## Heckmann, Otto Hermann Leopold

## Heckmann, Otto Hermann Leopold

# HECKMANN, OTTO HERMANN LEOPOLD

(*b*. Opladen, Germany, 23 June 1901;

*d*. Regensburg, Germany, 13 May 1983),

*astronomy, astrophysics, cosmology*

Heckmann was a German astronomer and astrophysicist active between 1925 and the 1970s who mastered the roles of observing astronomer, theoretical astrophysicist, and manager for the astronomical community in Europe, even worldwide. He was one of the founding members of the European Southern Observatory (ESO) in Chile, and left a particularly strong imprint. His scientific research covered astrometry (positions and proper motions of stars), photographic photometry of stellar clusters, the application of statistical mechanics to stellar dynamics, optical design, and theoretical cosmology.

**Formative Period: Astrometry** . Heckmann stemmed from a Catholic family in the Rhineland; his parents were Max Heckmann, a notary in Opladen, and Agnes Heckmann, née Grüter. He received his schooling at a gymnasium (secondary school) whose teaching was directed more toward the sciences than that of the regular humanistic schools. He studied mathematics, physics, and astronomy at the University of Bonn. Under the guidance of his astronomy teacher Karl Friedrich Küstner he wrote a doctoral thesis in astrometry concerning the precise determination of star positions in the open star cluster Praesepe in Cancer. He used photographic plates obtained at the Bonn Observatory. After graduation in 1925 he became an assistant to Küstner, who was involved in the planning for the star catalog AGK2 of the Northern Hemisphere, with the Bonn part then directed by Ernst Arnold Kohlschütter. (The catalog AGK1 had started in the 1870s and was completed in the 1930s.) Heckmann closely inspected the four-lens optical system of the Bonn telescope with the help of which this new star catalog was to be made. This time spent learning the optical adjustment of telescopes, one of his hobbies, later helped him with instruments in Göttingen, Hamburg, and at the ESO.

**Career Steps: Photometry, Stellar Dynamics, Cosmology** . Upon becoming, in 1927, an assistant of Hans Kienle, then director of the Göttingen Observatory, Heckmann, with his background in the determination of positions and proper motions of stars, shifted his focus to photographic photometry and its instrumental and observational problems. A program of measuring star colors in the red and blue band was carried through until 1935. Together with his collaborator Hans Haffner he succeeded in reaching an unprecedented astrometric precision for Praesepe by this photographic method. Both astronomers showed that, in the color-luminosity diagram, a sharp main sequence resulted. In 1929, Heckmann obtained his *venia legendi* (habilitation) at the University of Göttingen with an investigation of star positions in the star group Coma Berenices. In 1935, he received the honorary title of professor while remaining Kienle’s assistant. His observations in Göttingen were made with a new ASTROgraph in the Hainberg observatory, a telescope that cost Heckmann and Kienle two years of painstaking adjustment until it began working properly. In 1937 this instrument was supplemented by the first commercially built Schmidt telescope (by Zeiss-Jena) with an effective aperture of circa 38 centimeters.

While extending these empirical studies to weak-group stars in Praesepe, some of his theoretical interest went into the stellar statistics of globular clusters. These self-gravitating systems are intrinsically unstable; it is known that a final equilibrium state of maximal entropy cannot be reached. The treatment of clusters by statistical methods is notoriously difficult due to the long-range gravitational force. In a highly idealized model assuming energy conservation, and using the Boltzmann equation, Heckmann and Heinrich Siedentopf (1930) showed that by two-body interactions, that is, the scattering of point-like stars, an approximate isothermal state may be obtained. For this, the cluster must be thought to be embedded in an infinite system. (While this result had been encouraging then, it can at best be used as a solution of lowest approximation. Today, numerical integration of the Fokker-Planck equation is providing reasonable insight into the dynamics of the collapsing core and the postcollapse evolution of a globular cluster.) Heckmann and Siedentopf concluded that for their model of a globular cluster, Albert Einstein’s relativistic theory would give more natural results than Isaac Newton’s classical theory. (This belief is not backed by present observations restricting relativistic effects to the nuclei of galaxies.)

Also in Göttingen, papers with Hans Strassl (an astronomer also trained as an insurance mathematician) analyzed the detailed properties of the star stream, that is, the distribution of star velocities in our neighborhood in the Milky Way, by applying statistical mechanics (Heckmann and Strassl, 1934, 1935). By use of a Gaussian distribution, the authors succeeded in describing the differential rotation of the galaxy in addition to the effect of solar motion. Such investigations showed Heckmann’s interest and expertise in theoretical questions, and his ability to find competent collaborators.

A subject close to Heckmann’s heart during all of his scientific life, and understood by him as a “personal pleasure,” was cosmological theory. Already during his student days in Bonn he had given a mathematical seminar on Charlier’s hierarchical cosmological model. In the 1920s, with Einstein’s general relativity becoming the dominant theory of gravitation, Heckmann’s interest centered around cosmological models of this theory.

As a consequence of Einstein’s first cosmological model of 1917, until the 1930s physicists (Georges Lemaître, Einstein, Willem de Sitter, Arthur Eddington) dealt exclusively with world models with constant positive space curvature. At the time, apart from the introduction of the cosmological constant, this had been the sensational part of Einstein’s paper: that three-dimensional space in his cosmological model must be closed, that is, be a space with a finite volume but without boundaries. Up to then, infinitely extended Euclidean space with zero constant curvature had been assumed to describe reality. In addition, Alexander Friedmann had shown in 1924 that expanding homogeneous and isotropic cosmological models with constant negative space curvature do also exist. Provided with the simplest topology, they also are infinitely extended. In 1929 Howard P. Robertson independently rederived the world models with nonpositive curvature.

Heckmann, who had given his first course on general relativity in Göttingen during the winter term 1929–1930, was aware only of de Sitter’s and Lemaître’s results connected with positive space curvature. Thus in 1931, he again obtained Friedmann’s and Robertson’s geometries in his world models generated by an ideal fluid (with nonvanishing pressure) and a photon gas, both noninteracting, as their material source. The famous mathematician Hermann Weyl, who had also given contributions to cosmology, handed in this paper to the Göttingen Academy. While Robertson’s paper is conceptually very clear, Heckmann’s contribution seems more physical by stressing the differential form of the energy conservation law for matter (cf. also North, 1965). In Heckmann’s opinion, a decision for one of the three possibilities for positive, negative, or zero space curvature could not yet be made: the measurements of the matter density in the cosmos were too unprecise. He seemed to tend toward negative curvature (Heckmann, 1932, p. 105). De Sitter received a copy of Heckmann’s publication and wrote a joint paper with Einstein mentioning Heckmann’s name without giving a precise reference. In it, they acknowledged that “at the present time it is possible to represent the facts without assuming a curvature of 3-dimensional space” (Einstein & de Sitter, 1932, p. 214). Today, this particular world model is called Einstein–de Sitter space; the name Robertson–Heckmann solution would be also justified. In 1932, Heckmann gave a complete discussion and classification of the homogeneous and isotropic solutions of Friedmann’s equations (with cosmological constant).

Heckmann also was interested in theoretical optics: After Max Born had been fired by the Nazi authorities, Heckmann took over his course on theoretical optics at the University of Göttingen, apparently with Born’s encouragement (Heckmann, 1976, p. 30).

**Observatory Director: Cosmology, AGK3, and Schmidt Telescope** . In 1934, Edward Arthur Milne and William Hunter McCrea had developed what now is named Newtonian cosmology, that is, a cosmological theory based on Newton’s theory of gravitation supplemented with the so-called world postulate and a new definition of inertial systems. Heckmann, an astronomer continuously involved in painstaking observational work, was always inclined to test theories against practice. When he learned of Milne’s approach to cosmology in 1935, he compared it to Einstein’s and slightly expanded on it (Heckmann, 1940).

In 1937 the Hamburg University faculty’s call of Heckmann to the Observatory of Bergedorf near Hamburg was obstructed for years by the Government of the Reich, by the Reichsdozentenbund (an association of government lecturers dominated by the Nazis), and by various agencies within the Nazi Party, which, on the other side, ended the academic career of his coworker Strassl altogether. The call materialized only in April 1941, when Heckmann was named substitute director, and in January 1942 when he finally became director and full professor in Hamburg (Hentschel & Renneberg, 1995). In the meantime, since 1939, Heckmann had been administrative director of the Göttingen Observatory.

In his 1942 monograph *Theorien der Kosmologie*, Heckmann cleverly sandwiched his exposition of Einstein’s general relativity as applied to cosmology between Milne’s dynamical cosmology based on Newton, and Milne’s kinematical cosmology resting on the Lorentz group. The empirical material did not force him to take sides, and he left open his predilections (cf. Hentschel & Renneberg, 1995). However, he pointed out that Newtonian cosmology could properly describe light propagation only with difficulty. Werner Heisenberg and Friedrich Hund in Leipzig did a critical reading “of large parts of the manuscript.” Twenty years later, Heckmann’s conclusion was that Einstein’s theory should be preferred: “if we approach limits of velocity or of compression, or if we want to handle large-scale optical problems” (Heckmann, 1962, p. 430).

Heckmann continued to investigate both nonrelativistic and relativistic theories of gravitation after 1945. For some years, with Engelbert Schücking, he was able to maintain a research group for theoretical cosmology paralleling the much larger group of Pascual Jordan at Hamburg University. In a direct collaboration with Jordan and a younger astronomer, Walter Fricke, solutions of a generalization of general relativity, a scalar-tensor-theory of gravitation now called Jordan-Brans-Dicke theory, and corresponding to the Schwarzschild space-time, were obtained (Heckmann, Jordan, & Fricke, 1951). Within Newtonian cosmology, Schücking and Heckmann found cosmological models with rotation as analogs to the Gödel space-time in Einstein’s theory, and also more general rotating solutions with expansion and shear (Heckmann & Schücking, 1955, 1956). Heckmann hoped that the existence of rotation (and of shear) in cosmological models would avoid the occurrence of a big bang. With Schücking, he wrote two articles on cosmological theory in the new *Encyclopedia of Physics* (1959). In numerous lectures, contributions to conferences, and book articles, research concerning “world models,” both in Newtonian and in relativistic cosmology, remained a constant theme of Heckmann’s until the early 1960s. Heckmann’s contributions to cosmological modeling, although not pioneering work, are still appreciated by today’s cosmologists with an interest in the history of their field.

Heckmann had not been directly involved with observational work for the star catalog AGK2, started around 1930, but when in Hamburg he saw to it that this catalog was carried through to its completion. This was done until 1950 by the observatory’s director preceding him, Richard Schorr. Heckmann then initiated a new catalog of star positions comprising 180,000 stars in the Northern Hemisphere: AGK3. His motivation was to increase the precision for the positions of fixed stars in the Northern Hemisphere, and to possibly determine their proper motions since the measurements for AGK2. The work was performed between 1955 and 1975 under the scientific direction of Wilhelm Dieckvoss (Heckmann, 1975) who had already been involved in AGK2.

In the field of photographic photometry, among other investigations, Heckmann identified an assembly of stars around alpha Persei as an open star cluster, and determined proper motions for stars in the neighborhood of Praesepe. Since 1942, observations of the Hyades cluster had been made in Hamburg, from which, through a collaboration with Klaus Lübeck, a precise color-magnitude diagram was obtained.

Still another area of Heckmann’s unrelenting activity was the planning and acquisition of new telescopes. In 1931, the inventor Bernhard Schmidt had given the Hamburg Observatory its first Schmidt telescope. As early as 1937 a larger such instrument had been approved but remained unbuilt until the end of World War II. Due to Heckmann’s energetic prodding, a Schmidt telescope with an 80-centimeter free opening and a mirror diameter of 100 centimeters could be installed in 1954. It used an objective prism for getting spectra. The optical adjustment was done by Heckmann and his colleague Haffner, in Hamburg since 1937. His experience with this instrument helped Heckmann in the planning of the Schmidt telescope for ESO (100/120 cm) operating since the early 1970s. In the meantime, Heckmann’s advice had also been appreciated for the construction of an even larger Schmidt telescope at the Zentralinstitut für Astrophysik in Tautenburg in the German Democratic Republic.

**A Powerful Organizer** . After his indirect involvement in the star catalog AGK2, he successfully convinced a dozen European observatories to provide positions of reference stars by meridian observations for the new zonal star catalog AGK3. He also supported a similar reference project for the Southern Hemisphere (Southern Reference Stars Project, SRS), and provided the transit circle of the Bergedorf Observatory for astronomers in Perth, Australia. Since the original letter of intent, signed on 26 January 1954 by astronomers from six European countries, the idea of an ESO had taken up an increasing share of Heckmann’s many activities. He kept a close relationship with the influential astronomer Walter Baade, who had given an important first impulse to the idea of a joint European project in the Southern Hemisphere. In 1962, Heckmann obtained a leave of absence from Hamburg University in order to fill the first directorship of ESO, then still in its organizational gestation.

In the 1955–1963 period, the search for an appropriate observing site was focused on South Africa. Even before the final site in the Andes Mountains at La Silla (Chile) was formally approved by ESO on 26 May 1964, Heckmann, with the help of Chilean friends, in the fall of 1963 had been able to negotiate with the government of Chile. He had reached an agreement to be endorsed by the ESO Council and the Chilean Parliament. According to Heckmann, this unauthorized and later criticized but accepted move had speeded the project by circumventing lengthy formal debates among European governments. As to the installation of instruments, the time won in setting up the organization was partially lost, again, when the chief construction engineer for the Schmidt telescope chosen by Heckmann did not comply with the specifications.

Heckmann’s contributions to the planning of the observatory and the realization of ESO’s headquarters in Santiago and Garching is his greatest and lasting organizational achievement. He carried the main burden before full operation could be achieved; his French colleague in the instrumentation committee of ESO, Charles Fehrenbach, called him the “work horse” of the organization (Fehrenbach, 1984, p. 592). Indeed, he took on his responsibilities as the director of ESO until his mandatory retirement from the university in 1969, and continued as a consultant until 1972 (Blaauw, 1991). The period of his ESO directorship was filled with many honors: doctoral degrees from universities in France, Argentina, and the United Kingdom; medals from professional societies in the United States and France; membership in many scientific academies; and the presidency of the International Astronomical Union from 1967 to 1970.

## BIBLIOGRAPHY

### WORKS BY HECKMANN

With Heinrich Siedentopf. “Zur Dynamik kugelförmiger Sternhaufen.” *Zeitschrift für Astrophysik* 1 (1930): 67–97.

“Über die Metrik des sich ausdehnenden Universums.” *Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse*(1931): 126–130.

“Die Ausdehnung der Welt in ihrer Abhängigkeit von der Zeit.” *Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse*(1932): 97–106.

With Hans Strassl. “Zur Dynamik des Sternsystems.” *Nachrichten von der Gesellschaft der Wissenschaft zu Göttingen, Mathematisch-Physikalische Klasse, Fachgruppe II*(1934): 91–106; also *Veröffentlichungen der Universitätssternwarte Göttingen* no. 41: 191–206.

———. “Zur Dynamik des Sternsystems (Fortsetzung).” *Nachrichten von der Gesellschaft der Wissenschaft zu Göttingen, Mathematisch-Physikalische Klasse, Fachgruppe II*(1935): 153–170; also *Veröffentlichungen der Universitätssternwarte Göttingen*, no. 43: 217–221.

“Zur Kosmologie.” *Nachrichten von der Gesellschaft der Wissenschaft zu Göttingen, Mathematisch-Physikalische Klasse, Neue Folge*, *Fachgruppe II*, 3, no. 15 (1940); also *Veröffentlichungen der Universitätssternwarte Göttingen*, no. 68.

*Theorien der Kosmologie*. Fortschritte der Astronomie. Edited by P. Ten Bruggencate, vol. 2. Berlin: Springer, 1942. Reprinted Berlin: Springer, 1968, with an additional preface and new comments. The first monograph on cosmological theories in Germany.

With Pascual Jordan and Walter Fricke. “Zur erweiterten Gravitationstheorie. I.” *Zeitschrift für Astrophysik* 28 (1951): 113–149. Spherically symmetric solutions of Jordan-BransDicke theory.

“The Value of a Third AG Catalogue.” *Astrophysical Journal* 59 (1954): 31–34.

With Engelbert Schücking. “Bemerkungen zur Newtonschen Kosmologie. I.” *Zeitschrift für Astrophysik* 38 (1955): 95–109.

———. “Bemerkungen zur Newtonschen Kosmologie. II.” *Zeitschrift für Astrophysik* 40 (1956): 81–92.

With Klaus Lübeck. “Helligkeiten und Eigenbewegungen in den Hyaden.” *Zeitschrift für Astrophysik* 40 (1956): 1–20. A shorter English version is: “Photographic Photometry of the Hyades.” *Vistas of Astronomy* 2 (1956): 1115–1122.

With Engelbert Schücking. “Newtonsche und Einsteinsche Kosmologie” and “Andere kosmologische Theorien.” In *Encyclopedia of Physics*, edited by S. Flügge, vol. 53, *Astrophysics IV: Stellar Systems*. Berlin: Springer, 1959.

“General Review of Cosmological Theories.” In *Problems of Extra-Galactic Research*, edited by G. C. McVittie. IAU Symposium No. 15 (1961). London: Collier-Macmillan, 1962.

“On the Possible Influence of a General Rotation on the Expansion of the Universe.” *Astrophysical Journal* 66 (1961): 599–603.

*AGK3: Star Catalogue of Positions and Proper Motions North of –2.5 deg. Declination*. 8 vols. Edited by Wilhelm Dieckvoss. Hamburg and Bergedorf, Germany: Sternwarte, 1975. Planned by O. Heckmann; produced and edited by W. Dieckvoss, in collaboration with H. Kox, A. Günther, and E. Brosterhus.

*Sterne Kosmos Weltmodelle: Erlebte Astronomie*. Munich, Germany: Piper, 1976. An instructive autobiography.

### OTHER SOURCES

Blaauw, Adriaan. *ESO’s Early History: The European Southern Observatory from Concept to Reality*. Munich, Germany: ESO, 1991.

Einstein, Albert, and Willem de Sitter. “On the Relation between the Expansion and the Mean Density of the Universe.” *Proceedings of the National Academy of Sciences of the United States of America* 18 (1932): 213–214.

Fehrenbach, Charles. “La vie et l’oeuvre d’Otto Heckmann.” *Comptes rendus*, general series, 1 (1984): 591–593.

Friedmann, Alexander. “Über die Möglichkeit Einer welt mit Konstanter Negativer Krümmung des Raumes.” *Zeitschrift für Physik* 21 (1924): 326–332.

Hentschel, Klaus, and Monika Renneberg. “Eine Akademische Karriere: Der Astronom Otto Heckmann im Dritten Reich.” *Vierteljahreshefte für Zeitgeschichte* 43 (1995): 581–610. For those interested in political and administrative maneuverings during the Third Reich and the position Heckmann took.

North, John David. *The Measure of the Universe: A History of Modern Cosmology*. Oxford: Clarendon Press, 1965. A general presentation of the field in which Heckmann’s contributions to cosmology are evaluated.

Robertson, Howard P. “On the Foundations of Relativistic Cosmology.” *Proceedings of the National Academy of Sciences of the United States of America* 15 (1929): 822–829.

Voigt, Hans Heinrich. “Nachruf auf Otto Heckmann.” *Jahrbuch der Akademie der Wissenschaften in Göttingen* (1983): 80–87. An obituary written by the former director of the Göttingen Observatory, who was close to Heckmann.

*Hubert Goenner*

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