periods last for 10-30 thousand years; a very short time on the geological time scale.
It has been evident to scientists for many years that the ice-ages are periodic and many explanations for this periodicity have been proposed. One of the leading theories of the origin of the ice-ages was developed and published by Öpik at Armagh.
Öpik suggested that the rate of energy production in the centre of the sun varied from time to time, due to changes in the pattern of convection in the sun’s core. However, due to the way the sun’s atmosphere reacts to changes in energy production, the solar energy incident on the earth’s surface, would actually drop as the core of the sun heated up. It has not yet been possible to verify or reject Öpik’s theory although it received some further study recently when it was realised that it might be capable of explaining the low neutrino flux that has been observed from the sun by the new underground neutrino detectors.
Öpik's theory of the evolution of the Sun
Probably Öpik’s most important contribution to science was his study of the evolution of stars, published in 1938. In this pioneering paper he discussed the various processes that would follow the conversion of hydrogen into helium by thermonuclear reactions in the centre of the Sun and other stars. He showed how, when eventually the hydrogen had become exhausted, the central core would contract and its temperature rise above 10,000,000ºC. At the same time the outer atmosphere would expand until, in the case of the Sun, it filled the orbit of the planet Venus. By this time, with the increased energy radiated onto the earth and the close proximity of the solar surface, the oceans would boil and the earth would become a scorched and dead planet. It is now generally believed by astronomers that this overall picture is correct and that the expansion of the sun will terminate all life on earth in about 5,000 million years from the present.
Öpik’s remarkable study to show how stars evolve in time, was carried out using hand calculations. The results he obtained were finally confirmed about ten years later by Hoyle and Schwarzschild using electronic computers.
