Ernst Öpik

Ernst Julius Öpik was born on 22 October 1893 at Kunda, a small harbour and cement town on the Virumaa coast of northern Estonia. Some sources give 23 October and call the place Port Kunda; the difference in date is most likely the old and new calendars. His father, Karl Heinrich Öpik, was harbourmaster at Kunda, described elsewhere as the local customs official and harbour constable; his mother was Leontine Johanna, née Freiwald. There were six sons and one daughter. From 1888 to 1900 the family lived at Lontova, the settlement beside Kunda harbour, so the childhood home was really Lontova even though the birth is registered to Kunda.

In 1900 the family moved from Kunda to Tallinn. Ernst entered the Nikolai Gymnasium there, today the Gustav Adolf Gymnasium, and did so well in the entrance examinations that he was exempted from tuition fees. He graduated in 1911 with a gold medal. Astronomy started while he was still at school: with his classmates he founded a society called Vega, and they managed to get hold of a telescope with a three-inch lens. The other possibility was music. Öpik was a polished pianist with perfect pitch, and he considered a career in music seriously before he chose astronomy.

In the autumn of 1912 he entered Moscow University rather than Tartu, because in Moscow he could keep himself by giving private lessons. His astronomy professors were Vitold Tseraski and Pavel Sternberg, but he was largely self-taught and claimed he had nothing to learn from them. He had come to work on the minor bodies of the Solar System: asteroids, comets and meteors. Between 1912 and 1916, still an undergraduate, he produced 18 articles. In 1915 he determined the dynamical density of matter in the disk of the Galaxy. He took his degree in 1916 and then taught at Moscow University and its Observatory until 1919.

One of the student papers held. In The Densities of Visual Binary Stars (Astrophysical Journal, vol. 44, pp. 292–302, 1916) Öpik computed the densities of visual binary components from their orbits, masses and luminosities. The companion of Omicron-2 Eridani, 40 Eridani B, came out at over 25,000 times the mean density of the Sun. That is his own 1916 figure, not a modern measurement. He called his own answer impossible. The arithmetic held; the physics that would explain it did not yet exist. The star is now known to be a white dwarf. Armagh Observatory dates the calculation to 1915; the paper appeared in 1916.

At the end of January 1919 Öpik left starving Moscow. More than a hundred faculty members and their families set out to staff a new university at Tashkent; Öpik was the only astronomer among them. The journey was 3,000 km and took 70 days, ending in the first days of April. At Turkestan University he was associate professor and head of the astronomy department, and his main task was the revival of the Tashkent Observatory. He stayed in Central Asia from 1919 to 1921. He wrote his own account of the trek only in 1977, in the Annual Review of Astronomy and Astrophysics.

He came back in 1921. Opposed to communism, Öpik had volunteered for the White army during the Russian civil war; now he returned to an independent Estonia and was appointed astronomer and associate professor at Tartu University. From 1 December 1921 he held the post of astronomer-observer at the Tartu observatory, a position that let him give himself entirely to research. Moscow and Turkestan were behind him; Toome Hill lay ahead, until 1944. In 1922, a year after he came home, he worked out there the distance to the Andromeda nebula.

Dictionary of Irish Biography: Öpik, Ernst Julius · P. A. Wayman, D. J. Mullan: Royal Astronomical Society obituary, QJRAS 27 (1986), 508–512 (MacTutor transcript) · MacTutor (J. J. O’Connor, E. F. Robertson): Ernst Öpik biography · ERR Novaator, 2018, after Laurits Leedjärv’s article in Horisont: Ernst Öpik – oma ajast ees kõiketeadja astronoomias · Wikiquote: Ernst Öpik · IOP Spark: Surprisingly dense stars · Armagh Observatory and Planetarium: Professor Ernst Julius Öpik (compiled by J. Butler) · Eva Vabasalu, Estonian World Review (eesti.ca), 2008: Ernst Julius Öpik – the man · Wikipedia: Ernst Öpik

The Treaty of Tartu opened the way home. Taavet Rootsmäe, who was David Rootsman until 1936 and had been professor of astronomy and director of the observatory at Tartu since 1919, invited Öpik back in person. Öpik was then in Tashkent, where he had spent two years setting up the observatory of the newly founded Turkestan University. On 1 December 1921 he took up the post of astronomer-observer at the University of Tartu Observatory. His title was astronomer and associate professor, and he held it from 1921 to 1944, that is, until he left Estonia. The post gave him what he needed: time to devote himself fully to research, and to help restore the standing the observatory had held in the 19th century.

The observatory on Toome Hill was begun in 1808 and finished in 1810, and it was some years more before the instruments were in place. Friedrich Georg Wilhelm von Struve worked in the building from 1813, became professor and director in 1820, and was the first to measure accurately the distance to a star, Vega. The Fraunhofer refractor that arrived in 1824 was the best observational instrument in the world: a 24.4 cm objective, the first successful German equatorial mount, and clockwork accurate enough to hold an object in view for a long stretch. With it Struve compiled a catalogue of 3,112 double and multiple stars. By Öpik’s time that was inheritance rather than a working tool; the science in the house had gradually declined.

Rootsmäe gave the lectures; Öpik did the research. He was the man who brought scientific work at Tartu back to world level after it had gradually declined. In 1923 he took his doctorate at the University of Tartu with a thesis on meteor observations. At Tartu he honed his double-count method for meteors and designed the rocking-mirror meteor camera, the photographic version of the same idea. And from here, in 1922, came his estimate of the distance to the Andromeda nebula, which the next chapter takes up. Much of his Tartu work went into the Publications of the Tartu Observatory, a series running since 1817 and then the oldest astronomical periodical in the world.

In 1930 Harlow Shapley, director of the Harvard College Observatory, invited him to the United States; his double-count method for meteors may well have been the reason. From 1930 to 1934 Öpik was a visiting scientist and lecturer at Harvard while remaining on the Tartu staff, and there he founded the meteor research group. Öpik led the Arizona meteor expedition with Harlow Shapley and Samuel L. Boothroyd from October 1931 to July 1933. About 22,000 meteors were registered over the two desert seasons; other accounts of the same campaign give 26,000, and the two figures have never been reconciled. The material was reduced afterwards in Tartu, between 1932 and 1934. Harvard offered him a professorship. He declined it. Tartu had become dear to him.

The most important work of the Tartu period appeared in 1938: Stellar Structure, Source of Energy, and Evolution, pages 1 to 115 of volume 30 of the Publications of the Tartu Astronomical Observatory. It was not a note but a monograph, and every stellar model in it was computed by hand. Öpik was the first to introduce compound unmixed models: a convective core of helium with a hydrogen envelope in radiative equilibrium above it. He showed that when the hydrogen at the centre of a star runs out, the core contracts and its temperature rises above 10,000,000 degrees Celsius while the outer layers expand. That is the red giant, and he reached it fifteen years before Hoyle and Schwarzschild; other accounts put the gap at about ten years. The same year he was elected to the Estonian Academy of Sciences.

German was the working language of the series in the 1930s. The 1934 volume carried Rootsmäe’s paper on the Orionid streams, Ein Beitrag zur Kenntnis der Orionidenströme, still bylined D. Rootsman, so the director and the astronomer-observer were working on meteor questions side by side. Öpik’s 1938 monograph was reprinted in part, with commentary, in A Source Book in Astronomy and Astrophysics, 1900 to 1975, edited by Kenneth R. Lang and Owen Gingerich and published by Harvard University Press in 1979. A hundred and fifteen pages of Tartu paper ended up in the canonical anthology of the century’s key papers.

ERR Novaator, 2018, after Laurits Leedjärv’s article in Horisont: Ernst Öpik – oma ajast ees kõiketeadja astronoomias · Wikipedia: University of Tartu Old Observatory (wording adapted from this article, CC BY-SA 4.0) · Armagh Observatory and Planetarium: Professor Ernst Julius Öpik (compiled by J. Butler) · M. Jõeveer, Ernst Öpik 75-aastane (Tartu Observatory Calendar 45, 1969) · Dictionary of Irish Biography: Öpik, Ernst Julius · Ernst Öpik (Wikipedia) · J. Einasto, Two hundred years of galactic studies in Tartu Observatory (arXiv:1010.3977) · Laurits Leedjärv, Hingepõhjast välja tunginud teadus: Ernst Öpik 125 (Sirp, 2018; copy at opik.fyysika.ee) · Publications of the Tartu Observatory, the 1934 volume (Tartu Observatory Museum)

The method was his own. A mirror rocked at a known rate in front of the observer, turning a meteor’s straight track into a pseudo-cycloidal, looping trace. Counting the loops, or cusps, gave the length swept in one rocking period, and so the meteor’s angular velocity; from that came the geocentric and finally the heliocentric speed. No photography was needed. Every meteor seen by eye got a velocity. It was called the rocking mirror, or the double-pendulum method. Öpik also designed the photographic version of the same idea, a rocking-mirror meteor camera in which the rocking element chops the trail on the plate so the speed can be read off the image.

He had been on meteors since Tartu: his 1923 doctorate at the University of Tartu was a thesis on meteor observations. It may have been the double-count method that led Harlow Shapley to invite him to Harvard Observatory in 1930, where Öpik founded the meteor research group. From October 1931 to the end of July 1933 he led the Harvard Arizona meteor expedition together with Shapley and Samuel L. Boothroyd. Two ‘meteor houses’ went up in the desert: huts that sheltered the observers, each window fitted with a grid of lines against which a meteor’s position could be read. Several stations observed at the same time.

The high desert air gave exceptionally clear skies, and two seasons produced a large haul. How large is still open: one account gives about 22,000 meteors, another 26,000, and neither figure has been confirmed against the expedition’s own reports. The point of the expedition was specific. Widely spaced cameras were to record meteor trails stereoscopically, to improve knowledge of meteor orbits and above all to test whether a large fraction of them really were hyperbolic, and therefore came from deep space. The Arizona material was reduced afterwards in Tartu, between 1932 and 1934.

The answer he got was unexpected. The initial results, published in 1940, said that 57.2 per cent of the observed meteors were on hyperbolic orbits: moving too fast to be gravitationally bound to the Sun, and so, on the face of it, interstellar. For sporadic meteors the fraction was about 60 per cent. A few heliocentric velocities came out above 280 km/s. Figures quoted in the literature for his hyperbolic fraction run from 57.2 per cent up to nearly 70. He was still defending an interstellar origin in 1950.

The correction came in three parts, and only two of them were his. Whipple’s photographic meteor work of 1940 showed that the Taurids, a stream once thought to come from between the stars, circle the Sun and belong with Comet Encke. In 1956 Öpik re-determined the velocities and cut the hyperbolic fraction in his own data from well over half to about 3 per cent. In 1969 he admitted that the rocking-mirror method had rested on premises that did not hold: sporadic and shower meteors appear at different heights, and the eye does not judge the two kinds alike. The observations were sound. The velocity analysis was not. He put the true hyperbolic fraction below one per cent.

The other half of the work held. In 1958 Interscience Publishers of New York brought out ‘Physics of Meteor Flight in the Atmosphere’, number six in the Interscience Tracts on Physics and Astronomy. Its method is an energy balance: the meteoroid’s kinetic energy is spent on ablation, which Öpik divided into vaporization, fusion and spraying of the liquid, sputtering, and fragmentation. The vapour is excited, may be ionized, and radiates as it falls back to its ground state. That is the light we see. Luminous efficiency, the fraction of kinetic energy that becomes radiation, is his 1958 proposal, and it rests on an explicitly ad hoc assumption: that the measured brightness relates directly to the ablation. Meteoroid masses are still estimated from brightness inside that framework, and a 2017 Meteoritical Society abstract still opens its references with the book.

Paul A. Wiegert, Hyperbolic meteors: Interstellar or generated locally via the gravitational slingshot effect?, Icarus 242 (2014), 112–121 (author’s PDF) · Öpik, Physics of Meteor Flight in the Atmosphere (Interscience, New York, 1958) · The 1958 book, scanned at the Internet Archive · I. P. Williams, The velocity of meteoroids: a historical review (Atmospheric Chemistry and Physics 4, 471–475, 2004, PDF) · M. Jõeveer, Ernst Öpik 75-aastane (Tartu Observatory Calendar 45, 1969) · Meteoritical Society meeting abstract 6130 (2017, PDF)

He could not stay. Öpik had volunteered for the White army in the Russian civil war and was firmly against Bolshevik rule, and Armagh Observatory’s own account gives that as the reason he left when Soviet occupation of Estonia became imminent. In 1944, aged 50, he left Tartu Observatory behind, the post he had taken up on 1 December 1921. The MacTutor biography quotes a line of his from a London Evening Standard feature of 2007: ‘I would go to the sea and drown myself rather than live under the Russians.’ The Estonian account places the departure in the summer of 1944; no source used here gives a more exact date.

The departure itself was farm work. As the front came closer he harnessed a horse at his farm in Aakre parish, loaded the children and the essentials onto a cart, and drove to Tallinn, where the family boarded a ship for Germany. His second wife Alide and the children went with him, and he also helped two daughters from his first marriage, Maija and Inna, to get out. The Estonian account gives him six children in all, from two marriages. The home he left stood near Lake Võrtsjärv. The English retelling makes the cart journey hundreds of miles long, ending in a Hamburg refugee camp where conditions were poor.

In March 1946 the Baltic University was set up at Hamburg, in the British Zone of Occupation. It was founded in the displaced persons camps to educate refugees from Estonia, Latvia and Lithuania, with help from UNRRA and the Lutheran World Federation. Öpik became its Professor of Astronomy and Rector for the Estonian students, the university’s first Estonian rector. The university had a president assisted by three national rectors, Estonian, Latvian and Lithuanian; the presidents were Fricis Gulbis (1946–1948), Vladas Stanka (1948–1949) and Eduards Šturms (1949). His biography gives his term as 1945 to 1948, a year earlier than the founding.

In early 1947 the university was moved out of Hamburg to a former Luftwaffe barracks at Pinneberg, the Eggerstedt-Kaserne, and renamed the Displaced Persons’ Study Centre. Öpik organised the teaching there and was a professor of it; geology was taught by his younger brother Armin Öpik, who had fled Estonia in the same year, 1944. His son Uuno Öpik studied at the Baltic University from 1946 to 1948 and then moved with his parents to Northern Ireland. The university closed in 1949, by which time Öpik had already left for Armagh.

The way out came from an old acquaintance. Eric Lindsay, director of Armagh Observatory from 1937, had taken his PhD at Harvard while Öpik was there, and Öpik had been one of the examiners of his thesis. With the Baltic University facing closure, Lindsay obtained money from the Northern Ireland government to create a special post of Research Associate for him. The offer was made in December 1947. Öpik reached Armagh in June 1948, aged 54, having already been employed as an astronomer for 30 years. Estonian accounts call it the end of four hard years.

In Armagh they lived at 30 College Hill, the same street as the observatory: Öpik, Alide, Alide’s sister and three children. What he left in Estonia he had already chosen once: in the 1930s he was offered a professorship at Harvard and turned it down for Tartu, and in 1938 he was elected to the Estonian Academy of Sciences, six years before the flight. From 1956 he was also professor of astrophysics at Maryland, spending several months a year in the United States. He worked at Armagh until he retired in 1981 and died at Bangor on 10 September 1985. The obituary records that he was survived by Alide, one son and five daughters. No source used here records a return to Estonia.

MacTutor (J. J. O’Connor, E. F. Robertson): Ernst Öpik biography · P. A. Wayman, D. J. Mullan: Royal Astronomical Society obituary, QJRAS 27 (1986), 508–512 (MacTutor transcript) · Armagh Observatory and Planetarium: Ernst Julius Öpik (adapted from a biography by John Butler) · Wikipedia: Baltic University (wording adapted from this article, CC BY-SA 4.0) · Laurits Leedjärv, Universitas Tartuensis, 2018: Ernst Öpik 125 – astronoom, kes oli oma ajast ees · Auryn Cox, BBC News NI, 20 April 2026: Ernst Öpik: How a scientist fled the Red Army, turning displacement into discovery · Eva Vabasalu, Estonian World Review (eesti.ca), 2008: Ernst Julius Öpik – the man · Alex Preston, Astronotes (Armagh Observatory and Planetarium), 2021: A Reflection on Ernst Öpik – Part One · Australian Academy of Science: Armin Aleksander Öpik 1898-1983 · ERR Novaator, 2018, after Laurits Leedjärv’s article in Horisont: Ernst Öpik – oma ajast ees kõiketeadja astronoomias · Britannica: Ernst Julius Öpik