BIERMANN, LUDWIG FRANZ BENEDIKT

(b. Hamm, Westfalen, Germany, 13 March 1907; d. Göttingen, Germany, 12 January 1986)

comets, convection, interstellar medium, plasmas, stellar interiors, Sun.

Biermann was a German theoretical astrophysicist who conducted important research in the areas of convection in gases, stellar interiors, the Sun and its granulation and sunspots, comet tails as driven by the solar wind, comet nuclei and envelopes, the interplanetary medium, magnetic fields in the interstellar medium, and plasma physics. He was also the director of the Astrophysics Section of the Max Planck Institute for Astrophysics in Göttingen (1947–1958) and then in Munich (1958–1972). In those capacities he was one of the most influential German astronomers of the mid-twentieth century.

Biography. Biermann’s parents were Franz and Thea (née Schulte) Biermann. He was a student at the University of Munich in 1925–1927 and the University of Freiburg in 1927–1928. In 1932 he received a PhD from the University of Göttingen with a thesis on the topic of convection in stellar interiors. He married Ilse Wandel on 3 January 1942 and they had three children: Peter, Christine, and Sabine. Son Peter L. Biermann, following his father into the astronomy profession, became a productive observational radio astronomer at the Max Planck Institute for Radio Astronomy in Bonn, Germany. Ludwig Biermann died in 1986 at age seventy-eight.

Biographies in English can be found in Helmut A. Abt (1967) and Thomas G. Cowling and Louise Mestel (1986); biographies in German include those by Rudolf Kippenhahn, Arnulf Schlüter, Walter F. Huebner, and Heinz Billing, all collected by Biermann’s successor Gerd Buschhorn (1988).

Positions. Biermann held positions as an exchange scholar at the University of Edinburgh (1933–1934), lecturer in physics at the University of Jena (1934–1937), lecturer at Universität Sternwarte Berlin und Babelsberg (1937–1945), and lecturer at the University of Berlin (1938–1945). He was a professor at the University of Hamburg (1945–1947) and at the University of Göttingen (1948). Between 1947 and 1958 he was director of the Astrophysics Section of the Max Planck Institute for Astrophysics at the University of Göttingen. With the establishment of the Max Planck Institute for Physics and Astrophysics in Munich in 1958, he became director of its Institute of Astrophysics while R. Heinz F. Lüst was director of its Institute of Extraterrestrial Physics and Werner Heisenberg was simultaneously both director of its Institute of Physics and director of the entire Max Planck Institute. Biermann was also a visiting professor at the California Institute of Technology, Haverford College, and Princeton University in 1955 and 1961; at the University of California in Berkeley in 1959–1960; in Sydney and Canberra, Australia, in 1960; and at the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, in 1966–1967.

Research on Stellar Interiors. Biermann’s (1932) first published research concerned stellar interiors. That was at a time before the energy source for a star was known, but it was generally thought to be some sort of subatomic process. He used the usual equations for stellar interiors, except that of the unknown energy generation, and therefore obtained an infinite central temperature. He realized that the gas in the core of the star might be degenerate, that is, in a physical state in which the energy states are filled so that the gas does not behave in a classical manner in which the pressure increases when the temperature or density increases.

Biermann (1935, 1938) continued to work on convection in stars. The energy at any level in a star can be transported by radiation from one ion to another or by mass motion of globs of gas. Which process predominates depends on the local conditions. Cool stars tend to be convective throughout, particularly those that are still contracting, while hot stars are partly in radiation equilibrium. He discovered that an extensive zone below the photosphere of the Sun was in convection equilibrium. That research was partly done through lengthy correspondence (by letter) with Thomas G. Cowling in England before World War II. Biermann suggested (1938b) that the granulation in the solar atmosphere—the rice-grain pattern of bright and less-bright areas seen in highresolution photographs—is due to convection. That explanation is still regarded as valid. Later work (Biermann et al., 1959) extended that explanation.

Convection in a nonrotating star is one thing, but if a star is rotating rapidly there are meridional (i.e., along one meridian of a star, carrying material from the core to the surface and back again) currents that interfere with the convection. Biermann (1948) analyzed that complex situation. With certain radial zones being convective (well mixed) and others radiative (not mixed), there is the possibility of different zones having different compositions as the core changes from being composed nearly totally of hydrogen to being composed mostly of helium. This led Biermann (1943) to consider the composition of the Sun. This proved to be premature because the lack of knowledge about the opacities of gases at temperatures of millions of degrees inhibited a careful comparison with astronomical data. He calculated ionizations and opacities of such material.

Biermann (1938c) computed solar spectra and wondered whether it is possible to see the absorption edges for various metals. However, that was before Rupert Wildt (1939) discovered that the main source of opacity in the stellar atmosphere was H^-, the negative hydrogen ion. The predominance of that source made it unlikely that any absorption edges would be seen. Nevertheless, Biermann computed, often with students, the oscillator strengths (line strengths) for many ions of interest in the solar interior to compute the opacities within stars. This work was published in a half-dozen papers.

Biermann also investigated whether a totally convective star would be dynamically stable to unlimited expansion. Cowling (1938) proved that such a star would be stable because a star would always radiate as much energy as it produces inside, which provides an extra boundary condition that always makes it stable. That was later substantiated in a joint paper by Biermann and Cowling (1940).

One application of Biermann’s thinking of convection was his application to the nova phenomenon. Novas are stars that suddenly become brighter by a factor of 100,000 in a couple of days. Biermann wondered (1939) whether a star could suddenly change from being in radiative to convective equilibrium, thereby causing such an outburst. However, in 1964 Robert P. Kraft proved that all novae are in binaries, consisting of an expanding cool star that dumps material upon a white dwarf companion.

They have orbital periods around one day. The hydrogen-rich material ignites, much like throwing gasoline on the fire, in the white dwarf atmosphere, provoking a violent outburst.

In 1961, simultaneously with but independently of Martin Schwarzschild, Biermann suggested that waves in the outer convection zone of the Sun just below the atmosphere caused acoustic waves that heated the chromosphere and corona. That would mean that other late-type stars, later than spectral type F2, would also have hot coronas. That was confirmed later with the detection of x-rays from most late-type stars, stellar winds and mass loss, low rotational velocities, and calcium emission lines.

Studies of the big bang were very successful in predicting the abundance of helium in the early universe, and studies of stellar interiors, particularly by E. Margaret Burbidge et al. (1957), explained many of the abundance characteristics in most normal stars. However, there is a class of peculiar A-type stars (Ap) that have drastically different atmospheric abundances, for example, stars with 10⁶ times the normal mercury abundance. William A.

Fowler and others (1965) proposed that some nuclear reactions were occurring in subsurface regions initiated by strong magnetic fields (>1,000 gauss). That proposal was later replaced when Georges Michaud (1970) showed that a more natural and successful explanation was in diffusion. Ap stars have a still radiative zone between two outer convective zones just below the photosphere. In the quiet zone, heavy ions that exhibit few spectral lines will fall inward while light elements with rich spectra will be pushed outward by radiation pressure. To date that explanation has been accepted for the extreme Ap stars and the less extreme metallic-line (Am) stars that also have overabundances by factors of 10 and underabundances of helium, calcium, and scandium.

Comets. Sidney Chapman (1929), occasionally working with V. C. A. Ferraro (Chapman & Ferraro, 1930), proposed that a solar wind of neutral particles caused magnetic and electrical disturbances in Earth’s atmosphere and auroras during magnetic storms. Biermann (1951) wondered whether the same particles could be deflecting the gaseous tails of comets. Comets often have two tails: one of particles that trail behind due to the motion of the comet in its orbit, and a gaseous tail that points generally away from the Sun. He measured the structure in the tail of Whipple-Fedtke (1943 I) and computed the density and speed of the wind, getting 600 particles per cm³ and speeds of 500–1000 km sec^-1. Those numbers agreed with results from whistlers. (Whistlers are radio signals of audio frequencies heard at random times.) The explanation for them is that if lightning strokes occur at one place on Earth (e.g., in Annapolis, Maryland), at the other end of a geomagnetic line (Cape Horn, South Africa) a whistler or click occurs. The event can pass back and forth with one, three, five, or even seven passages between the two points. However, those numbers for the density and speed of the solar wind later had to be revised substantially downward because of a lack of consideration of the effects of magnetic fields. Biermann was the first to realize that the solar wind was acting continuously in time, not just during magnetic storms on Earth.

Biermann and Eleonore Trefftz (1964) speculated that comets should have extensive envelopes, up to 10,000 kilometers or more in diameter. Those should consist of neutral hydrogen and various molecules. These are the result of evaporation from Fred Whipple’s (1950) “icy conglomerate model.” Later observations from spacecraft in Lα (Lyman alpha at 1,216 angstroms) in the far ultraviolet region of the spectrum confirmed the presence of envelopes in Comet Bennett and other comets.

Biermann joined other astronomers in investigating the composition of the molecules in cometary nuclei. The physical conditions within the nuclei are not sufficient to produce the observed molecular abundances. Biermann and K. W. Michel (1978) considered whether the composition of cometary nuclei originated in the presolar nebula, the gaseous disk from which the Sun, its planets, asteroids, comets, and other constituents formed. Biermann and others (1982), working at Los Alamos National Laboratory in New Mexico, found that by suggesting a cometary origin in interstellar matter, the molecular abundances were consistent with observations.

The Interplanetary Medium. In 1968 Biermann delivered a series of lectures in the Department of Aerospace Engineering Sciences at the University of Colorado on the interplanetary medium that was reproduced in a book coauthored with Evry Schatzman, Cosmic Gas Dynamics(1974). This is a comprehensive and highly mathematical review, starting with the hydrodynamics and kinematics of the solar corona and solar wind, continuing through the magnetic fields and turbulence of the interplanetary medium, and concluding with the termination of the solar wind at the heliosphere, where it joins the interstellar medium.

The Interstellar Magnetic Field. Together with Leverett Davis Jr. (1960), Biermann calculated the magnetic fields that needed to be present in the halo and disk of Earth’s own galactic system in order to explain the in situ cosmic ray measurements in balloons by C. L. Critchfield and others (1952). That is, Biermann and Davis proposed that magnetic fields store the relativistic electrons produced by cosmic rays. For this to happen the magnetic fields must be greater than 5 x 10^-6 gauss in the halo and 2 x 10^-5 gauss in the disk. These values are consistent with the idea that the magnetic fields, cosmic rays, thermal pressure (pressure due to the gas temperature), and kinetic energy (due to the motions in the gas) are just able to counteract the inward force of gravity. These values are also consistent with other estimates on the interstellar magnetic field strengths.

Administrative Work. As noted above, Biermann was director of the Astrophysics Section of the Max Planck Institute in Göttingen from 1947 to 1958 and in Munich from 1958 to 1972. At both places he gathered around himself an outstanding group that was interested in cosmical electrodynamics and plasma physics in areas such as the solar chromosphere and corona, the solar wind, sunspots and flares, the interplanetary medium, stellar interiors, and the interstellar medium. He collaborated with Arnulf Schlüter, Eleonore Trefftz, Reimar and Rhea Lüst, Rudolf Kippenhahn, Friedrich Meyer, and Stefan Temesvary. He helped establish the parallel institutes of plasma physics and extraterrestrial physics. Together these made the Max Planck combined institute in Munich the outstanding theoretical astronomical center in continental Europe in the second half of the twentieth century.

Honors. Biermann was the recipient of the Copernicus Prize in 1943, member of the Bavarian Academy of Sciences and Humanities and of the International Academy of Astronautics, corresponding member of the Société Royale des Sciences de Liège, member of the Akademie der Naturforscher Leopoldina in Halle (East Germany), foreign associate of the National Academy of Sciences (United States), recipient of the C. W. Bruce Gold Medal of the Astronomical Society of the Pacific (United States) in 1967, associate of the Royal Astronomical Society (England) in 1964, and recipient of the Gold Medal of the same society in 1974. He was a member of the Astronomische Gesellschaft (Germany) and a German delegate to EURATOM.

BIBLIOGRAPHY

WORKS BY BIERMANN

“Untersuchungen über den inneren Aufbau der Sterne, IV.

Konvektionzonen im Innern der Sterne.” Zeitschrift für Astrophysik 5 (1932): 117–139. “Konvektion im Innern der Sterne.” Astronomische Nachrichten

257 (1935): 269–294. “Konvektion im Innern der Sterne (II).” Astronomische

Nachrichten264 (1938a): 361–395. “Zur Theorie der Granulation und der

Wasserstoffkonvektionszone der Sonne.” Astronomische Nachrichten264 (1938b): 395–398. “Über die Möglichkeit des Auftretens von

Metallabsorptionskanten im Spektrum der Sonne und der Sterne.” Zeitschrift für Astrophysik 16 (1938c): 291–296. “Über die dem Novaphänomen zugrunde liegenden physikalischen Vorgänge.” Zeitschrift für Astrophysik 18 (1939): 344–361.

With Thomas G. Cowling. “Chemische Zusammensetzung und dynamische Stabilität der Sterne, II.” Zeitschrift für Astrophysik 19 (1940): 1–10. Über die chemische Zusammensetzung der Sonne.” Zeitschrift für Astrophysik 22 (1943): 244–264. “Konvektion in rotierenden Sternen.” Zeitschrift für Astrophysik

25 (1948): 135–144. “Kometenschweife und solare Korpuskularstrahlung.” Zeitschrfit für Astrophysik 29 (1951): 274–286.

With Rudolf Kippenhahn, Rhea Lüst, and Stefan Temesvary.

“Beiträge zur Theorie der Sonnengranulation.” Zeitschrift für Astrophysik 48 (1959): 172–188.

With Leverett Davis Jr. “Considerations Bearing on the

Structure of the Galaxy.” Zeitschrift für Astrophysik 51 (1960): 19–31.

With Eleonore Trefftz. “Über die Mechanismen der Ionisation und der Anregung in Kometenatmosphären.” Zeitschrift für Astrophysik 59 (1964): 1–28.

With Evry Schatzman. Cosmic Gas Dynamics. Edited by

Mahinder S. Uberoi. New York: Wiley, 1974.

With K. W. Michel. “On the Origin of Cometary Nuclei in the

Presolar Nebula.” Moon and the Planets 18 (1978): 447–464.

With Paul T. Giguere and Walter F. Huebner. “A Model of a

Comet Coma with Interstellar Molecules in the Nucleus.” Astronomy & Astrophysics 108 (1982): 221–226.

OTHER WORKS

Abt, Helmut A. “Award of the Bruce Gold Medal to Professor

Ludwig Biermann.” Publications of the Astronomical Society of the Pacific 79 (1967): 197–200.

Burbidge, E. Margaret, et al. “Synthesis of the Elements in

Stars.” Reviews of Modern Physics29 (1957): 547–650. Buschhorn, Gerd. “Ludwig Biermann, 1907–1986.” Max-Planck-Gesellschaft Berichte und Mitteilungen 2 (1988): 5–80. Chapman, Sidney. “Solar Streams of Corpuscles: Their

Geometry, Absorption of Light, and Penetration.” Monthly Notices of the Royal Astronomical Society 89 (1929): 456–470. Chapman, Sidney, and Vincent C. A. Ferraro. “A New Theory of

Magnetic Storms.” Nature 126 (1930): 129–130. Cowling, Thomas G. “The Stability of Convective Stars.”

Monthly Notices of the Royal Astronomical Society 98 (1938): 528–535.

Cowling, Thomas G., and Louise Mestel. “Ludwig Franz

Benedict Biermann.” Quarterly Journal of the Royal Astronomical Society 27 (1986): 698–700.

Critchfield, Charles L., Edward P. Ney, and Sophie Oleksa. “Soft

Radiation at Balloon Altitudes.” Physical Review 85 (1952): 461–467.

Fowler, William A., et al. “The Synthesis and Destruction of

Elements in Peculiar Stars of Types A and B.” Astrophysical Journal142 (1965): 423–450.

Michaud, Georges. “Diffusion Processes in Peculiar A Stars.”

Astrophysical Journal 160 (1970): 641–658.

Whipple, Fred L. “A Comet Model I: The Acceleration of

Comet Encke.” Astrophysical Journal 111 (1950): 375–394. Wildt, Rupert. “Negative Ions of Hydrogen and the Opacity of

Stellar Atmospheres.” Astrophysical Journal 90 (1939): 611–620.

Helmut A. Abt