23 October 1893|
Kunda, Lääne-Viru, Estonia
|Died||10 September 1985
Bangor, Northern Ireland, United Kingdom
|Alma mater||University of Moscow
University of Tartu
|Notable awards||Gold Medal of the Royal Astronomical Society in 1975
Bruce Medal in 1976
Ernst Julius Öpik (23 October 1893 – 10 September 1985) was an Estonian astronomer and astrophysicist who spent the second half of his career (1948–1981) at the Armagh Observatory in Northern Ireland.
Öpik was born in Kunda, Estonia. He went to the University of Moscow to specialize in the study of minor bodies, such as asteroids, comets, and meteors. He completed his doctorate at the University of Tartu.
Astronomical work 
In 1916 Öpik published article in Astrophysical Journal, where he estimates the densities of visual binary stars. It is interesting to note that in his sample was ο2 Eridani, a white dwarf star. Öpik determined its density as 25,000 times the density of the Sun but concluded that the result is impossible.
In 1922, Ernst Öpik published a paper where he estimated the distance of the Andromeda Galaxy. He determined the distance using a novel astrophysical method based on the observed rotational velocities of the galaxy, which depends on the total mass around which stars are rotating, and on the assumption that the luminosity per unit mass was the same as that of our galaxy. He concluded that the distance was 450 kpc. His result was in good accordance with other estimates of these days (100 to 1000 kpc) and were closer to recent estimates (778 kpc) than Hubble's result (275 kpc). His method is still widely used.
In 1922 he correctly predicted the frequency of craters on Mars long before they were detected by space probes. In 1932 he postulated a theory concerning the origins of comets in our solar system. He believed that they originated in a cloud orbiting far beyond the orbit of Pluto. This cloud is now known as the Oort cloud or alternatively the Öpik-Oort Cloud in his honour. From October 1931 to the end of July 1933, Öpik headed, along with Harlow Shapley and Samuel L. Boothroyd, the Arizona meteor expedition, which detected approximately 22,000 meteors. He also invented a rocking camera for the study of meteors. In 1951 he published a paper concerning the triple-alpha process, describing the burning of helium-4 into carbon-12 in the cores of red giant stars. However, this achievement is often overlooked because Edwin Salpeter's paper on the same subject had already been published by the time Öpik's paper reached Britain and the United States.
In 1972, Öpik published a piece on the origin of the Moon by capture in the Irish Astronomical Journal (vol. 10, pp. 190-238). He included breakup and re-assembly of rings into which the Moon was broken.
Yarkovsky Effect connection 
The Yarkovsky Effect was discovered by the Russian civil engineer Ivan Osipovich Yarkovsky (1844–1902), who worked on scientific problems in his spare time. Writing in a pamphlet around the year 1900, Yarkovsky noted that the diurnal heating of a rotating object in space would cause it to experience a force that, while tiny, could lead to large long-term effects in the orbits of small bodies, especially meteoroids and small asteroids. Yarkovsky's remarkable insight would have been consigned to oblivion had it not been for Öpik, who read Yarkovsky's pamphlet sometime around 1909. Decades later, Öpik, recalling the pamphlet from memory, discussed the possible importance of the Yarkovsky Effect for moving meteoroids about the solar system.
Öpik fled his native country in 1944 because the approaching Red Army raised fear among Estonians. Living as a refugee in Germany, he became the Estonian chancellor of the Baltic University in Exile in the displaced persons camps. In 1948 he was offered a post in Armagh and remained there despite offers of lucrative jobs in America. From the early 1960s to the mid-1970s he also held a position at the University of Maryland, which he visited annually, typically for one semester. As air travel became more common, his refusal to fly made travel to the U.S. from Armagh systematically more difficult and he eventually ceased the annual ritual.
The asteroid 2099 Öpik is named in his honour. His grandson, Lembit Öpik, was formerly the Liberal Democrat Member of Parliament for Montgomeryshire, and has some astronomical connection in that he is a noted supporter of searching for asteroids that may collide with the Earth.
- Öpik, Ernst (12 1916). "The Densities of Visual Binary Stars". ApJ 44: 292–302. Bibcode:1916ApJ....44..292O. doi:10.1086/142296.
- Öpik, Ernst (1922). "An estimate of the distance of the Andromeda Nebula". ApJ 55: 406–410. Bibcode:1922ApJ....55..406O. doi:10.1086/142680.
- Samuel L. Boothroyd was a professor and chair of the department of astronomy at Cornell University from 1921 to 1942.
- Öpik, Ernst (1936). "Meteor Heights from the Arizona Expedition". Proc Natl Acad Sci U S A 22 (9). PMC 1076809.
- Rocking Camera - Armagh Observatory
- Rocking-mirror Meteor Camera - Armagh Observatory
- Salpeter, Edwin E. A Generalist Looks Back. Annual Review of Astronomy and Astrophysics. 2002. 40:1-25. pg. 9
- Sonoma State University on Ernst Öpik
- Armagh Observatory on Ernst Öpik
- Evening Standard (London); "Lembit and his Very Cheeky Family"