Ernst Julius Öpik, 1893-1985


{ M. de Groot Geodeet 6(30) 1994 11-14 }


Reflections on the life, works and personality of Estonia's greatest astronomer.


M. de Groot


Armagh Observatory, Armagh, Northern Ireland


Ernst Julius Öpik, 1893-1985, Eesti suurima astronoomi elu, töö ja isiksuse peegeldusi.

Autor, kes E.J. Öpiku elu lõpu-aastail puutus temaga ka isiklikult kokku, seostab edukust ülevaates loetletud laial skaalal E.J. Öpiku isiksuse omadustega. Hoidumine oma isiku ja vajaduste esiletõstmisest, tähelepanelik suhtumine teiste esitatud ideedesse, mis vajadust mööda oli humoorikas või ka järeleandmatu ja isegi terav, oma teemakäsitluste sihipärane kõigekülgsus (nn. morfoloogiline meetod) ja füüsikalise põhjendatuse eelistamine tihti subjektiivsele matemaatilisele elegantsile - need iseloomujooned võimaldasid E.J. Öpikul teha märkimisväärne panus paljudes valdkondades muusikast kosmoloogiani ja olla sealjuures mitme nähtuse mõistmisel hulk aastaid oma kolleegidest ees. Mitte kõigil juhtudel pole tema prioriteet üldtunnustatud, sest ta avaldas oma töid ka väljaandeis, mis polnud kõige loetavamad, uskudes, et tõde jääb tõeks igal pool. E.J. Öpiku mitmekülgsest tegevusest võib praktiliselt igaüks saada mõjutusi, nii et Eestimaa ühe suurima poja vaim elab sel viisil edasi.


Ernst Julius Öpik was bom on the 23rd of October 1893 and had an active part in the 20th century astronomy and astrophysics for some 70 years (1912 to ~ 1982). This was the century when our knowledge of the Universe evolved faster than at any other time or than in almost any other discipline. Right to the end of his active life he was able, not only to keep abreast of the many new astronomical disciplines that shot up like mushrooms after a rainy day, but also to comment on these developments in a way that gave evidence of deep insight into the mysteries of the Universe.


How did he do it? We would like to know. We fear, though, that, even if we knew his secret completely, we would be able to follow in his steps only at a very respectable distance behind him. I think his main secret was his personality. Let me try to paint a picture of his personality by reminding you of some of the work he did, some of the comments he made, and some of the things he said. I have made wide use of the contents of the two issues of The Irish Astronomical Journal mentioned in the references at the end of this paper.


Öpik the Musical Composer


An evaluation of Öpik's musical achievements is found in IAJ, Vol. 10, Special Issue, 1972. Characteristic are his independent style (was that because there is no record of his musical training, teachers, or similar circumstances?), the numerous changes in key, meter, and dynamic range (as if to say, "Now we have followed this idea for long enough, let's try something else"), the extensive notes for the performer, and the frequent pauses punctuating his ideas.


After leaving his native and beloved Estonia in 1944, and a few years of involvement with the Baltic University in West Germany, Öpik and his family came to Armagh in 1948. This was made possible through the efforts of the then Director of the Armagh Observatory, Dr. E.M. Lindsay. Lindsay had been a student of Öpik at Harvard and managed to obtain funds from the Northern Ireland Government for the creation of a special post of Research Associate at the Armagh Observatory. Öpik stayed here until the end of his life, despite various tempting offers to relocate in the USA. We find this "predicted" in the correspondence with Dr. Lindsay before his arrival at Armagh: "questions of salary and personal ambitions have never played a part in my life, at least have never influenced my decisions. Given the possibilities for research, I do not think that I shall feel inclined to look for anything else".


His work, including "his" News & Comments in The Irish Astronomical Journal, covered almost all areas of astronomy and astrophysics. A few examples follow.


The Physical Theory of Meteors


In 1933 Öpik wrote about "Atomic Collisions and Radiation of Meteors". After 1950 he refined his earlier ideas and wrote a monograph entitled "Physics of Meteor Flight in the Atmosphere" (1958). This required a number of different investigations like the calculation of the interaction potential between a meteoric atom and an atom of an air molecule, the calculation of the rate at which a meteoric atom loses energy because of the elastic collisions it suffers in passing through the air; and the calculation of the cross-section for excitation and ionization in collisions between the various species of atoms concerned. Öpik handled all of these with great skill, and when, in 1967, new observations were compared to his calculations based on the 1933 theory, the latter were found to be correct within 25%!


Large Meteorite Impact


His theory of large-meteorite impact was applied to terrestrial and lunar meteor craters as well as to the evaluation of meteor hazards for space vehicles. Not only for this reason was he frequently consulted by NASA on the dangers likely to be encountered by space missions. His wide interest and expertise in Solar System studies made him a valuable consultant in almost every area of space exploration.


Statistical Celestial Mechanics of Collision and Survival of Stray Bodies in the Solar System


Öpik's studies in this area had led to his "Note on Stellar Perturbations of Nearby Parabolic Orbits" in the Proceedings of the American Academy of Arts and Sciences (1932). In this paper he expressed his unbelief in hyperbolic orbits for meteoroids and made his first suggestions about what would later be known as the "Oort Cloud". It was partly because of the "obscurity" of these Proceedings for astronomers that this first suggestions about a comet cloud around the Solar System was not recognized and later (in 1950) proposed afresh by Oort.


These early dynamical studies of comets were replaced by more physical studies after 1950. This led to "Comet Nuclei as the Principal Source of Meteorites" (1966) which is an important contribution to the early history of the Solar System and, with other works on the subject, led to Whipple's remark that "no discussion of the origin of the Solar System can be adequate without full reference to Öpik's contributions on the subject of planetary perturbations of comets".


Studies of Planetary Atmospheres and Surface Conditions


1950 was an active year for Öpik's planetary studies. First he expressed his conviction that the surface of Venus should have the character of a desert. Then he followed up on one of his earliest involvements in the study of the Martian atmosphere (started in 1912) by stating his opinion that the surface of the Red Planet should contain hundreds of thousands of meteor craters larger than the one in Arizona. Both "predictions" were duly fulfilled by the Mariner spacecraft. Mariner II found Venus hot, dry, dusty and wind-swept, and Mariner IV found numerous craters on Mars.


Stellar Structure and Evolution


In 1916 Öpik had calculated the density of 40 Eri B as 25,000 times that of the Sun. This was within a factor three of today's value, but Öpik rejected this result and missed the "discovery" of white dwarfs. His further work on binaries showed in 1924 that the spectral sequence is not evolutionary as had been the general opinion until that time.


In 1922 Öpik predicted thermonuclear reactions as stellar energy sources. This was elaborated further in 1938, just before Bethe's more complete work. Through these studies Öpik was able to show that the mixing of the products of nuclear fusion in the interiors of stars led to the production of giant stars. Some of Öpik's ideas on stellar structure were not recognized as valid until the early 1950s when Schwarzschild and co-workers confirmed them through detailed calculations. These studies in stellar evolution led also to consideration of models for the Sun and the cause of the Ice Ages.


The Ice Ages


The multi-disciplinary aspects of the question of the cause of the Ice Ages allowed Öpik full swing for applying the Morphological Method ("taking into account all known data and factors which could influence a physical model, theoretical or experimental", Öpik, 1977a) of which he was very fond.


Öpik    considered    a    number    of    possible mechanisms as the cause of the Ice Ages. Continental drift as a main factor was discarded because it has not been sufficiently active over the last 500,000 years during which there were several glacial periods. The mechanism of atmospheric imbalance advocated by some also came under his criticism: Öpik argued for stabilizing atmospheric influences instead. Milankovitch's widely adhered-to "astronomical theory", involving the secular changes in the Earth's orbital parameters was rejected on a number of scores. Nor did he have much sympathy for the mechanism whereby changes in the interstellar medium would produce Ice Ages. He saw the resultant increase in solar luminosity as opposed to cooling of the Earth. Thus, already in 1938 he concluded that solar luminosity variations should be considered as the main contributing factor. The following statement is typical of his approach to complex problems like this:


"We believe that a mere qualitative picture, taking into account all the complexity of the conditions in stellar interiors, is still a better approximation to the truth than an exact mathematical theory based on simplifications which do not take into account certain most important factors of stellar structure and evolution."


Modern observations have shown clearly that the Sun is a variable star and that climatologists' silence about solar luminosity variations looks like some kind of reverence for the Sun as if it were a deity unaffected by change and decay. Öpik's more than 50 years of research on this subject alone, yielded more than 30 papers.




Two different estimates of the distance to M31 were given by Öpik in 1921 and 1922. The mean of these two values is 670,000 pc which is within 10% of the modern value; and this before the "Great Debate" and the subsequent work of Miss Leavitt! In 1933 Öpik arrived at the conclusion that there must have been a single event at the start of the present Universe. This was, again, before others came to the same conclusion through the work of Hubble and Tolman.


Öpik's use of the word present already betrayed his sympathy for the Oscillating Universe. But it was not until Bondi et al. launched their Steady-State Cosmology that the discussion about the origin of the Universe really took off. Öpik rejected the SSC as "a magnificent effort of intuition unsupported by experiment; .... unrivalled in modem science; it carries us back to the era of the great Greek philosophers who were constructing their own universes by pure imagination."


Thus, in Öpik's opinion, only the expanding and the oscillating Universe models had any validity. In a time when he saw the Hubble constant reduced from 580 to about 50 km/s/Mpc, corresponding to a ten-fold "expansion" of the Universe, Öpik still preferred the Oscillating Universe, and this was the subject of a popular book which appeared in 1960 and was subsequently translated into several languages. Öpik argued that a multi-universe model also favours the Oscillating Universe because with infinitely expanding universes in a multi-universe model, the various universes would merge and cause chaos. Nevertheless, he called the idea of the plurality of universes an "etymological paradox", but admitted that a "complete rethinking of all world models becomes necessary".


In a tribute to Öpik's all-round knowledge, especially as expressed in "his" News and Comments in The Irish Astronomical Journal, Dorrit Hoffleit (1972) commented that among the necessary ingredients to progress


"a greater need following upon dreams and extrapolations is relentless criticism by an expert, preferably one with a sense of humour (so often displayed in the pungent wording of Öpik's refutations), who cannot refrain from testing and exhibiting both the plausibility and the fallacies or inconsistencies of any new theory. Without such critical evaluations, advances in the science would be haphazard and uncertain."


Yes, Öpik could be very sharp, and as an expert critic   he   could  be   relentless   if  he   thought  the circumstances demanded it.


Other Factors


Öpik's disinterest in personal gain, already referred to above, was also apparent on other occasions. For instance in 1932 when Shapley offered him the post of professor at Harvard; Öpik recommended Menzel instead! The same attitude is also borne out by the fact that he was not afraid to publish in what others called "obscure" journals. He was not seeking in the first place his own honour, but was always loyally promoting his own institute. Even a quick look at the publications produced by the Armagh Observatory between 1950 and 1980 should convince anybody of this loyalty. He reasoned that truth is truth wherever it is published and that those who genuinely seek after truth must be prepared to do a lot of research, and that not only in the "popular" journals.


His plea for the free exchange of Institute Publications in his article "About Dogma in Science, and Other Recollections of an Astronomer" (1977b) also reveals the same attitude. Mainly due to economic pressures this practice disappeared almost completely shortly before the wide-spread introduction of desk-top publishing which, but for continually rising postal costs, might have saved this practice. The same attitude is also reflected in the way Öpik dealt with papers by new authors and with requests for reprints. He would consult the volumes of Astronomy and Astrophysics Abstracts to get a broader view of the interests of his correspondent, and then send a selection of his own reprints that he thought would be of interest. He called his reprints "seeds", mindful of the fact that there can be no growth if where there is no sowing.




I have tried to illustrate some characteristics of Dr. Öpik's personality from the way he dealt with various astronomical problems, from his reactions to efforts by other people to solve the same or different problems, and from the way he expressed his feelings in private conversation and in his musical compositions.


A final question to ponder: Have you found your Öpik connection? He covered so many subjects that something he wrote must have touched your research and, maybe, your life. He may even have caused a spark of inspiration in your mind. If in any way you have thus been touched by his life and works, then Ernst Öpik's life and works have not been in vain, and then the spirit of one of Estonia's greatest sons continues to live.




1. Hoffleit, D. 1972,1.A.J. 10 (Special Issue), 34.

2. Öpik, E.J. 1977a, I.A.J. 13, 95.

3. Öpik, E.J. 1977b, Ann. Rev. Astron. Astrophys. 15,1.


For further reading:


The Irish Astronomical Journal,10, Special Issue, 1972

The Irish Astronomical Journal,17, pp. 411 - 442, "Ernst Julius Öpik,   1893-1985, The Man and the Scientist"